In order to understand why resolution is critical, it is important to review how the speed controller works. The speed controller is typically a P-I controller, whose input is the difference between the speed set point and actual speed (encoder) values. The output of the speed controller generates the torque set point, which determines how much force the motor shaft will exert on its load. Therefore, the torque command to the motor is directly proportional to the difference between actual speed and speed set point. In order to smoothly control the motor's load, you never want the input to the speed controller to instantly have a large value.
Current servo drives have speed controllers that are updated in the 100-200Ásec range. For the example shown below, we will assume a 125Ásec speed controller on a motor running at 30rpm.
- Motor RPM = 30
- Deg/sec = 180
- Deg/125Ásec = 0.023
- Feedback resolution: 10,000/1 million
- Pulses/deg: 27.78/2.777.78
- Pulses/125Ásec: 0.625/62.5
The calculations show that, at this low speed, the resolver feedback is so low that consecutive scans of the speed controller can actually occur without the resolver registering a difference in angular position. Since actual speed is defined as ∆ Distance/∆ Time, this registers with the drive as zero speed over the previous 125Ásec. This causes the speed controller to immediately generate a large output to try and reduce the perceived difference at the input. During the next scan of the controller, there is an incremental change, and the controller then reduces its output, because the perceived difference at the input is gone or greatly reduced. It should be obvious how this behavior could cause erratic movements at this low speed.
To avoid this undesired consequence, the engineer tuning the drive is forced to keep the gain of the speed controller very low. A low gain slows the response time of the controller so that, when two scans occur on the same encoder increment, the controller delays its response time long enough to see the new pulse during its next scan. This stops the erratic movements but in some cases causes a new problem.
Suppose the motor needs to stop the load quickly. An example might be a machine where the inch button causes the machine to move at a low speed, but the operator needs the machine to stop the instant the inch button is released. Dynamically stopping a heavy load requires a fast injection of negative torque, and this requires a speed controller with a fast reaction time. If the feedback device is limiting the controller's gain, this abrupt stop may not be possible. Changing the encoder to a sin/cos can allow for the speed controller gain to be increased by as much as 300 percent.
Resolvers are less expensive and more durable than optical encoders, so there will always be a need for them with servomotors. However, when specifying the servomotor for an application, make sure all operating scenarios are considered before deciding on the feedback device. A good rule of thumb to use is to select a feedback device that can deliver 5-10 pulses at the lowest rpm required for the application for the scan time of the speed controller.