Kollmorgen
Commissioning
SILVER
LINE
®
Product Family
5-11
5.3.1.1 The Frequency Domain
A common way to specify response is measuring the response of the current loop to a sinusoidal command
over a wide range of frequencies. That is, measuring the loop response in the frequency domain. The
response of current loops gets poorer as the command frequency increases. This is expected: when the
command is at very low frequencies, below 10 Hz for example, the loop is nearly perfect; that is, the actual
current will look very much like the commanded current. At very high frequencies, above 20,000 Hz for
example, the system will not be able to respond at all--the command may represent substantial current, but
the actual current will be very small.
The basic measure of response is referred to as bandwidth. The bandwidth of a system is defined as the
frequency at which the command is attenuated to 70% (-3dB) of its low frequency response. Figure 3.6
shows the response of a properly compensated current loop with a bandwidth of 1000 Hz. This graph
illustrates a few key points to understanding response in the frequency domain. The frequency shown here is
the -3dB point--the point at which the response is 70% of the command. We assume here that the command
and response are scaled the same. However, in an actual system, you will need to adjust your scope so the
magnitudes show the same. Do this at a low frequency so you can be sure the loop is responding without
attenuation. After adjusting your oscilloscope, you can directly compare the two signals at higher
frequencies. Notice also in Figure 5-6 that the feedback (current) is lagging the command. Here, the lag is
about 1/8 of a revolution or 45
°
. This lag is typical for a well-behaved system. A large lag at the -3dB point,
especially over 90
°
, indicates a somewhat unstable system.
Equations 4.1 through 4.4 (see Section 4) were based on this model. In that case, we chose a midrange
bandwidth. Using the information provided here, you can select the bandwidth appropriate for your
application.
5.3.2 Measurement and Control
To observe current loop operation, lock the shaft. Apply a 10% on-time pulse as command (J6-3, -4). Place a current
probe on one of the two leads with current or measure current on TP-IFB (0.2V = continuous current).
If locking the shaft is impractical, configure the drive for a zero-torque position: remove J2 and J7 and move the
shaft so current is generated, but without torque. Always apply current in one direction or the shaft will turn. Use
very low current rather than zero current.
Observe the response to the pulse; it should be rapid and not over shoot more than 10%. If the response is not as you
desire, adjust RL and CL, one at a time and in increments of about 20%, until you achieve the desired response.
5.3.3 Current-Loop Model
For customers who wish to predict the performance of the current-loop operation, Figure 5-7 provides a simplified
frequency- domain model. Currently, RS (R69) is 10 kohms, RFB (R55) is 475 ohms, and RSAMPLE is either 0.050
ohms for 4 amp units or 0.025 ohms for 8 amp units. The PWM is a two-state modulator at 18 kHz.
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