User's Manual

PMAC2A PC104 Hardware Reference Manual
Software Setup 21
interrupt frequencies for better servo performance, but our default frequencies are typically more than fast
enough for many applications. We will discuss tuning parameter a bit later in this document.
I901= 29.44KHz/19.61KHz -1 = @0.5 set it at 1 or 14.72KHz
This is not exactly the same since I901 is an integer value but pretty close. Since we are doing any
commutation with a +/-10V signal it doesn’t make that much of a difference. The Servo Frequency we
will be able to get close though:
I902=14.72KHz/4.9 – 1 = 2.004 or 2 which is @4.9KHz
For a 10V max signal output:
Ix69=i900 + headroom = 2024
We must set I10 whenever we change the servo clock but since we kept it basically the same, I19 stays
pretty much the same. Without rounding it works out to the following:
I10 = 8388608/4.906613 = 1709653
For precise timing within your motion application it is important not to round off when calculating I10.
Effects of Output Resolution and Servo Interrupt Frequency on Servo
Gains
When you change your output resolution and/or servo interrupt timing your tuning parameters will no
longer respond the same. The system will have to be tuned again in order to achieve the desired
performance. There is an approximate relation of output resolution to servo loop gains . If you were
switching an application from a PMAC style 16bit Dac to a PMAC2Pc104 with default resolution of
about 11bits you can expect a change of your gains in order to get similar response.
The max output value of the output command with a 16bit Dac is 32767. With the PMAC2Pc104 at its
default parameters the max output value is 1001. If you had equal servo interrupt frequencies the
proportional gain on the PC104 system would have a proportional gain 1001/32767 or about 1/32 smaller.
This is more a rule of thumb than an exact formula. It is always recommended to go through a full tuning
procedure when changing output resolution.
If you decide to change the Servo Interrupt Frequency, then you are also changing the dynamics of the
servo filter and thus the system. You will need to retune the system in order to get the desired
performance. If you increase the servo frequency you will need to lower the proportional gain in order to
achieve similar performance. The reason you increased the frequency in the first place was more likely to
achieve a higher performance so relations here are not very helpful.
If you desire to change servo interrupt frequency in order to have your foreground PLCs execute more
often you can also adjust Ix60 to keep your gains the same, see the Pmac1/2 Software Reference Manual
for a further description of this parameter.