The Marconi servo loops have been very carefully tuned and have good stability margins. Unless there are gross changes in the telescope's parameters there will be no need to retune the circuits on the Process board. Changes in parameters can be assessed by performing a closed loop frequency response test and an autographic record test and comparing the results with those presented in the Site Tuning Report by Marconi.

Instability in the servo must be distinguished from tracking problems. The most likely cause of a tracking problem which may look like instability is a limit cycle. These can look quite sinusoidal because of the low bandwidth of the telescope response compared to the sampling rate (20Hz) or the velocity loop response (50Hz). They can be caused by a number of faults, but the most likely areas are:

Digital faults

Analogue faults

Adjusting the Analogue Processor board (APB)

See: Testing the ANALOG PROCESSING board

Component differences list of all  the Analogue Processor boards.

See:  Marconi-APB-component-differences-list.pdf

The Counter Board DAC

See:  Testing the COUNTER board

The Rate Generator board

See:  Testing the RATE GENERATOR board

Power Amplifiers

To be able to get access to the electronics of the P.A. module under test, it needs to be withdrawn from the rack. Then seperate the side panel with the input control and logic board by un doing the 8 screws along edge of the panel. Connect the amplifier with the special break-out box. Make sure that it is well insulated from all metalwork.

Power Amplifier test setup

Power Amplifier test 2

Connect the pos. termial of a DVM to TP8 and the neg. terminal to terminal 39 of the exposed input control and logic board. This is the last point before the analog  demand signal enters the comparator/modulator, were the signal is converted to a PWM (pulse width modulated) signal. Adjust RV1 as close as possible to +/- 0.00 volts. This has to be done in Engineering mode, with one of the manual push button pressed and the demand potentiometer fully anti clockwise.

This is the only adjustment that can be made. Re-assemble the P.A. module, and re-insert it into the rack.

n.b.  One point to note with offsets: When in Engineering Mode and with NO demand (velocity control pots fully anti clockwise) and pushing either the +/- direction buttons, the motor current meters should NOT give a small kick. If this happens, there is an offset somewhere which has drifted and needs to be nulled.

Servo Testing Procedures

The purpose of measuring the Transfer Function of a system is to determine it's stability. The result can be checked against previous measurements to determine whether the system parameters have changed. In the unlikely event that this has happened, appropriate compensation can be calculated and introduced into the system. Most servo problems which occur with a system which is known to be stable and properly tuned are the result of some component failure or non-linearity appearing in the closed loop.

Sometimes the problem can be due to a change in the control system  e.g.. increased friction, out of balance torque's, defective couplings and backlash etc. These failures often manifest themselves as Limit Cycles. These are oscillations of the system between two values and can be quite small or extremely large. The system is not unstable in the same sense of linear stability.

Measurements can be made in open loop in order to determine the response in closed loop, but this is not necessary when it is known that the closed loop system is stable. In fact, it is more convenient to make closed loop measurements and plot them on the closed loop coordinates of the Nicholls Chart. This is valid if the system is second order or if the dominant poles of the transfer function are sufficiently Dominant for the system to approximate second order. It is then easy to estimate the compensation necessary to optimise the response. Marconi used this method for on-site tuning of the telescope's axes and other drives. The results are in the Marconi book Tuning Report (at La Palma). This volume is a reference for the performance of the WHT. It also includes autographic records. These are charts of the servo response with time for different velocities, step and ramp functions. These tests would show up things like backlash, increased friction, loss of gain etc.

The Nicholls Chart is a convenient representation of a system's open loop, closed loop, gain and phase. The open loop response is measured on the orthogonal grid of phase and gain. The closed loop response uses the corresponding distorted set of coordinates. The locus of the response is the frequency axis and is non-linear so plotted points need to be identified with a frequency. If the locus passes through point 0,0 on the open loop coordinates or to the left of it when the gain is 0dB or greater, the system is unstable. The locus should not cross the +3dB closed loop gain circle as the system would be too under-damped and the settling time too long. Gain and phase margins are measured on the open loop coordinates from the point 0,0 to the locus along the corresponding axis. A change in open loop gain shifts the whole locus up or down the open loop gain axis. Increased inertia or friction generally tend to push the locus to the left at higher frequencies. Loops in the locus are the system's resonance and anti-resonance features. The lowest frequency loop is usually due to the drive compliance (the anti-resonance is called the Locked Rotor Resonance since this is the frequency that the load will resonate at when the rotor is locked solid). Higher frequency loops are structural resonance's feeding back into the servo.

Closed Loop Frequency Response Test

Equipment needed:
  1. Oscilloscope (to monitor demand and Tacho output).
  2. Chart recorder (for autographic records if required).
  3. Solartron 1250 Transfer Function Analyser (TFA) plus handbook.
  4. Printer with serial interface (if printout of values is required).

Setting Up

The general method for obtaining closed loop frequency response measurements using the TFA is to inject a disturbance signal, usually a sine wave at the summing junction and then to measure the amplitude and phase of the feedback signal at the same point before the injection.

The generator output of the TFA is connected to the injection point through a suitable resistor so as to preserve unity scaling of input/output signals. The generator output is also connected to channel 1 of the analyser section while the feedback signal is connected to channel 2. The analyser can thus be set to provide direct results by choosing to measure Ch2/Ch1.

In order that system non-linearities do not interfere with the measurement, it is usual to bias the motion of the system and then modulate this with a signal whose amplitude does not null the bias. There are a number of ways of doing this, but the preferred method is now described.

Measurement of Closed Position Loop

The TFA generator output is connected to the summing junction of the velocity loop at IC4 through a resistor equal to the sum of the resistors which connect the integrated position error to the same point. This ensures unity gain between the injected disturbance signal and the measured response at TP3 which is the position error. The generator is also input to Channel 1 and the response is input to Channel 2 so that the complex ratio of Ch2/Ch1 gives the system response directly  n.b. Ch2 inputs are inverted. This is because of the negative feedback connection of the position loop and in order to display the correct phase relationship, it is necessary to re-invert the response signal.

