Servomotor performance from low-cost step-motors!

The theory of electric machines was not completed until the mid-1980s, and most schools still teach the decades-old phasor approach. In applying field-oriented or vector control, we achieve from step-motors performance comparable to more costly "brushless dc" permanent-magnet synchronous motors.

Vector-Controlled Motor-Drive Technical Report Contents

Ultimate dynamic step-motor control

Industrial-size step-motors are essentially permanent-magnet synchronous (PMS) motors with negligible variable reluctance and many poles. They are generally less expensive than PMS "servo" motors with fewer poles and a round rotor. Innovatia offers significant advances over microstepping: field-oriented (vector) step-motor control.

Field-oriented control eliminates the resonances and loss of dynamic control due to open-loop microstepping. The motor instead behaves like a dc brush motor, even when changing speed. Instead of pulsing the rotor from one equilibrium position (step) to the next, field-oriented control continually maintains correct drive at all rotor positions.

Motor-drive field-oriented control theory

Torque control with no position sensors

The rotor position is sensed from the motor terminal voltages. This eliminates the need for a shaft encoder or resolver. The motor controller provides rotor position to the path controller, which sends it a torque command.

At low speeds (typically < 50 rpm), the motor-induced voltage is too small to reliably sense position. The mode of control defaults to microstepping or external position sensing. At low speed, if accelerations are also low for fine positioning, microstepping can often move the motor to the end position with acceptable motion performance.

Versatile DSP-Based Step-Motor Drive Design 

The example motor drive design of the tech report is based on a single-board, ADSP 21XX-series DSP-based motor-drive that controls speed or torque. DSP programming provides functional versatility for customization to a wide range of applications. Performance enhancement includes motor-impedance compensation, waveform drive optimization, and motor-sensed ("sensorless") control.

Servo-motor performance from low-cost step-motors. Discrete PWM generator DSP version shown above.

Examle motor-drive design features:

	DSP-based vector control for drive of two-phase-winding step-motors.
	Motion performance approaches theoretical electric-machine capability.
	Sinusoidal drive to motor windings for low torque ripple.
	Motor-terminal voltages and currents sensed for maximum control.
	Motor-terminal zero-crossing voltage sensing aligns optional position encoder absolute phase.
	9-bit dual digital PWM generator driven by DSP for sinusoidal drive of windings.
	Motor power-supply voltage sensed for feedforward supply compensation.
	On-board 5V, +12V converted down from 75 V motor supply.
	Winding-terminal impedance phase compensation.
	Analog or serial-digital torque or speed command, with emergency stop input.
	General-purpose hardware customized by reprogramming for different applications.

Vector-Controlled Motor Drives Tech Report:

Ordering Information

For more information, or to discuss semicustom design of vector controlled motor-drives for your application, contact Innovatia.