John Chandler
Technosoft US Inc.
Canton, Mich.

Over the past decade, digital signal processor (DSP) technology has significantly reduced the physical complexity of industrial drives by replacing hardware with real-time embedded software. Today, DSP technology is also being harnessed to minimize, or even replace, motor mounted feedback devices, making motion systems more robust and even less expensive. Here we explore several feedback schemes for ac servomotors made possible by DSP-based drives.

Count the cost

Ac servomotors are the workhorses of the motion control industry. But tapping into their performance has traditionally come at the cost of multiple feedback devices and increased wiring. The added expense is necessary because controllers must know position, velocity, and electrical angle (theta) in real-time to fully harness the capabilities of modern servo machines.

One of the more common feedback devices used with ac servomotors is a commutating encoder. Three commutation sensors (S1, S2, and S3) embedded in the device measure rotor position to within 60 electrical degrees. This information is read during startup, giving the controller a coarse (angular) reference as it initially applies current.

As the motor begins to rotate, the relative position count from encoder channels A+B is captured at the commutation sensor edge. This captured value is arbitrary and is used as an offset to align the signal theta with the rotor's absolute electrical angle. Once aligned, theta is then driven incrementally from channels A+B, providing an exact measurement of electrical angle.

The A+B derived signal theta is normally used as an input to drive field transformations. Field transformations behave like ac-to-dc and dctoac conversions, making it possible to measure motor current and apply voltage relative to the rotor's position, or electrical angle. This transformation-approach is often called field-oriented control. One of the benefits of field-oriented control is that it allows position, velocity, and current to be dynamically controlled in a dc manor, independent of the rotor's electrical angle.

Room for improvement

Although commutating encoders are widely used, there are some drawbacks to this approach. Specifically, after starting the motor, the commutation signals are normally not used. If these signals are driven differentially, this means that six additional wires are present only for the purpose of starting the motor. The additional wiring adds cost and can reduce reliability, especially in a mechanism where feedback cabling is routinely flexed.

Another issue is that not all encoders are provided with commutation signals. For this reason, some ac servomotors are fitted with an incremental encoder plus magnetic Hall sensors to monitor rotor position. This alternate approach takes a toll, adding component count, cost, and volume to the motor.