Stepper Motors: Driver Electronics
Stepper Motors: Driver Electronics
Bipolar stepper motors
Driver electronics consists of a Control circuitry and two H-Bridges, one for each of the windings.
The Control circuitry converts input signals to appropriate command signals for the H-Bridges.
Main input signals are
- Direction - CW or CCW direction of rotation
- Step Frequency (Step Clock) - number of steps per second.
Total number of clock pulses determines new rotor position
Other input signals might be
- Enable/Disable – switch on / off power supply
- Half/Full step - switch between half step operation and full step operation
- Type of microstepping (1/4 … 1/32 microsteps)
The H-Bridges handle the power, driving the currents through motor windings.
Current flows in both directions, alternating.
Due to the motor inductance, current rises according to the L/R ratio of the winding.
Picture shows operation in full step mode at a rather low step frequency.
After a period of rise, the current reaches its full level.
Once the motion has stopped and next position arrived, current is I= U/R.
With increasing step clock, the time for a current block gets shorter.
As an effect, torque generation is decreasing substantially.
The drive mode is called Constant Voltage Supply.
However, current rise can be improved considerably by applying another drive mode
Current controlled chopper driver
For use of this mode, windings are designed for low resistance, typically some few ohms.
Due to that small resistance (and also inductance), the current rise is very steep.
It is constantly measured, using sensing resistors Rsense (about 0.5 ohm).
Once reaching a set upper level, voltage supply is being switched off.
After falling below of a set lower level, voltage supply is being switched on again.
This switching procedure, called PWM control, is made with a very high frequency, much higher than the step clock (usually 10…20kHz).
The current is controlled to keep a level Irms (rms root mean square), according to thermal motor design.
Applicational notes for Constant Current Chopper Driver:
|Driver circuitry||H-Bridges, one for each winding. RSense for each winding necessary.|
|Costs||Higher costs of driver electronics, but fully-integrated circuits available on the market|
|Performance||Motor can operate at much higher step clocks, compared to constant voltage supply drive mode.
Highest output power and motor utilization for a given motor size.
Performance is restricted by increase of motor losses (iron losses and eddy current losses increase at high speed).
Low effect of supply voltage deviations and temperature on motor performance.
Unregulated voltage supply can be used.
|Electromag interference EMI||Higher than for constant voltage, due to superimposed high PWM frequency.|
Integrated Circuits (IC)
Several manufacturers offer ICs to control and drive stepper motors.
For instance Allegro Microsystems, Texas Instruments, Infineon, ST Microelectronics.
The ICs are available in different housings, including those for SMD technology.
Some additional components are necessary, e.g. sense resistors RSense and capacitors.
Basically, the IC can contain
- Fully integrated stepper motor driver IC, including both H-Bridges as well as control circuitry or
- H-Bridge only (One or Dual H-Bridge)
An example for a fully integrated stepper motor driver IC is L6228 from ST Microelectronics.
It contains a complete current controlled chopper driver and it is available in different package versions.
Input signals are CLOCK, Direction (CW/CCW) and Stepping Mode (HALF/FULL).
Sense resistors to connect to SENSEA and SENSEB.
Motor windings to connect to OUT1A … OUT2B.
Motor-integrated driver circuitry
Stepper motor driver electronics can be assembled to the motor, providing a compact and complete drive system.
Picture shows a JE unipolar driver electronics mounted to stepper motor size UC.
Control features are implemented in a small microcontroller.
Several control interface options can be implemented: