How can we control the speed of a stepper motor?
Stepper Motor Speed Control
Stepper motors are electromechanical devices that transform electrical energy into precise shaft rotation (or steps). This distinct function provides excellent repeatability, flexibility, accuracy, and torque, making stepper motors perfect for precise positioning applications such as telescopes, robotics, and antennas.
All of these benefits, however, will be seen only if stepper motors are suitably sized and speed control is implemented effectively. Efficient speed control methods allow the motor to travel more smoothly, reducing resonance and improving precision.
This article discusses how engineers can accomplish stepper motor speed control. It will also discuss the benefits and applicability of different speed control techniques. But, before we get started, it's important to grasp how does a stepper motor work.
How does a stepper motor work?
Like other electric motors, stepper motors include a moving element (called the rotor) and a fixed part (rotor). In contrast to conventional motors, the stator of a stepper motor is built with many teeth on which coils are wound. Furthermore, the rotor is a permanent magnet (or a variable reluctance iron core).
A magnetic field is formed by the current flowing in the coil simply by activating one (or more) of the stator phases with input pulses of energy. This magnetic field pulls the rotor one (or more) steps around to align with it, allowing engineers to obtain varying rotational angles.
The achieved rotation angle is related to the number of pulses and can be calculated as follows:
Where step angle is measured in (º/step)
When one pulse is applied to the input, the step angle is the angle at which the stepper motor shaft rotates. As a result, by increasing the number of pulses given to the input, engineers can accomplish larger rotation angles.
Stepper motor speed control methods
According to the equation below, the stepper motor's speed is proportional to the pulse rate:
The above equation explains how to adjust the speed of stepper motors by altering the input pulses. A higher input pulse rate (or frequency) causes the motor speed to increase correspondingly. Engineers accomplish this pulse variation by constructing a controller that generates pulses that are fed into a driver. This driver then regulates the amount of current flowing into the stepper motor, which in turn controls the motor's speed.
Here are various methods for controlling stepper motor speed.
Series resistance method
Because stepper motors rely on electric current in the stator coil to generate magnetic fields and move the motor, engineers can control the motor's speed by adjusting the electric current provided to the motor.
This type of speed control is accomplished by connecting a variable resistor (in series) to the motor windings. Increasing the resistance in the circuit causes a less electric current to flow in the motor coils, lowering the motor's speed.
Voltage regulation method
The voltage regulation method involves varying the voltage supply to the supply motor. To accomplish this job, several chips (or timers) have been designed. These chips function by converting the current signals supplied to the motor to a square wave. This signal's high time describes when the rotor revolves.
Stepper motor speed control, on the other hand, is not restricted to electrical approaches. Speed control can also be achieved by combining a gear assembly with a stepper motor.
Gearboxes and gear drives
Engineers can simply manage the available torque at the gearbox shaft to drive a load when gearboxes are used in combination with stepper motors, according to the equation below.
According to the sets of equations, the gearbox multiplies the torque at the stepper motor output by a factor proportional to the gear ratio and efficiency. The torque output at the gearbox shaft, on the other hand, has an opposite connection with the speed output at the gearbox shaft. Engineers can get higher speed output by reducing torque and vice versa.
Because of the high rigidity of the gearbox speed control mechanism, the motor is less susceptible to the effects of frictional loads and vibration.
Basics of Stepper Motors
Operation & Structure
A stepper motor, like the second hand of a clock, revolves with a fixed step angle. Because of the mechanical structure of the motor, highly accurate positioning may be achieved with open-loop control.
Accurate Positioning (Number of Steps)
The simple structure of stepper motors is achieved without the need for electrical components, such as an encoder within the motor, yet having full control of rotation and speed. As a result, stepper motors are extremely robust and reliable, with very few failures. When it comes to stopping accuracy, 0.05° (without cumulative pitch errors) is quite close. Stepper motors have a higher follow-up mechanism toward commands than servo motors because their placement is performed by open-loop control and is driven by a magnetized stator and magnetic rotor with small teeth. When stepper motors are stopped, there is no hunting. They are also excellent in low-rigidity belt drives.
Useful for Speed Control and Position Control
Stepper motors position according to the number of input pulses when pulses are fed to a driver via a pulse generator. The fundamental step angle of a 5-phase stepper motor is 0.72°, while a 2-phase stepper motor has a step angle of 1.8°. The stepper motor's rotational speed is defined by the pulse frequency (Hz) delivered to the driver, and the motor rotation can be freely changed by simply adjusting the number of input pulses or frequencies to the driver. Stepper motors are used not only for position control but also for speed control with good synchronization.
Stepper Motors Uses
Advantages and disadvantages of stepper motors
Stepper motors, despite being simple to control, do not handle rapid changes in load effectively. They are also susceptible to vibration and noise due to their design. These defects, however, are not deadly and can be solved with proper control.
While this article provides useful information regarding stepper motor speed control, engineers must consider several other factors when specifying stepper motors for a specific application. Engineers, for example, must still consider power consumption, stepper motor sizing, sealing, and step mode, among other things.