Enhancing Stepper Motor Systems with Linear Encoders

Tabletop automation is a trend that is gaining momentum, especially in the fields of medical laboratory automation and 3D printing. Both of these applications demand a level of linear positioning accuracy and speed that might suggest a servomotor as a solution, but market-driven cost constraints put most servos out of financial consideration. New advances in stepper motor design, including higher torque, higher power ratings, and the availability of closed-loop operation via integrated motor encoder feedback are enabling steppers to expand their application envelope to include many tasks that formerly demanded a servo system.

Meeting the Demand for Even More Accurate, More Reliable Positioning

As tabletop automation development progresses, performance demands are increasing to the point that stepper systems may struggle to meet requirements. Fortunately, the addition of an external linear encoder for direct position feedback can enhance a stepper system to enable the expected level of reliable accuracy. An external linear encoder puts drive-mechanism non-linearity inside the control loop, meaning any deviations caused by drive component inaccuracy are automatically corrected and compensated by the overall closed-loop positioning system. In addition, the external linear encoder provides another level of assurance that the driven element has actually moved to the position indicated by the number of stepper pulses and/or the movement reported by the motor encoder. This prevents position errors due to stepper motor stalling, lost counts on the motor encoder, someone manually moving the mechanism against motor torque, or drive mechanism malfunction, i.e. broken drive belt or sheared/skipped gearing.

Incremental, Absolute, or Hybrid Encoder Signals

bmlThe position signals from the external encoder are typically incremental, meaning a digital quadrature square wave train of pulses that are counted by the controller. To find a position, the system must be “homed” to a reference position and then moved the required number of counts to reach the command position. The next move requires starting with the position at the last move and computing the differential move to the next command position. Absolute position signals, typically SSI (synchronous serial interface) provide a unique data value for each position. This position is available upon power-up…no homing movement is required and there is no need for a pulse counter. A recent innovation is the hybrid encoder, where the encoder reads absolute position from the scale, but outputs a quadrature incremental pulse train in response to position moves. The hybrid encoder (sometimes referred to as “absolute quadrature”) can be programmed to deliver a continuous burst of pulses corresponding to absolute position at power up, upon request from the controller, or both.

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Pathways to More-Precise Linear Motion

In a previous SensorTech post, we discussed improving the accuracy of linear motion systems while lowering total system cost by employing external linear position encoders as secondary feedback.  The secondary feedback supplements the primary feedback provided by a rotary encoder mounted to the drive motor.

Now Clint Hayes, Sales and Product Manager for Linear Technologies at Bosch Rexroth, has written an excellent “How To” article for Machine Design magazine entitled “Six Keys to More-Precise Linear Motion.”  Mr. Hayes identifies precision as a combination of accuracy and repeatability, where accuracy is the discrepancy between target and actual position, and repeatability is the ability of a motion control system to return to a given position when repeatedly approaching that position from the same direction   He discusses the important effects of various mechanical design elements and operating conditions for linear guides that can influence these important motion control system specifications.

One of the important specifications discussed in the article is Positioning Accuracy.  Mr. Hayes points out that positioning accuracy is dependent on the capabilities and tolerances of the mechanical drive mechanism.  He also highlights the technique of implementing electronic position correction to compensate for rising mechanically-induced deviation as travel distance increases.

The reference measurement for this electronic correction can be derived from an externally mounted linear scale encoder.  The external encoder provides actual load position data that the motion controller uses to calculate the required amount of correction needed to compensate for the non-linear mechanical deviation over distance.

If you’d like to know more about the benefits of external position feedback, there’s a White Paper available called “Motion Control Primer: Direct load position sensing with secondary feedback encoders”.