Chapter 3- Alpha & Delta Drives

3.1 Servo Control System

3.1.1 Introduction

This section describes the main servo systems for the precise control of the telescope in both the polar axis (rotation of the horse shoe) and the declination axis (pivoting of the telescope tube). This movement is achieved using two independant, identical servo system. These system are normally controlled by VME LCU unit or, exceptionally, they may be completely controlled by hardware in one of two operating modes.

The hardware operating modes are provided as a "fall-back" in case of a computer malfunction. They also allow the telescope to be easily manoeuvred by the maintenance staff for testing and servicing work. The minimun hardware system which can be utilized is the basic analogue feedback system. (This minimum system incorporates current, velocity and acceleration feedbackloops in a novel arrangement to achieve the necessary degree of dynamic control and stability). In this mode the telescope can be moved at slewing speed only in both axes.

When the computer takes control of the servo systems, as in the normal case, all of the control facilities become available. In the fully automatic mode information is input on an interactive terminal. All of the telescope control facilities, and various back-up software, are available at the terminal. In the on-line mode the telescope can also be operated by means of a control panel and handset which interface directly with the computer.

This system philosophy prevents successive breakdowns in the more sophisticated control loops from causing a complete shut-down of the telescope. During a computer breakdown the manual control facilities remain active, but with successively restricted facilities.

3.1.2 The mechanical Drive System.

Both main axes of the telescope are driven by identical systems and the same mechanical drive system. Each servo drive uses two motors which are electrically and mechanically configured in a zero backlash arregement. The motors are Inland Servo's torque-motors, type T12022/B. These multipole 2 KW servomotors have very low ripple torque and a linear relationship between drive current and output torque. A schematic drawing of a complete motor drive arrangement for one axis is shown in the inset diagram.

Two motor-tachometer units are coupled via the gearbox to a large precision-ground gear-wheel. Each motor drives the gear wheel via three reduction stages. To eliminate backlash the two motors always oppose each other with a 'preload' torque. When the gear-wheel is driven in one direction drive-torque is provided by one motor while the other motor exerts an opposing torque. When the gear-wheel is driven in the other direction the functions of the motors are automatically reversed.

Both motors and the gearbox ride directly on the rim of the gear-wheel itself. The only rigid connection between the gear assembly and the telescope structure is by an articulated moment arm. This allows the gear meshing to be perfectly maintained, preventing any errors due to elastic deformation of the structure.

3.1.3 Analogue Feedback Loops

Figure number CS-E-0773 (below) shows the general layout of a complete servo system in a very simplified form. The dotted vertical line through the drawing shows the dividing line between the hardware and software controls of the servo system. All elements to the left of this line are a part of the servo control program; they do not exist in hardware. For simplicity only one motor is shown in the diagram. The functional operation of the servo system can be described at this point without considering the additional control circuitry for the second motor.

Three different types of feedback are used in the analogue part of the control system. A 2 kW linear power amplifier drives one motor through a series current sensing resistor R. This resistor develops a feedback voltage proportional to the motor current.

Using this current feedback signal the power amplifier is connected in a closed loop configuration. The input voltage is compared with the current feedback voltage at summing junction A. Any resultant difference signal is amplified by the power amplifier to correct the error. As output torque is proportional to the motor currcnt, the closed loop power amplifier/motor combination acts as a voltage to torque converter. Applied output torque is directly proportional to the input voltage at summing junction A.

Both velocity and acceleration feedback are used at summing junction B to provide tight loop control over the dynamic movement of the telescope tube. A multipole D.C. -generating tachometer is fitted directly to the motor shaft to provide a feedback voltage proportional to velocity. This signal establishes a direct analogue relationship between the input signal, at summing junction B, and the velocity with which the telescope moves. Acceleration feedback is also introduced at summing junction B from an accelerometer mounted directly on the telescope tube (or horse shoe, in the polar axis servo system) . As the motor does not drive the telescope directly, but through a gearbox, the mechanical coupling between the motor and telescope is not perfectly rigid. There will be a degree of elastic movement between the two. Acceleration feedback is necessary to ensure that movement of the telescope follows the input signal 'tightly', without dynamic errors due to the mechanical resonance of the telescope exaggerated by elastic deformation in the mechanical coupling.

This completes the basic analogue feedback configuration. The input voltage at summing junction B controls the velocity of the telescope in each direction. 'Tight' dynamic tracking exists between the rate of change of the input signal (slope) and the resulting acceleration of the telescope tube.

3.2 Analog Servo System

3.2.1 General Arrangement

Figure Number CS-E-0666 shows the basic layout of the servo system and in particular gives more detail on the analogue circuits.

The mode selector switch controls input relays which select an input signal from either a digital to analogue converter (DAC, digital feedback mode) or manually switched D.C. voltages (analogue mode). This signal is then processed by a ramp circuit to prevent the telescope from being driven at excessive velocity or acceleration. The output from this circuit feeds the closed 1oop analogue servo.

The first processing element within the analogue 1oop is the PID controller (Proportional Integral and Differential controller). This unit modifies the phase and amplitude response of the loop to ensure critical damping. Preset components are used to allow the loop response to be exactly matched to the various major system time-constants. The operation of this circuit is explained later.

