Stepper motors and stepper-based linear actuators are often selected for open-loop motion control devices and equipment. These can be found in a wide range of products and systems such as laboratory equipment, medical devices, vision systems, analytical equipment, office products, aerospace, communications systems, semiconductor equipment, and light industrial equipment.
Two Basic Types of Stepper Drives
The two major types of drives for stepper motors and stepper-based linear actuators are the L/R drive and the chopper drive. Some of the criteria for choosing the drive type include:
Cost of the drive
Physical size and configuration of the drive
Available power source
Rated output current of the drive
Motion duty cycle
Total loading on the motor
Required speed range of the motor
The L/R Drive
Think of the L/R drive as a “constant voltage” drive. For continuous-duty motor operation in a room-temperature environment, you essentially match up the available power source voltage for the L/R drive to the rated coil voltage of the motor. Regarding the name L/R Drive, the L is the electrical symbol for inductance and the R is the electrical symbol for resistance. Since the stepper motor torque is proportional to ampere-turns, it is the current through the motor windings that determines the output performance at any speed including zero.
At standstill, the maximum “holding current” current through the windings is limited by the coil resistance. As the stepping rate (motor speed) increases, the coil inductance becomes a major current-limiting factor (limiting the rate of change of coil current) along with the back-emf. Back-emf is a generated voltage proportional to the speed that is produced within the motor windings during rotation, which works against the source voltage because every motor is also a generator.
The motors operated with an L/R drive will have a relatively limited performance range when compared to using a chopper drive. The source-voltage-to-motor-voltage ratio with the L/R drives is basically 1:1 whereas with chopper drives, it can be many multiples such as 2:1, 4:1, 8:1, or more (see Figure 3).
Some of the reasons for selecting an L/R drive instead of a chopper drive might be a lower cost of the drive, smaller physical size, a relatively slow motor speed range, use of a unipolar motor, or the limitations of using a battery power source. A good example of a product utilizing many of these previously listed reasons for using an L/R drive with a small stepper-based linear actuator is a handheld electronic pipette as shown in Figure 1.
Typically, L/R driver performance curves published by stepper motor and stepper-based linear actuator manufacturers were developed with the full rated motor voltage available at the motor’s lead wires at zero steps per second. If there are any voltage drops through the drive circuitry, then the DC power supply voltage would be set slightly higher to compensate for the total voltage loss in the drive.
The Chopper Drive
Think of this type as a “constant current” drive. For continuous-duty motor operation in a room-temperature environment, you set the output RMS (Root Mean Square) current of the chopper drive to the rated RMS coil current of the motor. Regarding the name chopper drive, this technique for maintaining the proper motor phase current levels throughout a usable speed range is to rapidly turn on and off (i.e., chopping) a relatively high source voltage via a proportional duty cycle while circuitry monitors the current levels in the motor windings. Chopper drives can be separate standalone units or integrated with the motor. For an example of a compact standalone chopper drive, see Figure 2.
If the application has a fairly short duty cycle (i.e., the full powered ON or Run times relative to the OFF or lower-current zero motion Hold times) in a moderate-temperature environment, then a higher magnitude of run current can be used to increase the motion performance of the motor; however, care must be taken when using this higher-than-rated run current. The current levels and ON times versus hold or OFF times, as well as the ambient temperature and any motor cooling methods (conduction, convection, etc.,) will determine the internal coil temperatures. It is recommended to consult the motor manufacturer if significantly high phase currents are necessary.
The additional circuitry within chopper drives senses the magnitude of the phase currents and to control the voltage chopping, may increase their price (compared to an L/R drive) but it can help to maintain a high level of motor torque or force throughout a relatively wider speed range. The power supply voltage to a chopper drive is typically much higher than the rated voltage of the motor. As discussed in the L/R drive section, the source-voltage-to-motor voltage ratio for a chopper drive is usually significantly higher than 1:1 and is typically 8:1 or even higher; therefore, the relative performance range can be greatly improved (see Figure 3).
The inductance of relatively low-voltage stepper motors and actuators is significantly less than their mechanically equivalent motors of higher rated coil voltages. For very good motor performance over a wider speed range, a low-voltage motor operated with a chopper drive at a relatively high source voltage is selected. The relatively low inductance and lower back-emf characteristics of a low-voltage motor in conjunction with a high source voltage chopper drive can provide excellent performance results. The major requirement with these low-voltage motor configurations is that the drive has to be capable of providing higher levels of phase current.
As a cautionary note, some chopper drive manufacturers advertise their product’s output phase current levels as a peak value; using larger values is typically a marketing tactic. The continuous-duty phase currents for stepper motors and stepper-based linear actuators are typically rated as RMS values (RMS = Peak × 0.707)