Configure the servo parameters of one or more telescope axes.

WARNING

Beware that changing the servo parameters is potentially very dangerous. Servo tuning is not an easy thing to do, and getting it wrong can produce destructive oscillations. In particular, be very careful not to cancel too much of the derivative gain of the velocity servo, with a negative derivative gain, since this term in the velocity servo prevents the servo from going into unconstrained oscillations. Also note that the stability of the servo differs at different azimuths and elevations, and different tracking speeds, such that testing new parameters can involve days of tracking sources all over the sky, while watching the servo voltages for oscillations.

Arguments:

Axes axes

The set of telescope axes to configure.

Double track_vmax

The maximum tracking velocity (degrees/s).

Double slew_speed

The slew speed (degrees/second).

Double slew_volts

The signed servo control voltage that requests a positive topocentric velocity of the above slew speed. This should be negative if a positive voltage moves the axis towards a more negative topocentric position.

Double pgain

The proportional gain of the servo (1/seconds).

Double igain

The integral gain per second of the servo (1/seconds^2)

Double dgain

The derivative gain of the software portion of the servo.

Double vgain

The feed-forward velocity gain of the servo.

Double zero_offset

The servo control voltage that results in zero velocity.

Double accel_lim

The acceleration ceiling (degrees/s/s)

Example:

The following example tells the control system to set the transition between track and slew modes at 0.125 degrees/second, to set the slew speed to 0.25 degrees/second, indicates that this slew speed towards positive elevations can be requested by sending -10V to the velocity servo, tells the servo to use a proportional servo gain of 0.9, an integral servo gain per second of 0.2, to feed-forward the target velocity scaled by 0.9, to subtract a constant 0.04 volt zero-offset from the servo control voltage, and to limit the acceleration to 8.1 arc-minutes/s.

 servo_pars el, 0.125, 0.25, 10.0, 0.9, 0.2, -0.1, 0.9, -0.04, 0:8:06

Context:

The parameters set by the servo_pars command govern the behavior of the software part of the servo of the specified telescope axes. The servo of each telescope axis consists of a hardware velocity servo, commanded by a software position servo. Together these form a traditional PID (proportional, integral, differential) servo, with the addition of a velocity feed-forward term.

The proportional and integral gains of the servo are set in software, by the pgain and igain parameters. Nominally all of the differential gain is provided by the hardware servo. However small positive or negative values of the dgain parameter can be used judiciously to add or subtract from this. The velocity feed-forward gain is set by the vgain parameter.

The servo effectively has two modes. For speeds below track_vmax, the desired velocity, generated by the software servo is passed unchanged to the velocity servo. This provides good tracking when tracking sidereal sources. However above track_vmax and the maximum slew speed, slew_speed, the velocity output is progressively clipped, purportedly to smooth the transition between tracking and slew speeds.

Further information about the servo loop and its clipping behaviour, can be found in the 40m control system documentation.