Various link connections are used in the servo interface to set the operating mode, A or C, which determines the way in which the motors are controlled. The polarity separator is directly concerned with these different operating modes and a general explanation of the operation of the servo interface unit is given in the following section.

Each motor is driven by an individual 2 kW power amplifier. DC generating tacho generators coupled directly to the motor shafts are used for velocity feedback. Either one or both of these signals may be used, depending on the operating mode. The motor wiring arrangements and detailed circuit operation of the power amplifiers is described later.

3.2.2 Servo Interface Operating Modes

The Servo Interface components are shown enclosed in dotted lines on Figure Number CS-E-0666

The drive system can be connected in one of two possible operating modes, labeled as A and C. It is not intended that changes from one mode to another will be made for different applications. They were provided to allow alternative drive systems to be used during commissioning. In mode C (all C links bridged) the motors can exert an increased maximum drive torque on the telescope. In mode A (all A links bridged) the motors drive the telescope at normal torque levels with minimum gear loading. Mode A is generally preferable because the gear loading is much lower at all drive levels.

To eliminate backlash the two motors are always driven in opposition to each other with a fixed preload current. When the servo systems are being used a static signal is input to the preload circuit, 1C5, from the selector relays. This circuit operates as a linear integrator to prevent the preload signal from being suddenly applied. The delay, or time constant is determined by C4 and R18 and is approximately one second. The delayed output signal is then applied to both power amplifiers which cause a static preload current to flow in the motors. In mode C this current is approximately 10 amps (in each motor), while in mode A it can be made much smaller. As the motors are mechanically coupled in opposition they exert an opposing drive torque on the gears, thereby taking up the mechanical backlash

In mode C the output signal from the PID controller is fed directly to input 1 of PAl. In this mode the polarity separator is not used and input 1 of PA2 is grounded0 In response to a changing input signal the current in motor 1 is varied by direct control of PAl. On the other hand motor 2 is controlled indirectly by the inverted current 'image' signal of motor 1. To do this the current feedback voltage of PAl is applied to input 4 of PA2, causing PA2 to drive motor 2 with a current of the same magnitude but of opposite polarity. As the motors are mechanically coupled in opposition inversion of the current drive to motor 2 causes both motors to drive the telescope together. In this mode a high preload is necessary to maintain backlash-free operation throughout the operating range.

In mode A the polarity separator circuit, IC4, functions as a phase splitter. For positive going input signals the output of IC4 goes negative, D2 conducts and linear negative feedback takes place via R4. This output signal is applied to input 1 of PA2. For input signals of opposite polarity Dl conducts and positive going output signals are applied to input 1 of PAl. In this mode of operation only one motor drives the telescope at a time, the other exerts a small static opposing torque.

3.2.3 Drive Current Monitoring (See Figure Number CS-E-0666)

Because the motors are mechanically coupled in opposition, equal preload currents through the motors will not generate any applied torque to the telescope. An applied torque will only result when one of the motors is driven with a larger current than the other. In order to indicate the magnitude of this driving torque a difference amplifier, IC8, is used. The two current feedback voltages from PAl and PA2 are input to this amplifier. Provided the ratio of the divider resistors R31/R32 and R30/R29 is accurately matched the amplifier will have good common mode rejection. The amplifier output will be proportional to the 'drive current', or difference between the two motor currents, and insensitive to the preload current.

In addition to drive current monitoring an ammeter is included in series with each motor. These allow the preload currents to be monitored or adjusted.

3.2.4 Accelerometers

The accelerometers perform an important function in the control of the main servos. Because the motor drives the telescope through a gearbox and other elastic structural elements, the mechanical coupling between the motor and telescope is not rigid. There will be some elastic movement between the two. Acceleration feedback is used to prevent dynamic errors occuring due to the mechanical resonances of the telescope.

Two Brüel and Kjär type 8306 linear low level accelerometers are used on each axis. They are arranged in such a way that their in-phase signals are added and some side effects originating from variable gravity and centrifugal force components are cancelled. Each accelerometer is mounted on a rigid part of the structure to prevent it from being influenced by vibrations in minor structural parts. They are mounted on the horseshoe periphery (polar drive), and the tube center section (declination drive).

The most important signals generated by the accelerometers are those at the telescope's locked rotor resonant frequencies, (1.4 HZ for the polar axis, 3.4 HZ for the declination axis). The bandwidth of the feedback chain is limited above and below these frequencies to reduce unwanted interference signals. The 3 dB cut-off frequencies of this pass-band are approximately 0.7 HZ and 7.0 HZ, which covers both mechanical resonances with an octave margin at each end.

3.2.5 Strip Encoder

Documentation related can be found in the 3p6 Team Portal

3.3 General System Layout

3.3.1 General Equipment Layout

Click to enlarge

Upper figure shows the layout of equipment in the LCU room. All of the analogue control circuitry for each main servo drive system, including power distribution, is contained in two of the control room racks. Rack B houses the servo control system for the declination axis, rack C houses the polar axis system.

The basic analogue control loop for one axis consists of two power amplifiers, two servo-motors, the analogue control circuits and analogue feedback sensors (accelerometers, tacho - generators and current sensors). These components form the basic control loop which converts an analogue input voltage to movement of the telescope in one of its two axes. The two 2 kW power amplifiers for each axis are housed in lower chassis positions. Their outputs are wired to the telescope servo-motors via junction boxes mounted on the telescope itself.

