Configurations and Features
There can be other configurations and features for stepper drives:
- Bipolar Drives. For operating 4- or 8-lead bipolar stepper motors and stepper-based linear actuators.
- Unipolar Drives. For operating 6- or 8-lead unipolar steppers and actuators (typically L/R type drives).
- Non-programmable Drives. Require 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 Drives. These drives incorporate a microprocessor and can execute various motion control programs in addition to immediately executable motor commands. These drives 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.
- 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 tradeoff for these benefits is a reduction in motor step accuracy and repeatability, especially under loaded conditions.
- Encoder Input. There are many applications that may require a method of speed verification and/or positional verification such as certain medical devices, gas or liquid 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/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 accel/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. See Figure 4 for an example of a linear actuator non-ramped performance curve.
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 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. Figure 5 is 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 is 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, controllability 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.
This article was contributed by Haydon Kerk Pittman, Harleysville, PA. For more information, visit here.