An oscilloscope is connected to the Tacho signal in order the monitor the behaviour of the drive and ensure that the drive does not reverse or saturate on application of the disturbance signal.

For this test the bias is applied via the control computer since any bias applied by the generator would be counteracted by the action of the servo trying to maintain position. A battle which bias cannot win! It is also necessary that the telescope is operated in Computer Mode so that anti-backlash torque is applied.

The drive can now be set up to slew at a constant lowish speed by modifying the software velocity limit to a low value and then slewing to a position far enough away to allow time for the measurement to be made. The change to the velocity limit is made using GSEXAM and is described later.

With the drive slewing slowly, the stimulus or disturbance can be applied in small increments until a desirable level is reached  nb. The response will vary with frequency and a check must be made at each measurement that the bias is not being nulled by a high signal or that the response is so low as to cause problems for the correlator in the analyser section of the TFA.

A suitable integration time should be chosen such that results are consistent. This depends on the S/N of the response and the linearity of the system under test. On the WHT, an integration over 10 cycles has found to be adequate.

On the INT an integration of 100 cycles may be required although this is tedious at 0.1Hz.  n.b. This probably does not apply now as the INT now uses a WHT style TCS.

Care must be taken not to over-excite the system under test as this can cause damage let alone produce meaningless results. The stimulus should be kept small. Care with entering the amplitude and frequency parameters is needed and a close watch kept on the Tacho so as to avoid problems with resonance's of the system.

Connections for Velocity Loop Measurements

In this case, signal injection is at the same point, but via a resistor of the same value as the Tacho input resistor (R28). The response signal is measured at the Tacho input (TP5) and again the Ch2 connections are reversed. The position loop has to be disabled for the measurements to be valid and this is done by shorting the position error signal to 0v at the junction of R23 and R24. The bias can now be applied from the TFA until an appropriate rate is indicated by the Tacho output. The system is still in Computer mode for the test even though the position loop is disabled. All the precautions mentioned above are still applicable.

Plotting the Results

The best way is to photocopy the appropriate Nicholls Chart and then plot the spot frequencies onto it. Remember to plot using the closed loop coordinates.

The position loop is a Type 2 control system  i.e. A system with two pure integration's; one in the counter the other is the electronic integrator on the PRB. Hence the low frequency response will be asymptotic to the -180 degree open loop phase line. A departure from this indicates that the input resistors to the summing point are not matched. The locus should then sweep around the plug-hole in the middle, just grazing the 2dB contour at the highest frequency point possible before diverting into resonance loops.

n.b. A compromise has to be accepted in azimuth because of changing tube inertia with elevation. Thus two plots are made; one for the tube vertical and one for the tube at AP1. The closed loop bandwidth (at the -3dB contour is about 2.5Hz for azimuth and elevation.

The velocity loop is a Type 1 control system and thus the l.f. locus will be asymptotic with the -90 degree line. The closed loop bandwidth is about 25Hz for azimuth and elevation.


This should never need re-doing. If, however, changes to the system require it then the most direct method is to modify the locus on the Nicholls Chart. It is not a procedure that can automatically applied, but there are standard methods of compensation available to re-shape the locus if a simple gain shift is not appropriate. One has to understand both the theory and practice of control systems to be able to assess the system under test and devise appropriate compensation.

The Marconi PRB has the compensation networks on board so that re-tuning is a matter of simply introducing or changing components once the desired compensation has been calculated.


This is how you change the velocity limit for FRA tests or autographic records. You can avoid this by using the Marconi Test Box to inject a rate into the counter board.
  1. Run up the TCS.
  2. Log onto TCS at another terminal and type GSEXAM at the options prompt.
Once GSEXAM is running, you can examine the velocity limits. Type:


The value will be displayed in the upper window. The mechanism code in brackets refers to the axis.

These values are in radians per second  e.g..   ALT and AZ are: 1.745E-02

To change a value use the deposit command.  e.g.. To limit the altitude axis velocity to one hundredth of it's normal rate type:

MEC_AR_DAT(1).LIMIT.VEL = 1.745E-04

You can use examine to check the range. nb. The value is only changed in the Global Section for the duration of that TCS login. When the TCS is re-booted the value is re-initialised.

Autographic  Records

The Tuning Report describes these and how they are made. Simply connect the chart recorder, set up the velocity using the test box or GSEXAM and start recording. For a step response, you set up the required velocity, start the chart and then slew the telescope and stop the telescope. An indication of  how well the telescope servos  is given by the error at standstill. For azimuth and elevation this is usually +/- 1 bit, but for the rotators with the higher friction, it can be a lot more.


These tests can give a good indication of telescope performance changes and thus help to identify faults. They are more likely to be used to exonerate the telescope than to find problems! The frequency response test will not show up an offset in the system, but an autographic test should.

Basic closed loop performance can be checked using the Marconi Test Box. This eliminates the Rate Generator and also the computer closed position loop   i.e. In particular a faulty CAMAC encoder module. Gross oscillation of the telescope sometimes occurs on start-up and may be the result of the anti-backlash bias switching on too fast, a power amplifier coming on late or the brake sticking on a drive motor shaft. Switching power amplifiers on when already in Computer Mode nearly always induces this oscillation!

Telescope glitches are a phenomenon not understood. They usually occur in azimuth and are most likely due to a digital fault. If they become frequent, the test box could be used to track the telescope in azimuth (with a certain amount of manual dexterity) on a star. This might eliminate the major part of the Marconi electronics.

Last updated: 20th March 2015  rjp