Tags: Electronic Circuit, Embedded, Motor Control, Stepper Motor

Stepper motors (also called step motors) are electronic motors that offer precise rotation control . Stepper motors are very precise (an average stepper motor can turn in about 0.9 to 1.8 degrees in each direction) and in a relatively high rotational speed.

Stepper motors can be found in various types of hardware components: printers (the printer head moves left and right by using a stepper motor), scanners, computer hard disks, and so on.

Demonstration of a Stepper Motor

In this video you can see a short demonstration of a stepper motor movement. Notice the fine control of the speed and direction — it is achieved by the motors ability to move in very small steps.

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:

1. 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.
2. Step motors exhibit high torque at small angular velocities. This is useful for accelerating a payload up to speed.
3. Stepper motors have high holding torque -they have the property of being “self locking” when the rotor is stationary.
4. 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.
5. Stepper motors exhibit excellent positioning accuracy, and even more important, errors are non-cumulative.
6. 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.

Disadvantages of Stepper Motors

The main disadvantage of stepper motors are the open-loop operation – the lack of feedback for the motor’s position (Position Feedback) and it’s speed (Velocity Feedback). This disadvantage has a critical effect on the ability to reach high precision and it reduces the general ‘safety’ of the system.

How is a Stepper Motor built?

Stepper motors have many coils placed in a circle shape. When current passes through one of the coils, it becomes magnetic (the electro-magnetic principle) and thus moves the motor shaft to it’s direction. An example animation:

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.

Stepping Sequences

There are 4 kinds of stepping sequences: Full Stepping (also called Single Stepping, Double Stepping, Half Stepping, and Micro Stepping.

Controlling a Stepper Motor using the “Single Step” stepping method

This is the most basic method – turning on a single electromagnet every time.

animation:

This sequence requires the least amount of energy and generates the smoothest movement.

Controlling a Step Motor using the “Double-Step” stepping method

In this method two coils are turned on simultaneously.

animation:

This method does not generate a smooth movement as the previous method, and it requires double the current, but as return it generates double the torque.

Controlling a Stepper Motor using the “Half-Step” stepping method

Animation:

This method doubles the basic precision (in degrees) the motor can move at. For example, is the motor can move in 1.8 degrees for each step, than using Half-Stepping it is possible to move the motor in 0.9 degrees/step. The disadvantage in this control method is that in half of the sequence, double the current is required (when two coils are on instead of one).

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

Electronic Circuit Schematic for Stepper Motor Controller Circuit

CAD Model

Here you can see a simple computer CAD model of the controller – it uses a simple micro-controller that generates the stepping sequence, and 4 powerful MOSTFET transistors.

Links

• An example of a CNC Machine Controller which uses the Step\Direction method for controlling 3 stepper motors
• Jones on Stepping Motors -Detailed information about step motors

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Posted in Robotics by Lior Chen 5 Comments