3.3.2 The Control Chassis (Refer to Figure of Servo Signal Wiring)

Figure of Servo Signal Wiring is a block diagram of the internal circuitry of each control chassis. The unit contains four different plug-in printed circuit cards, each represented by one of the blocks. Each card has two edge connectors, labeled on the board as A and B. Pin connections to the printed circuits are labeled with the edge connector identifying letter followed by the pin identification (letter or number). For example A13 refers to PCB edge connector A, pin 13.

Depending on the operating mode selected, the relays switch one of the analogue control signals to the ramp circuit input, PCB connection AA on the relays, AN on the ramp circuit. In the analogue mode this signal is simply a switched D.C. level (of either polarity) which is used to move the telescope at slewing speed in either direction. Various interlock signals are used in the relay switching circuits. Their functions are described in the Interlock section (Chapter 5)

The ramp circuit and PID controller are concerned with processing the analogue input signal. The servo interface generates the various control signals for the power amplifiers. These connections are shown on the bottom left hand side of the drawing. Feedback signals from the accelerometer and the two tacho generators, Tl, T2 are shown entering the servo interface at bottom right.

3.3.3 Control Chassis Interlock (Refer to Figure of Servo Signal Wiring)

The operation of the main servo-drive motors, polar and declination axis, is governed by comprehensive interlock chains in the main servo drive interlock systems. These are fully described in Chapter 5. Unless the appropriate interlocks have been completed power will be removed from the corresponding servo-drive system by contactors in the power distribution units. These are located in rack D. When the power is cut-off by the interlock systems an 'interlock complete' indicator light on the power distribution unit front panel goes out.

In addition to this power switching interlock several other interlock signals are input to the control chassis from the servo drive interlocks

ENDSWITCH SIGNALS -If the telescope is driven too far in any direction it will encounter a limit switch. This causes power to be removed from the servo power amplifiers by de-energizing a contactor in the appropriate power distribution chassis. In order to re-energize the servo motors the analogue operating mode must be selected and a bypass keyswitch operated in the servo drive interlocks. When this is done the servo-motors may be re-energized and controlled using the manual slewing mode. The endswitch signals then ensure that the telescope can only be moved away from its limits.

For further details on the source of these endswitch signals see.

SERVO POWER ON - This signal is transmitted from the power distribution chassis when the contactors switch power to the servo power amplifiers. It is used to set certain pre-conditions within the analogue control circuits while the power is off.

When power is re-applied the preload current through each motor builds up gradually, with a time constant of about 1 second. This prevents the gear backlash from being taken up suddenly stressing the gears.

The signal is also used to short-circuit the input to the ramp circuit, PCB connection AN, whenever the servo power is off. This prevents the feedback loop from saturating and applying a sudden, large output current through the motors when the power is switched on.

15º BALL SWITCH - The 15º ballswitch signal is wired into the ramp circuits of both the polar and declination axis servo rack. When the telescope tube is driven below an elevation of 15º the maximum permitted slewing rate of the telescope is automatically reduced to approximately 0.25 degree/second. The tube then approaches the horizon at slow speed. The normal maximum slewing rate is 1 degree/second.

3.4 The Basic Analogue Servo System

3.4.1 Introduction and References

Upper section of this Chapter described the overall function and general layout of the main servo drives of the telescope, polar and declination axis. Further detailed information on the servo systems is divided into two major sections. This section describes the operation of the basic analogue servo system.

The servo systems are interconnected with various other equipment in the LCU room. External wiring between these units carries interlocking, mode selection, power distribution and data transmission signals.

3.4.2 General Description

The DCP/2000/60C power amplifiers are DC coupled, current limited power amplifiers housed in the lower chassis positions of racks C & D. Each power amplifier chassis is a self-contained unit consisting of an integral power supply, pre-amplifier and power amplifier with a single-ended output.

The power output stage consists of 72 silicon power transistors operating in class B. Current limiting is bi-directional with separate, independent adjustment of the limiting point for each polarity. The output stages are protected against thermal overload. If the temperature of the output stage rises above 100ºC, or if the continuous motor current exceeds 20 amps, the power supply is automatically switched off.

The power amplifier operates as a voltage to current converter. A current sensing resistor in series with the motor is used to produce a feedback voltage proportional to motor current. The amplifier will operate in all four quadrants of the load characteristic relating output current and output voltage. The amplifier is thus capable of delivering current into the motor or absorbing current from the motor at either polarity.

3.4.3 Specifications

3.4.4 Construction and Layout (Refer CS - E - 0365 4/4)

Each separate power amplifier occupies one chassis position There are two power amplifiers in each servo system. They are housed in the lower chassis positions of racks C & D. Refer to Fig. (CS - E - 0365 4/4) which shows the general layout of parts in each chassis.

The chassis contains two modular assemblies. The left-hand module is a combined heatsink/chassis which contains the power output stages complete with an electric cooling fan. The direction of airflow is shown by the arrows. The right-hand module consists of the power supply and the remainder of the amplifier circuitry. A single printed circuit board mounted above the transformer carries the output stage driving circuitry, preset adjustments and pre-amplifier. The top of this board is shielded by a steel plate which is bolted to the transformer.

The rear panel of the chassis carries all plug and socket interface connectors. These connectors are shown on the various drawings which follow, together with input-output connection pin numbers.

