Colleen Telling
Associate Editor

Elisabeth Eitel
Senior Editor

Manufacturing processes often march to the beat of short repetitive moves. These incremental or indexing cycles are prevalent on all sorts of production machinery, and they frequently hold the key to throughput and productivity.

Today, indexing speeds are determined largely by motors and drives and their ability to accelerate and decelerate quickly. Although many factors go into this, the main contributors are motor response, control techniques, and current generation. A little understanding here goes a long way toward increasing index speeds and making machines more valuable.

In many closed-loop motion control systems, acceleration is limited not so much by the motor, but by the way the motor is made to respond. And here, the one factor that s often overlooked is feedback resolution.

Too often, designers select encoder resolution on the basis of the positioning precision required, says Yves Villaret, Yet U.S. Inc., Manchester, N.H. If the resolution is too low, it will limit system cycle time. The answer, according to Villaret, is to select a higher resolution, which in turn, decreases settling time.

Rick Amendolea of Centricity Corp, Girard, Ohio echoes these thoughts. Relatively low resolution 4,096 post quadrature counts//rev can prevent systems from achieving a high response. Slight deviations in counts per update can generate errors from which the control loop must respond. These errors can be tuned out, however, using low-pass and notch filters.

Poor tuning improperly setting gains to axis mechanics can be a problem in and of itself, limiting motor control and response. To correct this, designers should connect motor feedback directly to the drive, ensuring quick velocity and current loop updates, says Rick Rey of Bosch Rexroth Corp., Hoffman Estates, Ill.

In applications with stepmotors, response times are optimized using both open and closed-loop control. With both types of control, a step-motor can run closed loop when it s about to lose steps, then revert to open loop once it regains control, says Nick Johantgen of Oriental Motor USA Corp., Torrance, Calif.

Steve Bartz of Emerson Control Techniques, Eden Prairie, Minn., describes a similar approach for servosystems. A servo drive employing state-space control with feedforward uses a position capture to mark the index s true beginning (in time and position). The control loop then uses this information to apply a gain to the algorithm, resulting in an accurate move.

Another source of limitation may be found in the drive controller. Drives have to process reference and feedback data, convert it to current in the motor, then deliver it in phase with shaft movements, explains Ed Lee of Powertec Industrial Motors Inc., Rock Hill, S.C. Response tends to suffer if these steps aren t executed quickly enough.

One way to counteract drive limitations is by improving control algorithms, modulation techniques, and current feedback precision. Increasing PWM and feedback resolutions strengthen response, as do advanced commutation methods such as sinusoidal and field oriented control (FOC), says Sergey Lototsky,, AC Tech Corp., Uxbridge, Mass.

John Chandler, Automotion Inc., Ann Arbor, Mich., adds this: The field alignment technique, whether trapezoidal full step, sinewave microstep, or FOC, is important to motor control. The goal in rapid indexing applications is to maximize acceleration, and from a motor control viewpoint, the challenge becomes how to maximize torque. The solution is to quickly drive current to a high level at an appropriate field angle.

Drive connection

Next in line when it comes to optimizing indexing speed is the motor drive. There are two main ways to drive synchronous motors. The first is setting the current phase by presetting a driver s output voltage to one of six possible states, explains Villaret. State selection depends on shaft angle and is sensed by pole-sensor switches. However, this trapezoidal method does not deliver precise phase control since the driving voltage changes step every 60°. Sinusoidal drives, a second option, employ a short cycle for the current control loop and continuously change the voltage output at each phase, keeping current at a desired amplitude and phase.