Basic Motion Control - An Introduction to Stepper Motors

Basic Motion Control - An Introduction to Stepper Motors

Stepper motors are brushless, synchronous electric motors that converts digital pulses into mechanical rotation. Due to their lower cost, high reliability, high torque at low speeds, and rugged construction, they are found in both industrial and commercial applications.

The Basic Idea

Every revolution of a stepper motor is divided into a discrete number of steps. The motor is sent a pulse for each step. Typically a single rotation is 200 steps or 1.8 degrees of rotation per step. Since a stepper motor can only take one step at a time, and each step is the same size, the motor's position can be controlled without feedback. Obviously, as the pulse frequency increases the discrete step movement will change into continuous rotation - with the speed directly proportional to the pulse frequency.

Why Use a Stepper Motor?

1) Open-loop control makes the motor simpler and less costly to control.

2) The rotation angle of the motor is proportional to the input pulse.

3) Precise positioning and repeatability of movement.

4) Wide ranges are available since the speed is proportional to the frequency of the input pulses.

5) Quality stepper motors have an accuracy of 3 to 5% of a step and this error is non-cumulative step to step.

6) Excellent response to starting/stopping/reversing.

7) If the windings are energized the motor has full torque at standstill.When the load is directly coupled it is possible to achieve very low speed synchronous rotation.

8) High MTBF - since there are no contact brushes in the motor.

Types of Step Motors

There are three basic types: variable reluctance, permanent magnet, and hybrid. Hybrid motors combine the best characteristics of the other two types. They are constructed with toothed stator poles and a permanent magnet rotor. Standard hybrids have 200 rotor teeth and rotate at 1.8 degrees per step. Because they exhibit high static torque, high dynamic torque and run at very high step rates, they are used in a wide variety of applications including: computer disk drives, printers/plotters, machine tools, pick and place machines, automated wire cutting and wire bonding machines.

Modes of Operation

Operating modes include Full, Half and Microstep. The step mode output of any stepping motor is dependent on the design of the driver.

Full Step: Standard hybrid stepping motors have 200 rotor teeth, or 200 full steps per revolution of the motor shaft which equals 1.8 degrees per step. Normally, full step mode is achieved by energizing both windings while alternately reversing the current. One pulse from the driver is equivalent to one step.

Half Step: The step motor rotates at 400 steps per revolution. One then two windings are alternately energized, causing the rotor to rotate half the distance, or 0.9 degrees. Half step mode will produce a smoother rotation than full step but the tradeoff is less torque. (approx. a 30% reduction).

Microstep: Microstepping drives are capable of dividing one step into 256 'microsteps', providing 51,200 steps per revolution or 0.007degrees per step. Microstepping is typically used in applications that require accurate positioning and smooth motion over a wide range of speeds. Again, improved motion control is traded off against torque.

Series or Parallel Winding Connection

Stepper motor windings may be connected in series or parallel. Series connection provides greater torque at low speeds. Parallel connection lowers the inductance providing increased torque at faster speeds.

Driver Output Limiting

The available motor torque vs. speed depends on the driver output voltage. Since the driver output can be rated up to twenty times higher than the motor voltage, the drive should be current limited to the step motor rating.

Indexer Overview

The indexer, or controller, provides step and direction outputs to the driver. Most applications require that the indexer also manage acceleration, deceleration, steps per second and distance.

The indexer is capable of receiving high-level commands from a host and generating the necessary step and direction pulses to the driver. Communication to the indexer is usually via a RS-232 or RS485 port. However, it can also monitor inputs from external Go, Jog, Home and Limit switches.

Indexer Stand-Alone Operation

An indexer can also operate independent of the host. Once a program is loaded into the controller it can be initiated from remote operator HMI's or auxiliary I/O.

Closed loop applications that require stall detection and exact motor position capability will often be pre-packaged with a driver, power supply and optional encoder

Multi-Axis Motion Control

Multi-axis systems are used where a motion control application will have more than one stepper motor. A typical multi-axis networking hub may have up to four stepper drives connected to it; each drive will be connected to a separate stepper motor. The hub provides coordinated movement for applications requiring a high degree of synchronization (i.e. circular or linear interpolation).

Converting Rotational to Linear Motion Control

Where linear motion is required a lead screw/worm gear drive system can be connected to a rotary stepper motor. If the lead screw pitch is equal to one inch per revolution and there are 200 full steps per revolution the resolution of the lead screw system is 0.005 inches per step. Finer resolution is possible by using microstepping mode.