58
HA0303T Rev A Feb 2014
Chapter 6
Display Intensity
- determines the brightness of the control panel display.
Enter a value from 0 to 255.
Control Loop
- When operating in Closed Loop mode, the proportional, integral and
differential (PID) constants can be used to fine tune the behaviour of the feedback
loop to changes in the output voltage or position. While closed loop operation allows
more precise control of the beam position, feedback loops need to be adjusted to suit
the different types of sensor that can be connected to the system.
The default values have been optimised to work with the sensor, in the opto-
mechanical set up shown in Fig. 5.2.
Proportional
- This parameter makes a change to the output which is proportional
to the current error value. A high proportional gain results in a large change in the
output for a given error. If set too high, the system can become unstable.
It accepts values in the range 0 to 10000 (default 10)
Integral
- This parameter accelerates the process towards the demanded set point
value, ensuring that the positional error is eventually reduced to zero. If set too
high, the output can overshoot the demand value.
It accepts values in the range 0 to 10000. (default 5).
Derivative
- This parameter damps the rate of change of the output, thereby
decreasing the overshoot which may be caused by the integral term. However, the
differential term also slows down system response. If set too high, it could lead to
instability due to signal noise amplification.
It accepts values in the range 0 to 10000 (default 0).
For more details on setting the PID parameters, see Appendix B
.
Derivative Low Pass Filter
- The output of the derivative (differential) part of the PID
controller can be passed through a tuneable low pass filter. Whilst the derivative
component of the PID loop often improves stability (as it acts as a retaining force
against abrupt changes in the system), it is prone to amplifying noise present in the
system, as the derivative component is sensitive to changes between adjacent
samples. To reduce this effect, a low pass filter can be applied to the samples. As
noise often tends to contain predominantly high frequency components, the low pass
filter can significantly decrease their contribution, often without diminishing the
beneficial, stabilizing effect of the derivative action. In some applications enabling this
filter can improve the overall closed loop performance.
The
Cut Off Frequency
is specified in Hz in the range 0 to 10000.
Notch Filter Settings -
Due to their construction, most actuators are prone to
mechanical resonance at well-defined frequencies. The underlying reason is that all
spring-mass systems are natural harmonic oscillators. This proneness to resonance
can be a problem in closed loop systems because, coupled with the effect of the
feedback, it can result in oscillations. With some actuators (for example the ASM003),
the resonance peak is either weak enough or at a high enough frequency for the
resonance not to be troublesome. With other actuators (for example the PGM100) the
resonance peak is very significant and needs to be eliminated for operation in a stable
