Stepper Motors Flashcards
What are Stepper Motors?
A simple and convenient ways of providing precise movement.
Stepper motors run at a speed proportional to step rate.
Open loop control - no feedback.
What are the methods used to increase resolution of a stepper motor?
Microstepping.
Toothed Rotors.
Hybrid stepper motors.
What are Hybrid Stepper Motors?
A combo of microstepping and toothed stators/ rotors.
They massively increase the resolution of the stepper motor.
Typical step angles 0.9 deg and 1.8 deg.
The teeth of the rotor are half a tooth out of step with the teeth on the stator.
What is a Biploar Driver?
Two H Bridges are connected to phase windings A and B.
Too reverse the polarity of the stator poles one must reverse the current.
Changing the position of the switches on the H Bridges can change the current.
What are the two methods of controlling the Stepper Motor?
Time per step.
Step per time.
Describe the Time per Step control method of a stepper motor.
Calculate the time until the next step is due.
Keep checking until that time has elapsed.
If the time has elapsed, make the step and recalculate the time interval and repeat.
Describe the Step per Time control method of a stepper motor.
Multiply the time since the last step by the current speed to get the desired distance moved.
Repeat until this distance exceeds one step.
Make the step, recalculate the speed and repeat.
What is Resonance in a Stepper Motor?
Resonance is a major problem with stepper motors.
The rotor is held at a given position by magnetic flux with finite stiffness.
When a torque is applied, the rotor can be displaced from the neutral position.
Essentially creates the rotational equivalent of a mass-spring system.
At resonance, the rotor oscillates instead of stepping neatly.
Resonance occurs when the step rate is the same as the natural frequency of the motor.
How can Resonance be prevented?
Microstepping.
Addition of frictional dampers.
What is a Darlington Pair?
Piece of circuitry used to amplify TTL signal when interfacing a solenoid to a computer.
Two transistors sit in piggy back and amplify the current.
A freewheeling diode must be included in the circuitry to stop the circuit burning out by stopping the back EMF punching a hole in the transistor when the circuit is switched off for the first time.
