The NTT Altitude and Azimuth (AL and AZ) electronics are each split into two racks: a power rack and a control rack. The power rack contains the power amplifiers, the brake amplifier and the velocity loop PI controller. The control rack contains the VME computer, the absolute encoder electronics units and the incremental encoder electronics units.

This document describes firstly the commercial electronic units in a brief form in order to give an overview. For further detailed information, refer to the maintenance information in Section 3.6. The velocity loop PI controller (which is manufactured by ESO) is described in some detail.

An operating procedure is described in Subsection 3.1.11 in order to provide information for writing control software.

3.1.1 Reference Information

The following sections provide the information specified:

3.1.2 Absolute Encoder

The absolute encoder system is identical for AL and AZ. It consists of a glass disc which is directly mounted on the telescope axis and four reading heads which are connected pair-vise to two IK320.1 incremental encoder counters and one SAE interface that latches the absolute encoder position.

The measured position of each reading head is available for the VME computer via the IK320.1 boards. The resolution is 25 bits (0.03862 arcsec/bit). The absolute accuracy after averaging the 4 heads, and after the correction cycle, is specified by Heidenhain as less than 0.5 arcsec for the 1 sigma value.

For detailed information about the absolute encoder, see the relevant Heidenhain ERO 7001 and IK320.1 documentation that is under chapter 14-'ENCODERS INFORMATION'.


If an absolute encoder reading head has to be changed, care has to be taken that the new head is mounted correctly, i.e. pointing exactly radially. This can be checked by observing the sinus and cosinus waves of the incremental track on a scope. The procedure is similar to that for the incremental encoder electronic unit (refer to Section 3.6) except that no adjustment is necessary. Only the amplitude level needs to be checked. The peak to peak level measured at the IK320 'P1/P2' test-points 2 and 3 board must be between 2.2V and 4.8V.

3.1.3 Incremental & absolute encoder interface:

The incremental encoder counter IK320.1 from Heidenhain with the SAE (absolute encoder value latch) are the required interfaces for the encoder resolution.

IK320.1 P1/P2 test points pin assignements:

PIN 1 2 3 4 5 6

3.1.4 Power Amplifiers

For AL as well as for AZ, the drive system consists of 4 identical motors and an amplifier for each motor. This is done to make it possible to give a constant torque to each motor in order to close the gear.

The amplifiers are Servowatt V204X-120.20, which are switched mode amplifiers with a switching frequency of 16 kHz. The amplifiers are used in current amplifier mode, and the transfer gain is 5 Amp/Volt. The current monitor signal (PACURx) has a scaling of 0.1 V/A.

3.1.5 Servodrive

Each servodrive consists of a servomotor, tachometer and emergency brake. The servomotors and tachometers are manufactured by Sierracin/Magnedyne.

The motor is a DC brush type servomotor with high efficiency samarium/cobalt magnets. The specified torque constant is 22.7 Nm/Amp.

Measured value at the acceptance test at Sierracin is 25.4 Nm/Amp. The tachometer sensitivity is specified as 10.5 V/rad/sec; (measured 11.2 V/rad/sec). Due to the high quality magnets, the resistance and inductance of the tachometers is much lower than specified: measured values are 2100 Ohm and 2.87 Henry. See further the "Technical Specification for Torque Motors and Tachometers" for details.

The brake is manufactured by Stromag, type EFM 41 T. This is an electromagnetic brake which needs to be energized to be disengaged (i.e. "free", not-braking). See the data sheet for details.

3.1.6 Brake Amplifier

The coils of the four brakes are connected in series in the brake amplifier unit, which is mounted in the power rack. The brake amplifier is manufactured by ESO. Important design items were to limit the heat dissipation in steady state and to have a fast switch-off time.

These requirements are fulfilled by switching the supply voltage to a low level, 1 second after disengaging (energizing) the brakes and by having a diode-resistor flyback circuit parallel to each brake coil.

In order to limit supply line transients during disengaging of the brakes, an inrush-current limiter is implemented.

The design defines, however, a maximum speed of engaging/ disengaging: Allow at least 2 seconds after each action for the system to stabilize. Too fast switching results in a blow-up of the current limiter!

3.1.7 Velocity PI Controller


The velocity proportional/integral (PI) controller has been designed by ESO. It accepts an analog speed command from the analog I/O VME board. A digital sensitivity selection input signal is provided in order to have no problems with the D/A quantization at low speeds. The level of each tacho signal is checked after the input buffer; if it is too high, an interlock is latched.

A torque reference analog input is available, which allows a proportional torque command to all four motors. This analog input is enabled with the digital input signal "Apply Torque Reference".

The velocity error signal is fed to the PI stage via a breakout point which allows the implementation of a filter in order to tune the open loop transfer function for the actual system. The PI stage output (Net Current Reference) is available at a testpoint. This Net Current Reference is summed with the pre-tension torque signal and the torque reference signal. This is done in a special way to provide an individual output signal for each of the four amplifiers.

Digital input signals are available to control the PI controller functions "Close Velocity Loop" and "Apply Pretension".

The following description of the PI controller refers to the schematic diagram.