A hinged front panel carries a Heinemann input circuit-breaker and a 30-0-30 ammeter for monitoring the motor current. By hinging down the front panel access can be made to various test points, and to the thermal current trip. This device has a calibrated trip-point setting which should be set to 18 amps. All test-points are labelled with their function. Most of these are obvious ; OP corresponds to the power amplifier output line, MC is the motor current feedback voltage. This is generated by an 0.1 ohm power resistor in series with the motor.

In order to examine the printed circuit board above the transformer, or to gain access to the preset adjustments, the steel protecting plate must be removed. Before attempting this switch off the power using the Heinemann circuit-breaker. A second internal power ON/OFF switch is included within the unit as a double level of safety. This small toggle switch is positioned above the test points. It is wired in series with the Siemen's relay contactor and can be used to isolate the trans- former. As 3 phase power input wiring from the Heinemann circuit breaker remains live it is preferable to switch this circuit-breaker off first.

3.4.5 Power Supply And Interlock Wiring
(Refer CS - E - 0365 4/4 and CS - E - 0365 1/4)

Fig. CS - E - 0365 1/4 shows the wiring of the power supply unit. An input power cable carrying 3 phase, 380 volt ac power enters the power amplifier chassis through the rear panel and is wired directly to the terminal block on the power supply module. From here the power is wired through a Heinemann circuit-breaker and a Siemen's relay contactor to the three phase power transformer.

The three phase power input cable is externally wired from the power distribution chassis, connector E or F. Contactors in that chassis are controlled by the main servo drive interlocks. The Siemen' s relay contactor is energized from a 24 volt supply in the local interlock chassis. This supply is not switched. The relay contactor is wired in this way to prevent the contactor from being energized unless the interlock output cable from Burndy connector C is properly terminated. If no supply is present across pins 6, 9, check the power wiring in the local interlock chassis, connector B or C.

Five indicator lamps are incorporated in the power wiring. They were shown physically on the mechanical layout drawing, Fig. CS - E - 0365 4/4 The three yellow lights, L3, L4, L5 are used to indicate a failure in any of the incoming supply phases. They should all be normally off. A red 'power on' light is wired between phase R and neutral, MP, to indicate when power has been switched on by the power distribution chassis. The fifth, white, indicator light, L2, is illuminated whenever the cooling fan is running. This is energized by two temperature switches, S5, S6, which are fitted to the power amplifier heatsink. They close at a temperature of 45ºC to switch the fan on.

Two additional temperature switches, S1, S2 are also attached to the power amplifier heatsink. These two contacts are wired in series to pins 1, 2 of the B5 terminal block. If the temperature of the power amplifier exceeds 100 ºC an interlock signal is transmitted out of the unit via pins 1,2 of the rear panel Burndy connector, C. This signal (PA>l00 C) is wired to the local interlock chassis.

Two further interlock signals are transmitted to the local interlock chassis. An auxillary contact on the Heinemann circuit-breaker is wired to pins 5, 7 of Burndy connector C, and an output from the thermal current trip is wired to pins 3, 4 of this connector (the thermal trip wiring is shown on the next drawing, Fig. CS - E - 0365 2/4). These two interlock signals are combined together in series within the local interlock chassis. If either the circuit breaker or the current trip transmits a signal, an interlock condition is generated in the local interlock chassis

3.4.6 Amplifier Input/Output Connections (Refer to Figure CS - E - 0365 2/4)

Figure CS - E - 0365 2/4 shows the general input/output wiring of the amplifier. All input lines are wired from rear panel Lemo connector A. This connector is externally wired to the control chassis, connector F or G, depending on the appropriate power amplifier, 1 or 2. The function and general wiring of each of these input lines was fully explained in Chapter 3, Section 3.2.2.

All amplifier output wiring is connected using 4 mm 2 heavy duty wire to carry the motor current. The 0.1 ohm series feedback resistor, R7-RlO, is shown wired using four separate resistors. This was necessary to handle the maximum output current.

All output test points are protected with a 1K series resistor to prevent any damage which might otherwise result from short-circuiting or incorrect meter range selection.

3.4.7 Amplifier Circuit Operation (Refer to Figure CS - E-0365 3/4)

Fig. CS - E-0365 3/4 shows the circuit wiring of the complete power amplifier. The dotted lines mark the sub-divisions which have been made for the purpose of describing the circuit operation. On the right-hand side of the drawing one of the power output sections is shown. There are an additional three identical stages connected in the same manner.

During the following descriptions nominal design voltages and currents are given at various points to clarify the operation of the circuits. As the complete power amplifier operates within various internal negative feedback loops, a component fault at any one point will cause erroneous operating conditions throughout the whole loop. The simplest method of tracing a circuit fault is with all external input lines grounded (I/P 1 to I/P 4). Under these operating conditions the power amplifier output voltage should be approximately zero and all bias voltages and currents are at their nominal values. These nominal operating values are used in the descriptions which follow in the next three sections. They are calculated from the design values and should be regarded as approximations only.

External input signal lines are all connected to the preamplifier which is shown at the far left of the drawing. Four input lines are required. They are externally interconnected as previously described in Part 1, section 5.2.2. Overall negative voltage feedback around the power amplifier loop is provided by the 619 K feedback resistor to input pin 3 of the pre-amplifier. Provided the power amplifier is functioning correctly pin 3 will be a virtual earth. It will deviate from zero by only a few millivolts to provide the necessary drive output to Tl.

