Introduction
The use of linear motors for precision motion applications has increased as technology has improved.
A linear motor typically uses a servo drive with sinusoidal commutation to minimize torque ripple and provide quiet smooth motion. It is also becoming more common for linear motor manufacturers to provide position feedback in the form of an analog sin/cos encoder as this method uses the signals from the motor magnets and is more cost-effective. Galil motion controllers now provide options for sinusoidal drives and interpolation of sin/cos analog feedback for smooth control of ultra-high performance applications.
Galil controllers use an intelligent interpolation algorithm allowing inexpensive sin/cos signals to be used successfully in high-precision applications. The AF command is used to set the analog feedback resolution at 2n counts/period. Galil drives perform sinusoidal commutation and are easily programmed using a choice of three different commutation methods. An example showing how to configure Galil controllers and sine drives for use with linear motors with sin/cos encoders is below.
An example of a Galil Controller used with a Copley ServoTube Linear Motor
Galil's DMC-31012 motion controller with an internal 800W sinusoidal drive and sin/cos feedback interpolation option was used in an application with a Copley ServoTube linear motor with sin/cos encoder. The goal was to move a stage and maintain a position accuracy of .04mm. The stage manufacturer specified a 25.6 mm/sin feedback period for the encoder signals which is equivalent to the motor’s magnetic cycle.
The AF command was used to interpolate the analog feedback at 2n counts/period. N= 10 was used for this application. AF 10 provided an interpolation of 1024 counts/period. The system position resolution can be calculated using the equation below:
Resolution = 25.6mm /1024 counts = .025mm/count
.025mm/count resolution is within our target accuracy of .04mm.
It’s important to note that sin/cos feedback sensors are analog and more prone to noise compared to a digital signal. The DMC-31012 allows an AF of 12 but AF 10 was chosen because it met the accuracy specifications while minimizing the affect of noise.
There are three methods to initialize sinusoidal commutation with Galil sinusoidal drives, BI, BZ and BX. For this application, the BZ command method was selected because it was better for high static friction. The BI command, which uses halls to initially commutate, is ideal in most cases but the Copley motor did not provide Hall sensors.
Another parameter used for commutation was the BM command. BM is the brushless modulus of the system or the length for which one magnetic cycle completes. For the Copley ServoTube, the motor's sin/cos period is the same length as the motors magnetic cycle. For AF 10, the setting for BM is 1024.
Once AF, BZ and BM were configured during initialization, the controller system was tuned for optimum performance using the GalilSuite tuning software.
Galil controllers with the sin/cos feedback option and sinusoidal drives provide a good solution for controlling linear motors with a high degree of accuracy and extremely smooth commutation. Sinusoidal drives and sin/cos feedback options are available on Galil's DMC40x0 multi-axis controller and the DMC-31012 single-axis controller.
For more detailed information, see application note #5523 “Connecting to a Linear Motor with Sinusoidal Commutation and Sin/Cos Feedback.” http://www.galil.com//download/application-note/note5523.pdf