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Kollmorgen
The velocity loop has the ability to be bandwidth limited for stability purposes. Resistor 8* and capacitor
16* are used for this purpose. Resistor 8* establishes a minimum gain while capacitor 16* determines the
roll-off frequency. The stability pot allows some adjustment in the AC gain portion of the compensator to
allow for changing external characteristics, such as inertia. Components resistor 7 and capacitor 14 allow
for even more roll off at higher frequencies, which is sometimes required to reduce higher frequency
noises.
The velocity compensator offers a feed-forward (or lead) network for the tachometer using components
12* and 15*. Resistor 12* is nominally chosen as 1/3 the value of R11 and C15 is adjusted (under test) to
allow best transient response to a step function while keeping the decel overshoot to a maximum of 10%.
While a lead network adds responsiveness, it can also add noise into the system and tends to tune the
system for a given inertia and makes the system less tolerant to changes in the inertia. It is perfectly
acceptable to not use the lead network. In cases with large load-to-motor inertia ratios or where there is
changing load inertia.
In systems requiring a starting spot for the lead-lag components the following method provides good
results:
1.
Determine the value for resistor 8*
With C 16* a short circuit, C 15* removed, and the stability pot about centered, apply a 1V
square wave to the input, allowing the motor to rotate freely. For best results, the frequency of the
square wave should be about ¼ the desired bandwidth. Start with resistor 8* at a low value
(1KΩ). Raise the value of resistor 8* until the response shows good, critically-damped response
(little to no overshoot). This establishes the value of resistor 8*.
2.
Determine the value of capacitor 16*
With the new value installed in R 8*, replace the short circuit with a capacitor substitution box.
Beginning with a very high capacitance value, lower the value until the response now shows
some overshoot. In systems not using the lead network, a 10% overshoot is usually optimal. In
systems using a lead network, 20% to 30% overshoot is usually optimal.
3.
Determine the value of resistor 12*
Omit this component if a lead network is not used. This resistor determines how much effect the
lead network has on the system. This resistor is simply set to ¼ the value of resistor 8* for most
systems. In systems ising some lead but more sensitive to noise issues or having larger inertial
variations, this resistor can be set higher.
4.
Determine the value of capacitor 15*
Omit this component if a lead network is not used. Install a capacitor substitution box. Start with
the lowest value and raise it until the tachometer shows a square response. (Remove the overshoot
inserted in step 2). This is the value to use for 15*.
5.
For systems requiring the best performance, it is possible to repeat steps 2 and 4 once, while
expecting only a marginal increase in overall bandwidth.
For best results, this procedure must be performed with a load on the motor that is very similar to the
actual machine load for the motor. The stability pot allows some adjustment in the actual system gain to
compensate for small differences. Higher frequency noise is eliminated using a resistor and capacitor in
locations 12* and 15*, if required. Resistor 12* is typically ¼ the value of resistor 8*.
Capacitor components 3* and 4* allow the ability to add torsional resonance filters and are not generally
used.
®
SERVOSTAR
S/CD Product Family
Operation Information
33
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