Circuit operation descriptions for the intermediate driver stage, power sections, biassing and current limiting circuitry will be found in the three sections which immediately follow.

3.4.8 Intermediate Driver Stage (Refer to Figure CS-E-0365 3/4)

The complete intermediate driver stage, Ti, T2, T3, operates as a voltage amplifier. It provides the high output voltage levels (+ 63 volts) which are required to drive the power sections.

The output of the pre-amplifier drives transistor T1, which operates as a voltage to current converter stage. The nominal collector current into this transistor is 2.9 mA, which requires an input voltage at the base of + 1.9 volts. A base voltage variation from this level of less than +/- 1.5 volts is sufficient to develop full power output. Negative feedback from the output stages to Tl base means that an increased drive level would be required at the pre-amplifier output pin.

Transistors T2 and T3 operate as constant current sources in opposition. T3 is an uncontrolled negative current 'sink' which requires a steady input collector current of 3.6 mA. This current is supplied from T2, which operates as the source current generator controlled, or modulated, by Ti. At an amplifier output voltage of zero T2, T3 are balanced. That is to say T2 supplies exactly the current required by T3 (3.6 mA). For a positive going output voltage T2 must supply more than 3.6 mA, the excess current provides a positive drive current to the upper power output sections. For negative going output voltages T2 provides less than 3.6 mA, the deficit current is then taken from the lower power output sections to generate negative power amplifier output drive.

Both T2 and T3 are biased by a temperature compensating series bias chain comprising D5-D8, a 47 K resistor and DlO-Dll. Diodes D3 and D4 are reverse biased during normal operation. They clamp the maximum output drive voltage (T2, T3 collector) to approximately +/- 63 volts.

A supply current of 6.5 mA enters the 100 ohm emitter resistor of T2 from the + 65 volt supply rail. A portion of this current must be deviated through Tl collector to set the nominal source current of T2 at 3.6 mA. This deviating current of 2.9 mA establishes the bias point of Tl, as described previously.

3.4.9 The Power Sections
(Refer to Figure CS-E-0365 3/4)

The output power sections provide the load current into the motor. Four identical power sections are used, only one is shown on the drawing (CS-E-0365 3/4). The four power sections are mounted on four different faces of the left-hand module, shown on Fig. CS - E - 0365 4/4 In the event of a failure in one of the output transistors it is preferable to replace an entire section. This is because the output transistors have been closely matched.

Output drive lines (a+l) to (a+4) from the intermediate driver stage provide positive going drive signals to the upper drive transistors of the power sections. Similarly (a-i) to (a-4) provide drive signals to the lower drive transistors of each power section. Signals Sl to 54 are concerned with current limiting feedback and are dealt with in the following section.

Each power section contains 16 output drive transistors, type 2N3773. Individual emitter feedback resistors of 0.68 ohms are used in the output transistors to distribute the load current equally through each transistor. To provide sufficient output drive current darlington pairs T9, TlO are used in the upper half of each section. The same function is provided Each power section contains 16 output drive transistors, type 2N3773. Individual emitter feedback resistors of 0.68 ohms are used in the output transistors to distribute the load current equally through each transistor. To provide sufficient output drive current darlington pairs T9, TlO are used in the upper half of each section. The same function is provided by cascaded PNP-NPN pairs in the lower half of each section.

Transistor TB is concerned with biassing. The voltage difference between input lines (a+l), (a-l), establishes the bias current through the output transistors of power section 1. Two bias point adjustments, ml and m2, are provided. Each of these potentiometers supplies the bias point setting for two power sections. The output from these bias adjustment potentiometers are connected via diodes D18 to transistors TB of the appropriate power sections. By adjusting these potentiometers the static collector current of TB can be varied to reduce the bias input voltage to the required level.

3.4.10 Current Limiting Circuitry
(Refer to Fig. CS-E-0365 3/4)

Current limiting is used to protect the output transistors against a short circuit. Feedback is taken from across one of the 0.68 ohm series emitter resistors in the case of the upper output transistors. Similar feedback is derived for the lower transistors as shown in the drawing. A pair of feedback lines for each power section are then connected to the points marked (s+l) to (s+4) in the case of the upper transistors, (s-l) to (s-4) for the lower transistors.

Current limiting is nominally adjusted to occur at an output current of + 20 amps. As this output current is shared between 32 output transistors (32 upper or 32 lower), the limiting point corresponds to a current of 625 mA in each output transistor. This current generates a feedback voltage of 425 mV across the 0.68 ohm series resistors. This voltage would normally be insufficient to turn on the limiting transistors T4, T5. To increase the feedback voltage to the required level 500 ohm adjustable resistors are inserted in series with the two feedback lines. Current source transistors T6, T7 generate a steady bleed current of approximately 1.4 mA through these adjustable resistors. The resulting voltage-drop adds a fixed offset voltage (adjustable between 0 and 900 my) to the feedback voltages.

Each adjustment potentiometer is set so that transistors T4, T5 limit the input drive voltage to the power sections, between a+ and a-, at an output current of 20 amps. A description of this adjustment procedure is included in section 3.4.11.

