1. Stepping motors
Jari Kostamo

2. Outline Introduction Stepping motor structures
Properties of stepping motor
Control of stepping motors

3. Stepping motor
Stepping motor is an electromechanical device which converts electrical pulses to discrete mechanical translation.
Axle of the stepping motor turns in discrete steps when the coils are pulsed with electric current in proper sequence
Direction of rotation depends on the sequence of the pulses
Angle of rotation is determined by the number of pulses
Rotation speed is determined by the frequency of the pulses

4. Stepping motor
Typically for small power applications (holding torque in a range of 1 – Ncm or power less than 750 W)
Low cost
Easy to interface with digital control with microcontrollers.
Precise control without feedback (step angle as low as 0,5o)
Can be used with feedback
Stepping rated possible up to 40 kHz (drive) or 3 kHz (pull in freq)
Dynamical properties are only moderate due to the operating principle
Can lose steps when loaded heavily
Operating range may contain problematic areas
Achieved position is not perfectly precise but the motor can be displaced with a small torque.
Operating is slightly shaking
Audible noise due to the stepping

5. Stepping motors
Typically 12, 24, 72, 100, 144, 180 or 200 steps per revolution (equal to 30, 15, 5, 3.6, 2.5, 2 or 1.8 degree step angle).
With microstepping even steps per revolution can be achieved.
Unipolar and bipolar motors.
Open control is used if losing steps does not cause critical problems.

6. Magnetic circuit
Translation in stepping motor is produced by magnetic forces
Magnetic circuit is generated with coils
Magnetic field strength depends on the nuber of ampere turns, current and geometry
Inductance of the coil creates an opposing voltage proportional to the rate of change in current.
Inductance has a tendency to limit the chages in current.
Larger voltages are needed if rapid changes are desired.
 Current control

7. Pulse width modulated control (PWM)
Efficient and modern way to control currents of the motor coils is PWM
PWM amplifier, chopper, switched mode amplifier...
Transistors are rapidly switched on and off
ON-state has low resistance – OFF state has high resistance
Yields to small losses
Current is defined by the proportion of the ON and OFF states
Can be implemented with uC
Dedicated circuits available.

8. Stepping motors
Permanent magnet motor
Reluctance motor
Hybrid motor

9. Variable reluctance stepping motor; VR motor
Kenjo, T., Stepping motors and their microprocessor controls

10. Permanent magnet stepping motor; PM motor
Kenjo, T., Stepping motors and their microprocessor controls

11. Hybrid stepping motor
Kenjo, T., Stepping motors and their microprocessor controls

12. Linear stepping motor Functional principle similar to hybrid motor,
Parker
Functional principle similar to hybrid motor,
2D motions possible
2D stage: Baldor

13. Stepping motor control example
Full stepping
(Kirjasta Bolton, Mechatronics) A1 A2 B1 B2
Half stepping A1 A2 B1 B2

14. Half stepping

15. (Book: Fraser & Milne, Integrated Electrical and Electronic Engineering)

16. Dynamical properties of stepping motors
Kenjo, T., Stepping motors and their micro-processor controls

17. Holding torque
Kenjo, T., Stepping motors and their micro-processor controls

18. Settling time
Kenjo, T., Stepping motors and their micro-processor controls
“If vibration at low speeds is an issue, consider the geared types. Not only does changing gears increase torque, but also these models improve starting and stopping responsiveness.”

19. Resonance frequencies
Lawrence & Mauch, Real-Time
Microcomputer System Design
Speed-torque graph

20. Stepping rate
Typical stepping profile

21. Control systems for stepping motors
Lawrence & Mauch, Real-Time
Microcomputer System Design
Direct computer control
Computer control
with stepping circuit
Computer control with
intelligent indexer

22. Digital linear actuator