Stepper motors (also called step motors) are electronic motors that offer precise rotation control. The size of the increment is measured in degrees and can vary depending on the application. Typical increments are 0.9 or 1.8 degrees, with 400 or 200 increments thus representing a full circle.
Stepper motors have been used in computer hard drive s, because they can be moved and positioned with precision. They have also been used in various robot ic devices and as antenna rotators.
Characteristics and Advantages of Stepper Motors
There are several characteristics of stepper motors which have made them the actuator of choice in a large number of applications:
- The device can be operated in an open-loop with a positioning accuracy of +-1 step. Thus to rotate in a certain angular distance, the motor can be commanded to rotate a certain number of steps and the mechanical element coupled to the shaft will move the required distance.
- Step motors exhibit high torque at small angular velocities. This is useful for accelerating a payload up to speed.
- Stepper motors have high holding torque -they have the property of being “self locking” when the rotor is stationary.
- Stepper motors are directly compatible with digital control techniques, and can be easily interfaced to a digital Step\Direction controller, a microprocessor, or a computer.
- Stepper motors exhibit excellent positioning accuracy, and even more important, errors are non-cumulative.
- Motor construction is simple and rugged. There are usually only two bearings, and the motor generally has a long maintenance-free life. For this reason, it is a cost-effective actuator.
Many of these advantages make the stepper motor useful in certain types of robots or machines. The price gap between Servo motors also gives stepper motors an advantage.
How does a Stepper Motor work?
DC brush motors rotate continuously when voltage is applied to their terminals. Stepper motors, on the other hand, effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by an external control circuit, such as a microcontroller. To make the motor shaft turn, first, one electromagnet is given power, which magnetically attracts the gear’s teeth. When the gear’s teeth are aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those rotations is called a “step”, with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle.
Types of Stepper Motors
There are two main kinds of stepper motors: Unipolar, and Bipolar. The difference between the two kinds is in the way the electromagnets are connected. The advantage of unipolar motors are their simplified control, but on the other hand, their torque (force) is less than that of bipolar motors.The advantage of bipolar motors is greater torque for the same size of motor, but on the other hand a more complex control circuit is needed, one that can change the direction of the current in every step.
There are hybrid motors which can work both in unipolar and bipolar modes, by using the motor’s wires differently.
Controlling a Stepper Motor
As opposed to regular DC motors, controlling stepper motors is much more complex. Here there are no two leads that need to be connected to a power source in order to spin the motor. In order to move the motor in a certain direction, a Stepping Sequence should be generated. A stepping sequence is a controlled switching of the motors coils. When a coil is on, current flows through the coils wire’s, and it becomes an electromagnet. Then the motor’s head is being magnetized to the coil, and a movement is created.
Building a Stepper Motor Controller Circuit
The easiest way to control a stepper motor is using a Step/Direction controller. This kind of controller gets only two inputs – the desired direction of the rotation (1=clockwise, 0=counterclockwise), and an indication of whether to step or stay in the current position(step=1, step=0, accordingly). The controller itself then generates the stepping sequences, as described in the previous section.
The controller uses powerful MOSFET transistors to switch the current to the coils.
The controller has 5 inputs and 6 outputs:
- 2 inputs for logic voltage source (5v, ground)
- 2 inputs for the motors voltage source (up to 50v 10ampere, ground)
- 2 inputs for controlling the step and direction
- 5 outputs for connecting the step motors