Bipolar Stepper Motors: How They Work and How to Control Them

A stepper motor is a brushless DC motor that rotates in discrete steps by changing the magnetic field in its stator coils. Unlike conventional DC motors, stepper motors don’t need position sensors or feedback loops for control. Instead, an external controller sends pulses to switch the current direction in the coils, moving the rotor to the desired position.

There are two main types of stepper motors: unipolar and bipolar. They differ in the number of wires and how their coils are connected.

A unipolar stepper motor has two windings per phase, each with a center tap that is connected to a common power supply. The current can flow in either direction through each half of the winding, creating four possible states for each phase. A unipolar stepper motor can be driven by a simple transistor circuit that switches the current on and off in each half of the winding.

A bipolar stepper motor has one winding per phase, without a center tap. The current must be reversed in the whole winding to change the polarity of the magnetic field. A bipolar stepper motor requires a more complex driver circuit that can reverse the current direction in each winding, usually using an H-bridge arrangement.

This article focuses on bipolar stepper motors, covering their advantages, disadvantages, basic operation, control modes, and interfacing with microcontrollers.

What is a Bipolar Stepper Motor?

A bipolar stepper motor is defined as a stepper motor with one winding per phase and no center tap. A typical bipolar stepper motor has four wires, corresponding to the two ends of each winding.

The advantage of a bipolar stepper motor is that it can produce more torque than a unipolar stepper motor of the same size because it uses the full winding rather than half of it. The disadvantage is that it requires a more complicated driver circuit that can reverse the current direction in each winding.

The following diagram shows the internal structure of a bipolar stepper motor:

The rotor consists of a permanent magnet with north (N) and south (S) poles, while the stator has four electromagnets (A, B, C, D) arranged in pairs (AB and CD). Each pair forms one phase of the motor.

When current flows through one of the windings, it creates a magnetic field that attracts or repels the rotor poles, depending on the polarity of the current. By switching the current direction in each winding in a specific sequence, the rotor can be made to rotate in steps.

How to Control a Bipolar Stepper Motor?

To control a bipolar stepper motor, we need to provide two signals for each phase: one to control the current direction (direction signal) and one to control the current magnitude (step signal). The direction signal determines whether the current flows from A to B or from B to A in phase AB, and from C to D or from D to C in phase CD. The step signal determines when to switch the current on or off in each winding.

There are different ways to generate these signals, depending on the desired speed, torque, resolution, and power consumption of the motor. These are called control modes, and they include:

• Full-step mode
• Half-step mode
• Micro-step mode

Full-Step Mode

In full-step mode, both windings of each phase are energized simultaneously, creating maximum torque. The rotor aligns with the stator poles’ magnetic field, making one full step per pulse.

The following table shows an example of a full-step mode sequence for clockwise rotation:

| Step | Phase AB | Phase CD | | —- | | 1 | + | – | | 2 | – | – | | 3 | – | + | | 4 | + | + |

The direction of the current is indicated by the sign (+ or -), and the absence of current is indicated by a blank. The step signal is assumed to be high when the current is present and low when the current is absent.

The following diagram illustrates the full-step mode sequence:

The advantage of the full-step mode is that it provides maximum torque and simple control. The disadvantage is that it has low resolution and can cause vibration and noise.

The resolution of a stepper motor is the smallest angle that it can rotate in one step. In full-step mode, the resolution is equal to the step angle of the motor, which is usually 1.8° or 0.9°. This means that a motor with a 1.8° step angle can rotate in 200 steps per revolution, while a motor with a 0.9° step angle can rotate in 400 steps per revolution.

To increase the resolution and smoothness of the motion, we can use half-step mode or micro-step mode.

Half-Step Mode

In half-step mode, only one winding of each phase is energized at a time, except for the intermediate steps, where both windings are energized. The rotor aligns itself halfway between the stator poles, resulting in half a step per pulse.

The following table shows an example of a half-step mode sequence for clockwise rotation:

| Step | Phase AB | Phase CD | | —- | | 1 | + | | | 2 | + | – | | 3 | | – | | 4 | – | – | | 5 | – | | | 6 | – | + | | 7 | | + | | 8 | + | + |

The advantage of the half-step mode is that it doubles the resolution and reduces the vibration and noise. The disadvantage is that it reduces the torque and increases the power consumption.

The resolution of a stepper motor in half-step mode is half of the step angle of the motor. For example, a motor with a 1.8° step angle can rotate in 400 steps per revolution, while a motor with a 0.9° step angle can rotate in 800 steps per revolution.

To further increase the resolution and smoothness of the motion, we can use micro-step mode.

Micro-Step Mode

In micro-step mode, both windings of each phase are energized with varying current levels, creating intermediate positions between the full and half steps. The rotor moves in small increments, resulting in a fraction of a step per pulse.

The following table shows an example of a micro-step mode sequence for clockwise rotation, with 8 micro-steps per full step:

| Step | Phase AB Current | Phase CD Current | | —- | | 1 | 100% | 0% | | 2 | 92% | -38% | | 3 | 71% | -71% | | 4 | 38% | -92% | | 5 | 0% | -100% | | 6 | -38% | -92% | | 7 | -71% | -71% | | 8 | -92% | -38% | | 9 | -100% | 0% | | 10 | -92% | 38% | | 11 | -71% | 71% | | 12 | -38% | 92% | | 13 | 0% | 100% | | 14 | 38% | 92% | | 15 | 71% | 71% | | 16 | 92% | 38% |

The following diagram illustrates the micro-step mode sequence:

The advantage of micro-step mode is that it increases the resolution and smoothness of the motion, as well as reduces the resonance and noise. The disadvantage is that it requires a more sophisticated driver circuit that can control the current levels in each winding and that it reduces the torque and accuracy.

The resolution of a stepper motor in micro-step mode depends on the number of micro-steps per full step. For example, a motor with a 1.8° step angle can rotate in up to 6400 steps per revolution, if the driver can provide 32 micro-steps per full step.

Conclusion

A bipolar stepper motor has one winding per phase and no center tap. It requires a driver circuit, typically using an H-bridge, to reverse current direction in each winding. These motors produce more torque than unipolar stepper motors of the same size but consume more power and have more complex wiring.

A bipolar stepper motor can be controlled in different modes, such as full-step, half-step, and micro-step, depending on the desired speed, torque, resolution, and smoothness of the motion. Each mode has its own advantages and disadvantages and requires a different sequence of signals to switch the current in each winding.

Bipolar stepper motors are widely used in applications that require precise positioning and speed control, such as printers, scanners, CNC machines, robots, and cameras. They are also suitable for applications that require high torque at low speeds, such as pumps, valves, and actuators.