3.4.11 Preset Adjustments
(Refer to Fig. CS-E-0365 3/4)

This section describes the different procedures which are used for setting the preset adjustments in each power amplifier.

CURRENT LIMITATION (± 20 amps) this adjustment is made with both the motor and the thermal current trip short-circuited. When this is done the amplifier is effectively shorted to ground through the 0.1 ohm, 50 watt current shunt. Output current may be measured on the front panel ammeter, or a voltmeter may be connected across the current shunt. In this case a reading of + 2 volts corresponds to +/- 20 amps.

Two separate adjustments are required because positive and negative current limits are independantly adjustable. If the input lines (I/P1 to I/P4) cannot be controlled easily to swing the power amplifier output in both positive and negative directions, disconnect them from the external system. Leave input lines 2, 3 and 4 disconnected. I/P1 can now be connected to either the + 15 volt or - 15 volt pre-amplifier supply as required. The power amplifier inverts the input signal and an input of + 15 volts allows the negative output current limit to be adjusted, and vice versa.

Refer to Figure CS - E - 0365 4/4 which shows the position of the potentiometers. It is advisable to turn them fully counter clockwise if they have been disturbed or replacement power sections are being fitted. Clockwise adjustment of the appropriate preset will then allow each current limit to be increased to 20 amps.

BIAS ADJUSTMENT (100 mA) - to make this adjustment disconnect the amplifier output line to the motor so that the amplifier is unloaded. Disconnect all of the incoming input lines (I/P1 to I/P4) from the external system, then short-circuit these inputs to ground on the pre-amplifier connector, Bornes Bl.

Identify the four red wires from the power supply capacitors which supply the power output sections with + 65 volts D.C. Connect an ammeter in series with these supply lines to measure the total bias current.

The ammeter should read 100 mA. If it does not then initially adjust both ml and m2 for zero current, then gradually increase ml until the ammeter reads 50 mA. Now increase m2 until the ammeter shows a total bias current of 100 mA. This is the correct bias setting point.

ZERO ADJUSTMENT - with each of the input lines shorted to ground, as described in the bias adjustment procedure, adjust the offset potentiometer for zero output from the power amplifier.

3.5 Accelerometers

3.5.1 Location And General Wiring
(Refer to Fig. CS-E-0574 1/2)

Chapter 3, section 3.2.4 described the general function of the accelerometers. Two accelerometers are used on each axis. For the polar drive they are mounted on opposite sides of the horseshoe periphery, and on opposite sides of the centre section for the declination axis.

Refer to Fig. CS-E-0574 1/2 which shows the general wiring and interconnection of each pair of accelerometer units. The acceleration sensors themselves are Briiel and Kjaer type 8306. They are housed in cylindrical metal containers which also contain some internal circuitry in addition to the basic piezoelectric sensor. Fig. CS-E-0574 1/2 shows the circuit diagram of this internal circuitry at the bottom left-hand corner. If a fault develops here it is advisable to replace the entire sensor.

Each accelerometer unit is mounted in a small box which is rigidly attached to the telescope. It is important that the body of the sensor is electrically isolated from the telescope, as it floats at approximately - 14 volts from the telescope ground. For this reason the base of each accelerometer unit is made of rigid plastic. The sensor is bolted directly onto this base.

Accelerometer unit 2 only contains a sensor. A single double-screened cable connects it to accelerometer unit 1 which contains the other sensor and a pre-amplifier. The pre-amplifier is mounted on a small plug-in printed circuit board. An output cable from unit 1 connects the pre-amplifier output signal to the appropriate servo rack. Although this connection is shown running directly to the servo rack, in practice a junction box is used on the route. For the polar axis this is junction box GlJl, for the declination axis G2Jl.

3.5.2 Circuit Operation
(Refer to Fig. CS-E-0574 1/2)

Refer to the wiring diagram of the accelerometer unit, Fig. CS-E-0574 1/2. Each B & K 8306 sensor requires a single + 28 volt supply. This is obtained from the ± 15 volt supplies, PCB connections C, D on the pre-amplifier board. An AAZ 15.input diode and a lMF/35V capacitor are used for power-supply filtering and protection against reverse-connection. Two additional diodes are wired in series with the supplies to the sensors. These are used to drop the total sensor supply voltage to approximately 28 volts. Because of this floating ground arrangement and the small signal levels involved, it is important that the cable ground screens are interconnected strictly according to the wiring diagram.

The LH0O42CH integrated circuit is connected as an ac pre-amplifier with a gain of 10. The lower cut-off frequency of 0.7 Hz is determined by the series input components (0.22 MF capacitor, 1 M resistor). The only attenuation of high frequencies is provided by the 15 pF feedback capacitor. This attenuates frequencies above 1KHz to eliminate any radio-frequency interference. Additional high-frequency filtering is included in the servo interface circuits.

3.5.3 Fault-Finding (Refer to Figure CS-E-0574 1/2)

If the supplies to the accelerometers have been checked and found correct, connect an oscilloscope between the screen ground and the amplifier output on PCB connections E and F. The D.C. level at this point is not important because a-c coupling is used within the servo interface circuits. Various high-frequency noise signals will be apparent as the sensor has a bandwidth of 0.2 Hz to 1KHz. These noises are primarily generated by the telescope hydraulic systems and by anyone moving on the telescope. They are not important as they are rejected later. The operation and general wiring of each accelerometer can be separately checked by tapping the sensor housing. A strong similar signal should be generated by both sensors.

