VSERVO Command

You must complete the VSERVO questions for both the azimuth (Vservo AZ) and elevation (Pservero EL).

Once the velocity servo parameters are set to an initial-guess value, you must to exercise the servo on each axis to check for proper behavior. When properly setup, the servo should rapidly, and without overshoot, bring the antenna velocity to any requested rate as entered using the local TTY at and et commands. The tachometer readings should be reasonably stable, plus or minus 0.1 T-Units, and the output drive should exhibit minimal oscillation around the mean level necessary to obtain the requested velocity.

Use the following suggestions to help resolve problems in the servo setup.

Servo Possible Problems and Solutions

Problem	Solution
Servo is completely unstable	Recheck the tach sign to make sure the positive velocities correspond to the positive position increments.
Tach sign is correct	Check the drive sign: If the velocity overshoots, the tachometer and drive filtering time constants may be too long. If the antenna is sluggish and does not quickly reach the desired velocity, the feedback gain is too low.
Antenna chatters Output drive oscillates around its mean value	Feedback gain is probably too high.
equilibrium velocities differ slightly from the requested rates For exampke, request 50 but get 51.	Nominal drive slope and/or sustaining drives are incorrect.

Motor positive sustaining drive: 15.00 D-Units

Motor negative sustaining drive: -15.00 D-Units

These numbers indicate the drives that are required to just overcome the friction of the motor during positive (CW or upward) and negative (CCW or downward) motion. These are given in D-units, ranging from -100 to +100. To determine the proper values, use the local TTY control ad and ed commands. Start from initial rest and gradually increase the drive until the motor suddenly starts to move. Then decrease the drive until the motion stops due to friction. Enter the smallest drive values for which continuous motion could be sustained.

Nominal positive drive slope: 0.800 D/T-Units

Nominal negative drive slope: 0.800 D/T-Units

These parameters are used along with the sustaining drive levels to make an initial guess of the drive required to maintain a given velocity in the steady state.

Use the local TTY control to determine the following values:

1. Output a drive level that results in a velocity that is approximately 75 % of full speed.

2. When a steady tachometer reading has been achieved, record the drive and tach readings from the TTY .

   The required slope is (Drive - Sustain) / Tach, where Sustain refers to the sustaining drives that were measured in the previously.

If the motor amplifier has a different gain in each direction, 2 different slopes are permitted - the first value for positive (CW or upward) motion and the second one for negative (CCW or downward) motion.

Velocity feedback slope: 25.000 D/dT-Units

The tachometer error feedback slope controls the tightness of the velocity servo. The velocity servo is stable for most values of this parameter. If the value is too small, the motion is sluggish with relatively large errors in the final achieved velocity. If the value is too large, the currents thrash wildly as the servo attempts to maintain the exact requested tachometer level. The appropriate value must be determined empirically.

Connect an oscilloscope to the drive and tachometer signals and use the local TTY control to select different servo rates - the at and et commands. Choose the largest value of the parameter that brings the antenna rapidly to the requested velocities without excessive drive oscillation around its equilibrium value. If a scope is unavailable, you can make a fair judgment by observing the drive values that are displayed on the TTY. The feedback slope has units of Drive/TachError, typical values range from 10 ... 200.

Velocity feedback deadzone: 0.10 T-Units

A deadzone is built into the tachometer feedback path to ensure that the uncertainty of the low bits, of the A/D converters, does not result in motor chatter. Typical values are 0.1...0.5 T-units. If values above these limits are necessary to control the chatter, then this is indicative of the excessive noise on the tachometer inputs.

Apply velocity error integral correction: YES New Value:

Characteristic time of the integral: 2.00 sec New Value:

Maximum resulting drive bias: +/-25.00 D-Units New Value:

RCP8 velocity servos include a velocity error integral feedback term, in addition to the proportional error feedback term and bias terms that have always been available. The error integral effectively removes any remaining steady-state velocity bias from the servo, and guarantees that scans runs at precisely their requested speed. These questions to configure the velocity error integral feedback term. The feature is switched On/Off using the first question.

The second question establishes the characteristic time T₀ of the integral, which is defined as follows. Suppose that a fixed velocity error E was sustained for a period of time. The proportional feedback term would produce a drive D=SE, where S is the velocity feedback slope. Then, if that same error E were applied to the integrator for T₀ seconds, the same drive term D would also result.

The gain of the integrator effectively is established by T₀ ; larger times produce smaller gains. One rule of thumb (Ziegler-Nichols) for a first guess of S and T₀ is to disable the integral feedback, and increase S until reaching a value S_u , at which the antenna goes into unstable oscillation with an observed period P_u. Reasonable first settings are then obtained with S = S_u / 2.2, and T0 = 2.2 P_u.