Transfer Function

The PI controller has the following main time constants:

Velocity Sensitivity

The measured sensitivity of the AL and AZ tachos (Kt) is 11.2 V/rad/sec. The four tacho signals are summed, which sets the total sensitivity of the four tachos to 44.8 V/rad/sec. This refers to the motor axis. For the telescope axis, this value has to be multiplied by the gear ratio "i" for AL and "i+1" for AZ.

Moreover, the sensitivity of the velocity reference input is dependent upon the signal PIVRS8 and the values of R3 to R6, R8 to R11. If PIVRS8 is negated, the sensitivity results in the following:

For AL:

= 335V / rad/ sec = 5.85v/ degree/ sec = 1.63 x 10E-3V/arcesc/sec

(full scale = 1.5 deg/ sec = 8.78V)

And for AZ:

= 365V/ rad/ sec = 6.21V/ degree/ sec = 1.73 x 10E-3V/arcsec/sec

(full scale = 1.5 deg/ sec = 9.32V)

If the signal PIVRS8 is asserted, the sensitivity is 13.0 x 10E-3V/ arcsec/ sec for AL and 13.8 x 10E-3V/ arcsec/ sec for AZ.

Note that these values are dependent upon a lot of component values and are accurate to approximately +/-3%.

Velocity Interlock

The circuit around IC13, 14 and 15 provides a velocity interlock for each of the four tachometer inputs. The threshold voltage is determined by R45 through R48 and is set to 7.5 Volts. So the speed at which the system interlocks is :

For AL:

where w = rotational speed of telescope axis

= 4.16E-2 rad/ sec = 2.39 degree/ sec

And for AZ:

= 3.92E-2 rad/ sec = 2.25 degree/ sec.

If the maximum speed is exceeded, a flip-flop is latched and RL1 is released. This switches the servo amplifier off, engages the brakes and also asserts the computer signal ILPI. The light emitting diode LED1 on the PI controller front panel lights up.

A reset pulse signal via pin C26 (PIRESET) resets the interlock latch.


The circuit around IC10 and IC16 provides the feature to feed a pre-tension signal to the motors. The outputs OUT1 and OUT2 provide the negative signal while OUT3 and OUT4 provide the positive signal. The voltage is adjusted by P1 and is enabled to the outputs by asserting "Apply Pretension" (PIAPT).

Note however that the four outputs are connected differently to the four amplifiers for AL and AZ, see the schematic diagram of the power amplifier connections for details. This is done because the mounting of the motors is different for AL and AZ, which results in different positive rotation directions (refer to drawing CS-E-1654).

The level of the pre-tension is adjusted to the half of the continuous torque (300 Nm referred to the motor axis) of the motors. Given a motor torque constant of 25 Nm/Amp and an amplifier gain of 5, this results in a required voltage of 1.2 Volt at IC11 pin 14.

Torque Reference

The circuit IC6 incorporates the analog torque reference input PITORQUE. Asserting of the signal PIATR (Apply Torque Reference) feeds the torque reference signal to the output stages. The scaling is the same as for the pre-tension torque:

25 Nm/Volt (referred to motor axis).

The implementation of this torque reference is not foreseen at the time of installation for safety reasons in the mechanical structure. When this function needs to be implemented, IC6 must be mounted in its socket.

Other PI Controller Signals

This paragraph is meant as a summary of the PI controller signals which are not described in the previous paragraphs.


If the PI controller has to be replaced for a spare one, the velocity loop has to be adjusted. In order to do this safely, use the following procedure.

3.1.8 Interlock Chain

The interlock chain is kept simple due to the altitude/azimuth mounting of the telescope. The chain is similar for AL and AZ and provides a computer input for each possible interlock. This gives a service friendly system.

The description of the interlock chain refers to the corresponding schematic diagrams.

3.1.9 Interlock Chain for AL

Following the chain from the bottom, the power amplifiers will be enabled if:

3.1.10 Interlock Chain for AZ

Following the chain from the bottom, the power amplifiers will be enabled if:

3.1.11 Operating Procedure

In this chapter, the following operating procedures are described:

3.1.12 Start-up

At this stage, the system is in velocity controlled loop mode, i.e. it can drift very slowly due to offset and temperature effects in the electronics. The system accepts velocity reference commands from the VME computer and a next step can be to initialize the absolute encoder and to close the position loop.

Checks During Operation

The following checks must be performed during operation, it is suggested on a 100 millisec cyclic basis. If any of the conditions mentioned below occurs, the software action has to be an immediate stop, (see Sub-section 3.1.13 ).

3.1.13 Stop

In order to stop the telescope, the following sequence has to be followed:

At this stage, the telescope brakes are engaged.

Special Precautions

The previous paragraphs contain enough information for programming, but one item is critical in operating the PI controller:

During start-up, PICVL has to be asserted shortly after asserting PIAPT (see the start-up sequence in Subsection 3.1.12), otherwise the telescope can accelerate because of unbalance or in the case of failure of an amplifier.

3.1.14 Connections to Other Subsystems

For both AL and AZ, the following connections to other subsystems exist:

Moreover for AZ: