Features of Stepper Motor Drivers

Bipolar stepper motor drivers

For operating 4 or 8 lead bipolar stepper motors and stepper-based linear actuators.

Unipolar stepper motor drivers

For operating 6 or 8 lead unipolar steppers and actuators (typically L/R type drives).

Non-programmable stepper drivers

Requiring digital “pulse” and “direction” inputs from a controller (and some include an “output enable” digital input). The controller outputs a single pulse to the drive for each motor step and outputs a stream of pulses to the drive for the motor to ‘Index’ a precise amount. The quantity of pulses to the drive determines the amount of rotational or linear movement and the frequency of the pulse train determines the rotational or linear speed of the stepper motor or linear actuator respectively.

Programmable stepper drivers

These drives incorporate a microprocessor and can execute various motion control programs in addition to immediately executable motor commands. These stepper motor drivers can have the motor ‘index’ virtually any amount in either direction and at various speeds in real time or under user-specified program control. most programmable drives have some general purpose (GP), digital Inputs/Outputs (I/O) for ‘talking’ with or controlling other equipment (and thus providing coordinated system motion control), and also have some conditional functions based upon the GP inputs, the relative motor position, and/or encoder feedback data.

Digital stepper motor driver

Half step mode in addition to standard full step mode

In the half step mode, the drive can electronically divide each full step of a stepper motor in half. For example, a stepper motor with a 15° full step rotation can operate at a 7.5° step angle with the drive in the half step mode. A 1.8° stepper motor can be run with 0.9° half step increments, and so forth. Similarly the linear resolution of a stepper-based linear actuator can be divided in half using the half step mode.

Micro-Stepping Modes

Micro-stepping drives can electronically divide each full step of a stepper motor or actuator into finer discrete step angles than half stepping. Typical division factors are 1/4, 1/8, 1/16, 1/32, etc, and/or 1/5, 1/10, 1/25, 1/50, etc. The four major benefits of micro-stepping the motor are increased rotary or linear resolution, smoother operation, reduced audible dynamic noise and a reduction of dynamic resonance. The trade-off for these benefits is a reduction in motor step accuracy and repeatability, especially under loaded conditions.

Encoder input

There are many applications which may require a method of speed verification and/or positional verification such as certain medical devices, gases or liquids flow regulation, communications equipment or microelectronics. To ‘close-the-loop’ of a stepper-based system an integrated motor-mounted or a load-attached rotary or linear encoder can essentially ‘tell’ this type of drive if the motor is successfully achieving the commanded step rates and/or has achieved the true commanded position for every move. An encoder can also recover the significant loss of motor step accuracy when using fine micro-stepping modes as described above.

Acceleration and deceleration ramping

To help get a relatively greater load moving and/or achieve higher motor step rates (possibly without having to change to a physically larger motor), the use of acce/decel ramping can often be implemented with many stepper drives. As shown in typical published (non-ramped) speed, versus torque or speed versus force performance curves for stepper motors or step-per- based actuators, the slower the motor speed the higher the output torque or output force respectively.

To benefit from ‘lower speed, higher force levels’ the rotary or linear move profiles can include an initial start, from standstill, at a relatively low base speed and then immediately begin ramping up to the desired ‘high velocity, and to then reverse this technique if a deceleration ramp is also required. Just as we have to accelerate heavy motor vehicles up to speed from a dead stop, stepper motors and actuators can usually get relatively large loads moving with the use of ramping. To continue with this analogy, it requires extra power (i.e. engine mechanical horsepower for a conventional vehicle or electrical power for a motor) to get moving up to speed and then, depending upon the type of loading, it may take significantly less power to maintain motion at a constant velocity. Refer to Figure 5 for an example of a possible performance benefit with ramping.

A phase current ‘boost’ feature

Some Chopper Drives offer an option to set a boosted phase current (higher magnitude than continuous rated current) during part of, or possibly all of, any acceleration and/or deceleration ramp. Typically the ‘active time’ of boosted current levels are of limited duration during a ramp to prevent overheating the motor windings. This boosted current during an acceleration ramp can increase the internal torque of the motor allowing the motor to get a relatively larger load moving from the rest position. Similarly a boosted current during a deceleration ramp can help to stop a relatively larger moving load.

In summary, there may be many factors to consider when designing a motion control device or system using stepper motors or stepper- based linear actuators. One of the critical components is the stepper drive and its selection is best determined by various factors such as the type, physical size, voltage and current ratings, available step modes, corntollability and programmability, ramping and/or current boosting options, as well as cost and delivery lead time. Depending upon the loading and duty cycle significantly improved performance from, or the increase in energy efficiency of a stepper motor or stepper- based actuator can often be achieved by the proper selection of the drive type (along with any optional features of the drive) and the power source.

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