PSERVO Command

PSERVO provides acess to the legacy position servo parameters. You must complete the questions for both the azimuth (Pservo AZ) and elevation (Pservero EL).

The set-up questions listed in your configuration vary depending upon your set-up and how your have responded to previous questions.

Hysteresis inner zone: 0.051 deg

Hysteresis outer zone: 0.020 deg

These represent the position errors, in degrees, within which the position servo does not attempt to correct the antennas location. Two values are specified — one for the lower limit and one for the upper limit. When the actual position error is less than the lower limit, the position servo does not drive the antenna (that is, it requests 0 velocity from the velocity servo.) Likewise, when the actual position error is greater than the upper limit, the servo always drives the antenna to correct it. There is hysteresis between these limits which helps to prevent antenna chatter once it has reached the desired position (the servos state between the limits is whatever it was at the time the limits were entered.)

It is important that the inner limit is greater than half the weight of the least significant bit that encodes the antenna position. For example, for 12-bit binary encoding the inner limit must be at least 0.045 º. If it were smaller, the position servo might not realize that the final position had been reached and would continually move the antenna around a single LSB interval. As an initial guess, use an inner zone that is 10 % larger than half the LSB weight. The outer zone should then be set somewhat larger, perhaps by 50 %.

Servo type: Legacy

There are 2 types of position servo loops, Legacy and Feedfwd. Legacy has a desired speed which ramps to zero at the desired position, using multiple slopes shown here:

First position break point: 1.00 deg

Second position break point: 5.00 deg

These represent the values of the 2 position-error break points, in a piecewise linear definition, of the desired velocity-versus-position error.

First interval slope: 12.00 (T–units)/deg

Second interval slope: 3.00 (T–units)/deg

Third interval slope: 1.00 (T–units)/deg

These represent the 3 piecewise linear definition slopes of a desired velocity-versus-position error.

The following 3 intervals are defined as:
  1. Zero to first-break point,
  2. first-break point to second-break point, and
  3. second-break point to infinity.

Servo type: Feedfwd

The Feedfwd servo uses a feed forward servo to correct for the effects of a stretching rubber belt used on Vaisala pedestals, for example. Its tuning parameters are:

Period of mechanical resonance: 0.50 sec

This is a mechanical property of the antenna–pedestal combination. Different for azimuth and elevation.

Drive constant: 0.5000 deg/sec/D–Unit

This is a property of the motor driver and gear ratio, defining how many deg/sec drive is produced by a D-unit. This is different for azimuth and elevation in the Vaisala pedestal.

Drive end wait factor (n*T/2+T/4): 1

At the end of the drive, wait for a period of time before reading the current position to check it. T is the period of mechanical resonance, and here you enter the n.

Maximum acceleration: 6.0 deg/sec/sec

This is the maximum acceleration the antenna-pedestal is capable of doing.

  1. The position servo cannot work properly with incorrect settings. Make sure the velocity servo has been thoroughly checked, in accordance with VSERVO Command.
  2. Set the inner and outer zones of the hysteresis as described below.
  3. For Legacy servos,
    1. Set the first position break point to a small value P, such as 1.0 º , and attempt several values of first interval slope.

      Find the largest slope that results in no overshoot when steps of P-degrees or less are performed. Use the ap and ep angle monitor commands to test the slope values.

    2. Choose a larger second position break point and find the largest second interval slope that accomplishes larger steps without overshoot.
    3. Find the largest third interval slope that permits large steps of any size to be travelled without overshoot.
  4. Exercise the servo on each axis to check for proper behavior.

    Try to move the antenna using the local TTY ap and ep commands and verify that:

    • Any position step can be requested without overshooting the final mark.
    • Very small steps cause antenna motion to occur. If the position feedback curve is incorrect, it is possible for the servo to work properly for some step sizes but not for others. Test a range of steps.

    This step attempts to tune the antenna for maximum performance (that is, the antenna arrives at a requested position as rapidly as possible.) On some systems, delays—usually in the response to a drive voltage—can lead to small position overshoots that can usually be eliminated by detuning the antenna performance. Detuning is accomplished by lowering the slopes, for the first and second endpoints, so the approximation to the braking curve lies below the observed curve. This usually eliminates any position overshoot, with a slight performance penalty.

  5. Iterate the procedure as needed, making sure to complete the questions for both the azimuth (Pservo AZ) and elevation (Pservero EL).