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
The 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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