Stepper Motor Speed Control
Stepper motor speed control: An overview
Stepper motors are electromechanical devices that convert electrical energy into shaft rotation at precise angles (or steps). This unique operation offers high repeatability, flexibility, accuracy, and torque, making stepper motors ideal in applications that require precise positioning like telescopes, robotics, and antennas.
However, all these advantages will only be felt if stepper motors are correctly sized, and speed control is implemented accurately. Efficient speed control methods allow the motor to move more smoothly, decreasing resonance and ensuring higher accuracy.
This article presents methods by which engineers can achieve stepper motor speed control. It will also present some of the advantages and suitability of these speed control methods. But before delving right into it, it’s essential to understand how a stepper motor works.
How does a stepper motor work?
As with all electric motors, stepper motors feature a moving part (called the rotor) and a stationary part (rotor). However, unlike the typical motors, the stator of a stepper motor is designed to have several teeth on which coils are wound. In addition, the rotor is a permanent magnet (or a variable reluctance iron core).
By simply energizing one (or more) of the stator phases using input pulses of electricity, a magnetic field is generated by the current flowing in the coil. This magnetic field pulls the rotor around by one (or more) steps to align with this field, allowing engineers to achieve different rotational angles.
The achieved rotation angle depends on the number of pulses and can be calculated using:
Step angle is measured in (º/step)
The step angle is the angle that the stepper motor shaft rotates when one pulse is applied to the input. Therefore, engineers can achieve greater rotation angles by increasing the number of pulses applied to the input.
Stepper motor speed control methods
The speed of the stepper motor is proportional to the pulse rate according to the equation below:
Stepper motor speed is measured in revolutions per minute (rpm)
Pulse rate is measured in Hz
Step angle is measured in (º/step)
The equation above shows that the speed of stepper motors can be controlled by varying the input pulses. A higher input pulse rate (or higher pulse frequency) causes the speed of the motor to increase proportionally. Engineers achieve this pulse variation by implementing a controller that generates pulses input to a driver. This driver then controls the amount of current flow into the stepper motor, which in turn controls the speed of the motor.
Here are some methods of achieving stepper motor speed controls.
#1 Series resistance method
Because stepper motors rely on electric current in the stator coil to create magnetic fields and move the motor, engineers can regulate the speed of these motors by regulating the electric current supplied to the motor.
This speed control method can be achieved by incorporating a variable resistor (in series connection) with the motor windings. Increasing the resistance in the circuit causes a lesser amount of electric current to flow in the motor coils, thereby reducing the speed of the motor.
#2 Voltage regulation method
The voltage regulation method involves varying the voltage supply to the supply motor. Several chips (or timers) have been developed to achieve this function. And these chips work by changing the current signals supplied to the motor into a square wave. The high time of this signal describes when the rotor rotates.
However, stepper motor speed control is not limited to electrical methods. Engineers can also achieve speed control using a combination of gear assembly with the stepper motor.
#3 Gearboxes and gear drives
When gearboxes are used in combination with stepper motors, engineers can easily control the available torque at the gearbox shaft to drive a load, according to the equation below
To = torque output at the gearbox shaft
Tm = torque output at the stepper motor shaft
n = gearbox efficiency
No = speed output at the gearbox shaft
Nm = speed output at the stepper motor shaft
From the sets of equation, one can tell that the gearbox multiplies the torque at the stepper motor output by an amount proportional to the gear ratio and efficiency. However, the torque output at the gearbox shaft shares an inverse relationship with the speed output at the gearbox shaft. Therefore, by reducing the torque, engineers can achieve greater speed output and vice versa.
The gearbox speed control method offers high rigidity, making the motor less prone to the effects of frictional loads and vibration.
While this article presents helpful information about stepper motor speed control, there are several other things that engineers must consider when specifying stepper motors for a particular application. For example, engineers still have to determine power consumption, stepper motor sizing, sealing, and step mode, among other things.