Stepper Drives

Stepper Drives

Stepper systems (motor + drive) are open-loop systems which accept digital step & direction inputs provided by an "indexer" or "motion controller" which is basically a programmable pulse generator. Step & direction commands are typically generated from a PLC (programmable logic controller), signal generator, PC or programmable motion controller. The sequence of command pulses is "translated" into motion of the motor by the drive ("translator"). The result is a very cost effective all-digital Smart Motion System.

The function of the stepper drive is to sequentially regulate the current into the motor phase windings in order to produce the desired motion. The switching scheme used in a drive (full-step, half-step, mini-step or micro-step) in combination with the mechanical construction of the motor determines the system resolution in discrete motor steps per revolution (steps/rev). While heat considerations ultimately limit the maximum torque from a given motor/drive system, the torque generated at speed is largely a function of the drive's ability to overcome the inductance of the motor windings and push the maximum current into the phase windings as quickly as possible without over-heating. 

There are many different types of stepper drives designed to accomplish this task (L/R, unipolar, bipolar, PWM/chopper, recirculating chopper, etc.). When using a hybrid step motor, it is not recommended to use either a L/R or voltage drive. These drives provide constant voltage which create heat during operation and as a result will increase the motor's resistance. Any change to the motor's resistance will change the current supplied. Using a constant current drive, such as PWM/chopper drive, for all applications with hybrid motors is recommended. Note: when using tin-can (can-stack) motors, these limits do not apply.

  • Microstep Drivers: Step motors are commonly driven by microstepping drives. A Microstep Driver applies power to the appropriate step motor winding to produce torque. It precisely divides the current between the motor phases thus positioning the step motor at smaller increments between full steps. It provides higher resolution but with less torque. Microstepping does not increase step accuracy, but will allow a motor to run with less noise, minimize low speed resonance effects and produce smooth rotation over a wide speed range. Microstepping is commonly used to increase a motor's resolution. The degree of the improvement depends on the step accuracy of the motor.
  • Drivers with Oscillators: An oscillator is a device that is used to produce pulses for driving a step motor at a preset speed. In microstep drivers, the oscillator is usually incorporated into the hardware construction of the stepper drive. In oscillator mode, the pulse input becomes a run/stop signal: when this signal is set low, the motor accelerates to a preset speed and slews. Raising the input high (to 5 volts) causes the step driver to decelerate to rest. 
  • Full/Half Step Drivers: In full step operation, a hybrid stepper motor steps through the normal step angle (e.g. 200 step/revolution) causing the motor to rotate 1.8° per full step, while in half step operation the motor rotates 0.9° per full step. Full step mode is typically used in applications where torque and speed performances are less important, wherein the motor operates at a fixed speed and load conditions are well defined. Typically, stepper motors are used in full step mode as replacements in existing stepper motor systems, and not used in new applications. Half step mode generates steps that are half the normal step size. Therefore, this mode provides twice the resolution of full step mode. 

Step motor systems can be Open Loop or Closed Loop: 

  • Open Loop System - A system that does not use feedback to verify the desired result, or output, has been reached. Most step motor systems are operated open loop.
  • Closed Loop System - A system that uses feedback to verify the desired result, or output, has been reached. As an example, a feedback device such as an encoder is commonly used to provide position or velocity information to a motion controller. Closing the position loop with an encoder in an open loop step motor system benefits overall system performance since it provides stall detection, position verification and/or path correction.