What is a Bipolar Stepper Motor?
A stepper motor is a brushless DC motor that divides a full rotation into several equal steps. The motor’s position can then be directed to move and hold at one of these steps without any position sensor for feedback (an open-loop controller). Bipolar stepper motors are a type of stepper motor with a single winding per phase and no center tap (unlike a unipolar stepper motor).
The DC current in a winding needs to be reversed to reverse a magnetic pole and allow the motor to function. As there is no center tap in a Bipolar Stepper Motor, the driving circuit must be more complicated. Typically a H-bridge arrangement is used.
Stepper motors are available in three basic configurations:
- Universal Stepper Motor – A universal stepper motor can be connected as a bipolar or a unipolar stepper motor.
- Unipolar Stepper Motor – A unipolar stepper motor can be used as a unipolar or a bipolar stepper motor.
- Bipolar Stepper Motor – A bipolar stepper motor can only be used as a bipolar.
Bipolar Stepper Basics
A bipolar stepper motor has one winding per stator phase. A two phase bipolar stepper motor will have 4 leads. In a bipolar stepper, we don’t have a common lead like in a unipolar stepper motor.
Hence, there is no natural reversal of the current direction through the winding.
A bipolar stepper motor has easy wiring arrangement but its operation is little complex. In order to drive a bipolar stepper, we need a driver IC with an internal H bridge circuit.
This is because, to reverse the polarity of stator poles, the current needs to be reversed. This can only be done through a H bridge.
There are two other reasons to use an H Bridge IC
- The current drawn by a stepper motor is quite high. The micro-controller pin can only provide up to 15 mA at maximum. The stepper needs current which is around ten times this value. An external driver IC is capable of handling such high currents.
- Another reason why H Bridge is used is that the stator coils are nothing but an inductor. When coil current changes direction a spike is generated. A normal micro-controller pin cannot tolerate such high spikes without damaging itself. Hence to protect micro-controller pins, an H bridge is necessary.
The most common H Bridge IC used in most Bipolar stepper interfacing projects is L293D.
Interfacing to Micro-Controller
4 micro-controller pins are required to control the motor. We need to provide the L293D with a 5 V supply as well as the voltage at which the motor needs to operate. Since we will be using both the drivers of the IC, we will assert the enable pin for both of them.
Bipolar Motor Driver Circuit Interfacing Diagram
A bipolar motor circuit diagram is shown below:
There are three different ways in which we can drive the bipolar stepper motor:
- Only one of the phase winding is energized at a time. That is, either AB or CD is energized. Of course, the coils will be energized in such a way that we get correct polarity. But only one phase is energized. This type of stepping will give less holding torque because only one phase is energized.
- In this method, both the phases are activated at the same time. The rotor will align itself between two poles. This arrangement will give higher holding torque than the previous method.
- The third method is used for half stepping. This method is used generally to improve the stepping angle. Here, in step 1, only 1 phase is ON, then in step 2, 2 phases are ON, then again, only one phase is ON, and the sequence continues.
Bipolar Stepper Drives
Many companies have started assembling their own bipolar stepper drives. Care must be taken that you connect the stepper motor correctly to the drive.
Also, the drive must be able to supply sufficient current for your stepper. The micro-controller must only provide the step and direction signal to the drive.
This method will occupy only two micro-controller pins and help projects that require a large number of micro-controller pins for other functions.
Unipolar Stepper vs Bipolar Stepper
Both uni-polar and Bipolar steppers are used widely in projects. However, they have their own advantages and disadvantages from the application point of view.
The advantage of a unipolar motor is that we do not have to use a complex H bridge circuitry to control the stepper motor.
Only a simple driver like ULN2003A will do the task satisfactorily. But, there is one disadvantage of unipolar motors. The torque generated by them is quite less. This is because the current is flowing only through half the winding. Hence they are used in low torque applications.
On the other hand, bipolar stepper motors are a little complex to wire as we have to use a current reversing H bridge driver IC like an L293D.
But the advantage is that the current will flow through the full coil. The resulting torque generated by the motor is larger as compared to a unipolar motor.