Linear motor control without the math

Pramod Ranade, SPJ Embedded Technologies

April 01, 2009

Pramod Ranade, SPJ Embedded TechnologiesApril 01, 2009

To run a stepper motor smoothly, it's necessary to apply linear acceleration. However, conventional algorithms to generate linear acceleration require complicated mathematics, which makes it difficult to implement the algorithms in a field-programmable gate array (FPGA). My colleagues and I, instead,

have used a new algorithm that requires only addition and subtraction, yet produces smooth linear acceleration. This algorithm was first implemented as a C program and tested. Then it was implemented in an FPGA for use in a real application.

The algorithm had its genesis in a project we were doing that required a dedicated controller for a six-axis machine. Each axis was driven by a stepper motor, with speeds up to 100,000 steps per second (with microstepping). For smooth and noiseless movements, stepper motors need to be accelerated (and decelerated) linearly. In other words, the motor speed should increase or decrease linearly, until the desired speed is reached.

Apart from the six axes, the machine also needed many more I/O. So in any case, we needed a good microcontroller. We then tried to look for reference designs for generating linear speed profiles for stepper motors.

We came across a very good article-- "Generate stepper-motor speed profiles in real time" by David Austin (Embedded Systems Programming, January 2005, www.embedded.com/56800129). It suggested a new algorithm that can run on a simple (even 8 bit) microcontroller to generate linear speed profile for stepper motors. However, it probably could not be used to run six motors at once.


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