Wed 19th Jul 2017 - Choosing the Right Rotary Servo Motor Feedback Device - Part 2
Before getting started, you may find it helpful to review our earlier post on basic operational theory for three types of feedback devices: resolvers, optical absolute encoders and optical incremental encoders.
Consider the operational environment for your application when selecting a feedback device. The ambient temperature and shock/vibration ratings required for your application can quickly determine if you need a resolver or if an optical encoder will suffice.
- Ambient temperature - Optical encoders are typically rated for up to 90 degrees C while resolvers can be rated for up to 200 degrees C
- Shock and vibration - Optical encoders are typically rated for up to 100g of shock and 30g of vibration; resolvers can be rated for up to 200g of shock and 40g vibration
Speed control, position control and torque control are performance requirements that factor into selecting the right feedback device for your application.
Speed control – The resolution you choose will be influenced by the update rate (velocity loop) and input frequency of your control.
For instance, if you are operating at a very slow speed, you need to make sure that your resolution is not too coarse or the control update may occur prior to your next count.
Example - slow speed operation
- Velocity loop of Drive = .000125 seconds
- Motor RPM = 50
- 50 rpm = .83 rev/sec = 300 deg./sec
- 300 deg./sec x .000125 sec = .0375 deg. of shaft rotation for every update by the control
- Resolution = 1,000 counts/rev
- 1,000/360 = 2.8 counts/deg. = 1 count every .357 deg.
On the other hand, if you are operating at a very high rpm, you need to make sure that your drive can process all of the feedback device resolution relative to its input frequency.
Example - high speed operation
- Input frequency of Drive = 20 Mhz = 20,000,000 counts/sec capability
- Motor RPM = 2,000 = 33.33 rev/sec
- 20 bit encoder = 1,048,576 counts/rev
- Resultant counts = 33.33 x 1,048,576 = 35 million counts/sec
Formulas for speed performance requirements
- Minimum resolution (pulse/rev) for low speed requirement
- Maximum resolution (pulse/rev) for high speed requirement
For instance, a 5,000 pulse/rev. encoder will allow you to control your motor shaft to .07 deg of rotation. If your encoder’s accuracy is .07 deg then your resultant level of shaft control is .14 deg.
After that you need to take into account the accuracies of the components that your motor is driving. If you are now adding a gearhead with .08 deg. of backlash to your motor with the 5,000 pule/rev. encoder, then your resultant level of shaft control at the output of your gearhead is .14 deg. .08 deg. = .22 deg.
As with speed control, you need to look at the update rate (position loop) of your control device.
- Slow speed operation - If you are running at a very slow rpm, and your resolution is too coarse, your controller will see this as an error and will try to correct.
- High speed operation - If you have a high speed application and you choose too much resolution relative to your controller input frequency, you will cause your controller to fault.
Here are a few considerations to help you choose between BISS, EnDat and Hiperface communication protocols.
- If you are designing your own drive and controls you may want to investigate which protocol offers the best performance relative to clock frequencies, transmission rates, etc.
- From a motor manufacturer viewpoint, we look at resolution, permissible speed and ease of integration.
- From an end user standpoint, make sure the communication protocol of your motor feedback device is a match to your controller, especially when you mix and match components from various manufacturers.
These formulas and examples show what your minimum pulse/rev (PPR) feedback resolution should be when using a screw or belt driven linear stage. Round the value up to the closest standard resolution offered by your feedback device. Remember to understand your controller update rate and input frequency relative to the speed that you want to run your motor at.
PPR = Screw Lead/Move Requirement
Lead = 10mm
Move = .00254mm
PPR = 10/.00254 = 3937 PPR
Belt driven stage
PPR = Circumference of Pulley/Move Requirement
Pulley circumference = 15.7 inch
Move = .008 inch
PPR = 15.7/.008 = 1,962 PPR
Watch this video to learn more about the features of our SM slotless servo motor. Consider how environmental and performance requirements will apply to the feedback choices offered for this servo motor.
Article reposted with Parker Hannifin's permission.