Farrand Encoder Electronics Tuning Procedure

Calypso Telescope

Gary J. McGrath

March 24, 1998

The Calypso Telescope uses Farrand encoders on the Azimuth and Altitude axes for an accurate absolute position reference. Tuning the Farrand encoder electronics is performed to optimize the encoder accuracy. This procedure involves adjusting the amplitude of the excitation signal, matching the phase of the cosine signal to the excitation signal, and balancing the sin and cosine preamplifier gains. These procedures must be performed in this order.

The following sections describe the purpose and procedure for each step of the tuning.

Excitation Amplitude Adjustment

Phase Adjustment

Sin and Cosine Balancing

The following references may also be used as a guide for these procedures:

  1. Calypso Telescope Servo Controller Users Manual.
  2. Farrand Controls Engineering Report 19001 "Using Inductosyn Transducers with Analog Devices, Incorporated 2S80 Converter"

Excitation Amplitude Adjustment

This procedure adjusts the amplitude of the 10 kHz excitation signal. This amplitude must be within the manufacturers specification.
    1. Display the cosine signal on an oscilloscope by probing the COS-HI input terminal on the Converter-Oscillator board (or COS Pre-amp output). The signal will change in amplitude as the encoder is moved. Note the maximum amplitude.
    2. Adjust the "DRIVE" potentiometer on the Converter-Oscillator board until the maximum amplitude is 5.7 volts peak to peak (or 2.0 volts rms) +/-5%.

 

Phase Adjustment

This procedure adjusts the phase of the cosine signal so that it is in phase with the excitation signal. The relative phase between the sin and cosine signals is not affected by this procedure, as their phase is determined by the physical position of the sin and cosine windings in the encoder.
    1. Connect one input of an oscilloscope to the "COS-HI" input on the Converter-Oscillator board (or the COS output on the Preamplifier board).
    2. Connect another input of the oscilloscope to the phase test point TP-phi.
    3. Displaying both inputs, adjust the "PHASE" potentiometer until both signals are in phase. The encoder position will cause the COS signal to appear either in phase or 180 degrees out of phase with the reference signal. You may need to move the encoder slightly to display the signals in phase.

Sin and Cosine Balancing

This procedure balances the preamplifier stages of the sin and cosine signals. It is important that these gains are accurately matched. The first step roughly sets the encoder position to about 135 (or 315) degrees of position within one sector of the encoder. At either of these positions the sin and cosine signals should be equal and opposite. The second step more accurately positions the encoder to 135 (or 315) degrees. In the third step, the gain of the cosine preamplifier channel is adjusted to match the fixed gain of the sin preamplifier channel.
  1. Rough Mechanical Positioning
    1. Connect probe 1 of the oscilloscope on "COS-HI" input and probe 2 on "SIN-HI" input.
    2. Turn the power on to the NEMA boxes. Configure the servo to use the Heidenhain encoder instead of the Farrand.
    3. Configure the servo to use constant velocity input with a command velocity of 0.0.
    4. Start the MASTER and SERVO controller programs.
    5. Press F2 (Start) on the Maser screen. The command and encoder positions will both be displayed as zero on the computer.
    6. Log in to the VAX via telnet. Any computer with telnet may be used. Although, it may be convenient to use the laptop since it can be plugged into the Ethernet jack at the telescope level. Open a Telnet window and connect to pluto. At the VMS prompt, login as TCC. Then type the command TELRUN. You will now be at the TCC command line interpreter.
    7. Use the TALK command to issue commands directly to the controllers. From the TCC, type "talk tcc_az". This gets you into the TALK mode. You can then enter commands to the controllers until you press CTRL-Z or there is an error.
    8. Type "AZ SEEK.REF" (you can use either AZ or ALT) to initialize the axis.
    9. Use the MOVE command to position the encoder so that the sin and cosine signals are equal and out of phase. Since the value of sin and cosine are equal and opposite at both 135 and 315 degrees, this position puts the encoder discs at the physical position of approximately either 135 or 315 degrees of motion within one of the 512 sectors of a revolution.
    10. Apply the brakes and stop the servo by pressing F3 (Halt) for the desired axis on the MASTER controller.
  2. Fine Mechanical Positioning
    1. Turn the main power off on the control panel.
    2. Disconnect the encoder winding connector from the input of the preamplifier. Connect the series-winding adapter between the encoder winding connector and the preamplifier inputs. This connects the sin and cosine windings together in series and drives the sin channel of the preamplifier. If the encoder is positioned at either 135 or 315 degrees the sin and cosine signals will cancel each other.
    3. Turn the power back on.
    4. Run the master and servo controller programs.
    5. Press F2 (Start) on the Maser screen. The command and encoder positions will both be displayed as zero on the computer.
    6. Use the HP3562A signal analyzer to measure the voltage at TP-S. The power spectrum setting should be used on the analyzer. Adjust the position of the axis with the servo by sending very small MOVE commands until the null is below 50 microvolts rms (or 2.5x10-9 V^2 as displayed on the analyzer). The null should be made as small as possible.
    7. Lock the axis by applying the brakes and stopping the servo.
  3. Preamplifier Balancing
    1. Turn the main power off on the control panel.
    2. Disconnect the series-winding adapter. Reconnect the encoder winding connector to the preamplifier input for normal operation.
    3. Disconnect the digital output connector (40-pin header) on the digital interface board. Connect the LED readout test board in series between the digital ribbon cable coming from the resolver board, and the 40-pin header on the digital interface board. This is a straight through connection so the orientation of the LED test board does not matter. The black line on the edge of the LED readouts indicates the 16-bits of the resolver digital word.
    4. Turn the main power back on.
    5. With the axis still locked, adjust the balancing potentiometer until the LED indicators read either 0110000000000000 (135 degrees), or 1110000000000000 (315 degrees).
    6. Without moving the potentiometer setting, remove the potentiometer from the circuit.
    7. Accurately measure the resistance of the potentiometer.
    8. Using 1% resistors, assemble a resistor network to accurately match the potentiometer resistance.
    9. Solder the resistor set on the preamplifier board in place of the balancing potentiometer.