If the sensors are removed from their mounting they should be handled with extreme care and protected from shocks. After removing leave the sensor connected and try turning it rapidly from the horizontal to the vertical position. This should produce a strong output signal transition of about 10 volts on the voltage output lead of the sensor.

It is important that the accelerometers are rigidly mounted with no free play. Check the mounting of the sensor to the plastic base, and the base to telescope fixings.

3.6 Analogue Control Circuits.

3.6.1 Location and General Wiring

The analogue control circuits consit of the ramp input circuit, PID controller and servo interface circuits. They are all located in the control chassis. section 3.3.2 described the layout and general interconnection of control chassis printed circuits. The general block wiring diagram is also repeated here as Fig. Servo Signal Wiring for convenience.

Three plug-in printed circuit cards carry the analogue control circuits. Various test points are wired on each of these cards to simplify test and calibration. General descriptions of the function of each preset calibration will be found in the sections which follow. A summary of the calibration procedures and preset adjustments will be found at the end of this Section

3.6.2 Speed Limitation
(Refer to Fig. CS-E-0531)

Fig. CS-E-0531 shows the circuit diagram of the ramp circuit. This circuit limits both the magnitude and the rate of change of analogue input signals. At telescope elevations of less, than 150 the magnitude of the input signal is limited to reduce the maximum speed, or slewing rate, of the telescope to approximately 0.25 degrees per second. The normal maximum slewing rate is 1 degree per second.

An analogue input signal arrives on PCB connector A, pins N, P, from either the MDAC (ON LINE mode), the DAC (OFF LINE digital servo) or from manual switches (OFF LINE slewing). A spare amplifier input is provided on pins J, K. This is not norraally used and input pin J should be externally connected to ground. The speed limiter only acts on the input signal when the telescope is at an elevation of less than 150. Above this elevation the amplifier operates linearly with unity gain. An input signal from the 15º ball switch arrives on PCB connector A, pins R, S. At elevations of greater than 15º this input signal is at + 24 volts, turning TR2 hard on and energizing relay RLl. This opens a normally-closed contact and switches the speed limit potentiometer, P3, out of circuit. TlL 209 is a light-emitting diode which illuminates to signal that the ball switch signal has been received.

When RLl is de-energized non-linear feedback takes place through potentiometer P3 and two pairs of shunt feedback diodes (one diode-pair for each polarity). The potentiometer is normally positioned at approximately the mid-point of its travel. At this setting an amplifier output voltage of 2.5 volts generates a potentiometer output of 1.25 volts. This voltage is just sufficient to overcome the threshold voltage of a diode-pair and negative limiting feedback takes place.

The overall gain of the analogue control system is such that a voltage of 10 volts corresponds to a telescope velocity of 1 degree per second. A reduced voltage limit of 2.5 volts causes the maximum slewing rate to be reduced to 0.25 degrees per second.

3.6.3 Acceleration Limitation
(Refer to Figure CS-E-0531)

The rate of change of analogue input signals is limited to reduce the maximum acceleration and decceleration of the telescope to 1/4 degree/second/second. If the telescope was initially motionless then this acceleration would allow the maximum slewing rate of 1 degree/second to be reached in 4 seconds.

Two integrated circuits are used in the acceleration limiter. The first is a fast comparator which compares the input voltage Ul, with the output voltage Ur, and generates an output error voltage Uo,to correct any difference. For slowly changing input signals output Ur will follow the input signal with very small errors, Ui = Ur.

The error voltage Uo is internally clamped by a resistive divider chain and two diodes to approximately + 10 volts. Due to the high gain of the comparator small input errors will cause the output to switch rapidly between these limits. In the steady state condition, when Ui is constant, the acceleration limiter will oscillate at a high frequency. This produces a small, triangular output signal on Ur. This high frequency component is irrelevant as it is filtered out by the low bandwidth of the PID controller.

The second amplifier is connected as a linear integrator. The four series feedback diodes have a special purpose in the 'ON LINE' mode only. This is described further on. In the 'OFF LINE' mode these diodes are short-circuited by a normally closed contact on relay RL2. Any difference between Ui and Ur causes an error signal Uo, of + 10 volts to be generated. This causes an adjustable, preset current of approximately 100 micro-amps to flow into the summing junction of the second amplifier. The amplifier will exactly balance this input current by drawing an equal feedback current through the 40 MF feedback capacitor (cl). The output 'slews', or changes to its new valve, at a rate of 2.5 volts/second (V/t = I/c).

This voltage slew rate corresponds to the required acceleration limit of 1/4 degree/second/second.

3.6.4 The PID Unit
(Refer Figure CS-E-0589)

Fig. CS-E-0589 shows the circuit diagram of the PID unit (Proportional Integral and Ufferential controller). The main function of this circuit is to modify the phase and amplitude response of the servo loop, to ensure best damping with maximum gain.

The input terminal (IN-) of the first integrated circuit amplifier serves as the input summing junction of the complete servo loop. An analogue input signal (SPEED l/P) arrives on PCB connector B, pin B from the ramp circuit. This control input signal defines the required telescope speed. Feedback signals from the accelerometer and tachogenerators are compared with this speed control input, any difference signal is amplified and output to the power amplifiers, PCB connector A, pin L, via the polarity separator.