The integral can be clamped (the so called anti-windup feature) to prevent it from drifting into large values when the antenna is not in equilibrium. This clamp value is expressed as the maximum drive correction that can be contributed from the integral term alone. If your antenna is well characterized by its sustaining drives and nominal drive slopes, then this clamp value can be reduced (since the nominal guesses do not need to be adjusted very much). This helps reduce brief overshoots that can be caused by the integral feedback.

Generate stepper motor drive control signals: NO NO

Maximum absolute velocity: 95.00 T-Units

This value represents the tachometer level which corresponds to the maximum antenna rotation rate that is considered safe. The lower-rate limit is the negative of the upper-rate limit. If the A/D converter hardware components have been set properly, the maximum value should be at least halfway through the converters full range - at least 50. If the safe value is less than 50, then the A/D range should be altered to make better use of the available 12 bits. The local TTY control, or an external manual control, can be used to cause the antenna to spin at the maximum safe rate while the tach levels are noted from the local TTY angle display.

Velocity shutdown safe margin: 4.00 T-Units

Velocity shutdown check time: 1.00 sec

RCP8 has provisions to shutdown if the observed velocity on either antenna axis exceeds the internal maximum velocity limits that are enforced by the velocity servo. If the velocity overshoots in the vicinity of these limits, the shutdown criterion can sometimes lead to false shutdowns when no actual problem exists. Ideally, the velocity servo is setup to ensure that overshoots does not occur however, given the influences of motor damping and wind gusts, this strict condition is difficult to enforce.

To minimize false shutdowns due to temporary velocity overshoots, the shutdown criterion is expressed in terms of a velocity tolerance and a time limit. If this condition persist longer than the specified time, shutdown occurs if the absolute value of the measured velocity exceeds the sum of the maximum limit and the specified tolerance. As an initial guess, the tolerance should be set slightly higher than the maximum sustained velocity error that is ever observed, under normal operating conditions, while taking into account wind loading and other operational effects. The time should then be set slightly longer than the time by which the longest transient overshoot exceeds the specified tolerance.

Tach zero-delay-smoother window:0.010 sec

Model order within the window:3

Tach filter time constant: 0.025 sec

The tachometer inputs can be filtered with a simple, exponentially-weighted smoothing filter prior to applying to the velocity servo. This filtering is intended to remove spurious components from the digital tachometer samples. The filter time constant is typically set at approximately one-third the reciprocal of the antennas upper-frequency response limit.

The filter time constant is entered directly in seconds but the exact value must be determined by trial and error from an initial approximation. If the time constant is too large, the velocity servo becomes unstable and oscillates around the desired velocities before settling. If the time constant is too small, then no significant smoothing or spurious rejection is attained. The value should be increased until the velocity overshoots become noticeable on an oscilloscope display of the tachometer signals. The final time-constant value should be slightly less than this level. Velocity overshoots can also be detected by the human eye by requesting zero velocity and observing how the antenna comes to rest.

Drive filter time constant: 0.025 sec

This drive filter behaves much like the tachometer filter, as described in the previous paragraph, except this is applied to the output drive levels prior to D/A conversion. The purpose of the filter is to smooth the motor drive signal and to remove the high frequency feedback components that can be generated by the velocity servo. Although these components most likely would be filtered by the motor and mechanical system, the users power-drive electronics might be adversely affected by sudden changes in the motor current. The filter time constant should be set as large as possible, consistent with preventing velocity overshoots as described in the previous paragraph. The drive and tachometer filters has similar time constants however, from this common value, improved performance is usually obtained if the tachometer constant is decreased and the drive constant is increased.

Drive slew rate limit for Zero --> Max: 0.200 sec

A slew rate limit can also be imposed on the output drive signals. The limit is expressed as the number of seconds required for the drive to slew from 0 ... 100. For example, a value of 0.2 seconds restricts the rate of change of the output drive to 500 D-Units/second. The slew rate limit is useful in preventing abrupt changes in motor drive. In some cases, such fluctuations can bring about unwanted oscillations in the antenna/pedestal mechanical system. The slew rate limiter is applied by the RCP8 software after the output filter.

The Zero-to-Max drive slew rate time can be set as large as 15 seconds. This allows RCP8 servos to work more gracefully with external motor controllers that incorporate a velocity feedback loop of their own. In such cases, the RCP8 velocity feedback slope should be set to 0, and internal (model based) acceleration limiting should be disabled. Acceleration limiting can be accomplished instead using RCP8s drive slew rate limiter, which can now work over a longer time span.

Note that the drive filtering and slew rate limiting are both overruled by the detection of shutdown conditions, and the enforcement of soft limits of travel. If the antenna is heading rapidly toward a soft limit, the drive is immediately adjusted to stop before the limit is reached.