Potentiometers P1, P2 are wired in series with two of the input lines. P1 allows the input gain of the servo system to be adjusted so that a SPEED I/P signal of 10 volts corresponds to a telescope velocity of 1 degree per second. P2 adjusts the return gain of the acceleration feedback loop and is used to optimize mechanical resonance damping performance. Potentiometer P3 is used to adjust the zero offset of the input amplifier. It is used as a final adjustment of the servo system voltage offset.

Two field effect transistors are used as amplifier input protection diodes. They have superior leakage characteristics compared with ordinary diodes. Zener diodes are used for boundary limiting in parallel with the feedback components of both amplifiers. They prevent the amplifiers from saturating and its consequences.

Various resistor-capacitor networks are used in the PID circuit to modify its phase-amplitude response at different frequencies. Potentiometer P4 allows the overall loop gain of the system to be adjusted for minimum static velocity error and optimum transient response. The setting of P2, P4 and the values of the preset network components are optimally set during commissioning. They should not be disturbed or altered.

MOS field effect transistor BSV8l is used as an ON/OFF switch. The gate of this transistor is normally biased at approximately + 5.3 volts. In this condition the transistor conducts and exhibits a low drain to source impedance which has no effect on the amplifier circuit. When the input signal on PCB connector A, pin 5, is switched to 0 volts the transistor gate bias is reduced. This condition turns the transistor off and reduces the internal loop gain of the servo system. This input control facility is not used at present, but may be incorporated at a later date by a special interlock system.

3.6.5 Servo Interface Circuits
(Refer to Figures CS-E-0591 1/2 & CS-E-0591 2/2)

Figures CS-E-0591 1/2 & CS-E-0591 2/2 show the individual circuits which are all mounted on the servo interface printed circuit. Most of these circuits were described in Part 1. Detailed information to enable the adjustment of preset potentiometers will be found in the following section (3.6.6).

POLARITY SEPAPATOR : the function of the polarity separator circuit and the function of the A/C mode selection links was fully explained in Section 3.2.2.

TACHO SUMMING BUFFER see Section 3.2.1.

PRELOAD CIRCUIT : see Section 3.2.2.

DRIVE CURRENT SUMMING BUFFER see Section 3.2.3.

ACCELERATION SIGNAL SHAPER this circuit is shown in Figure CS-E-0591 2/2. The acceleration signal circuit helps to define the frequency pass band over which accelerometer signals are accepted. It has an overall gain of approximately unity within the pass-band 0.7 Hz to 7 Hz.

The first amplifier is connected as a low pass filter. An output signal from the pre-amplifier arrives on PCB connector B, pin E. Shunt feedback components CS, R24 set the upper frequency limit of the pass-band at 7 Hz. Increased rejection beyond 10 Hz is provided by input filtering, R26, R27 and C7.

The second amplifier is ac-coupled and has a gain of unity. Input components CS, R23 set the lower frequency limit of the pass-band at 0.7 Hz. The acceleration feedback output signal on PCB connector B, pin D, is connected into the PID unit.

3.6.6 Adjustment And Calibration Levels
(Refer To Table 5a.4.6)

Table 5a.4.6 lists sufficient data for all preset adjustments to be carried out on the analogue control circuits. All measurements given in the table are made with respect to the system ground. They must be carried out in the order given. If the power amplifiers are also being calibrated carry out all adjustments to the power amplifiers first. In any case whenever a power amplifier zero setting is changed the servo system offset must be re-adjusted afterwards as described in step 8. This particular adjustment may be made independantly of the other steps as and when required.

All measurements are made with the servo system in its OFF LINE ANALOGUE operating mode. The handset control pushbuttons are used as a source of calibration signals. These buttons generate an input of + 5 volts to the ramp circuit with sufficient accuracy to enable all preset adjustments to be carried out. The +/- alpha, ± delta buttons on the handset are represented by + S in the last column of the preset adjustment table. S = 0 indicates that the parameter given in this step is adjusted with no buttons pushed. + S indicates that comparative measurements should be made by alternately pushing the +S and -S buttons. The setting of the preset in these steps is a compromise to achieve the best accuracy in both directions.

Adjustment of the acceleration ramp in step 9 involves measurement of the total ramp build-up time on an oscilloscope. Allow the system to settle initially with no buttons pushed (S = 0), then push either +5, or -5, to measure the ramp build-up time in one direction. Repeat in the opposite direction and adjust the preset for a 2 second duration, or the best compromise between the two directions.

All measurement points have been given for each step. As an example step 1 calls for the use of a meter to measure the offset in the tacho summing buffer. Connection is made between ground and connector A, pin K (AK) of the servo interface card.

Step 2. to 3 call for the power supply to the power amplifiers to be switched off. Ensure that the power supply capacitors are fully discharged before these measurements are made.

3.7 Engineering Panel Drawings

Auxiliary Functions Control (Page 1)

Auxiliary Functions Control (Page 2)

Auxiliary Functions Control (Page 3)

Auxiliary Functions Control (Page 5) (Handset)

Auxiliary Functions Control (Page 6)

Auxiliary Functions Control (Page 7)

At the rear of Engineering Panel there are a ADAM module 6066 used for remote control from NOB of different functions.

Send Comments to: Agustin Macchino Last update: August 28, 2001