An Overview of Stepper Motor Drivers and Their Types

A stepper motor is a brushless DC electric motor that turns in steps, with input in the form of programmed energization of the stator windings and output in the form of discrete angular rotation. The defining characteristic of a stepper is its shaft, which rotates by performing steps; that is, by moving a fixed number of degrees. Stepper motors can also divide a full rotation into an equal number of steps. If the motor is appropriately sized for the application, its position can be commanded accurately without any feedback mechanism. This is attributed to the stepper’s internal structure, where the exact angular position of the shaft is known by simply counting how many steps have been performed, with no need for a sensor. This feature also makes it fit for a wide range of applications, including medical devices, robotic arms, and aerospace and defense applications.

Stepper Motor Drivers Explained

A stepper motor driver provides the required voltage and current to a stepper motor so that the motor can perform the required operation. In order for the motor to rotate, its coils must be energized in a specific sequence to generate the magnetic field with which the rotor is going to align; this is accomplished by the driver. 

A few key elements comprise a stepper motor driver: a transistor bridge, a pre-driver, and an MCU. The transistor bridge physically controls the electrical connection of the motor coils, with one transistor bridge being needed for each motor phase. A pre-driver is a device that controls the activation of the transistors, providing the required voltage and current. In turn, the pre-driver is controlled by an MCU, or a microcontroller unit. The MCU is typically programmed by the motor user and generates specific signals for the pre-driver in order to obtain the desired motor behavior.

There are a few issues that are inherent in a normal drive circuit. These can be understood by considering each phase winding as an RL (Resistive Inductive) circuit, subject to repetitive switching. When a behavior step voltage is applied, the current rises exponentially as i(t) = I(1-e).

In practice, the electrical time constant (or t) limits the rise and fall of current in the winding. At a low stepping rate, the current rises to the rated value in each ON interval and falls to zero value in each OFF interval. However, as the switching rate increases, the current is unable to rise to the steady state, nor fall to zero value within the on/off time intervals set by the pulse waveform. This smooths the winding current, reducing the swing of the current waveform; as a result, the torque developed by the motor is reduced considerably. In fact, for higher frequencies, the motor simply vibrates or oscillates within one step of the current mechanical position.

 

Figure 1:Equivalent Circuit of Motor with Switch

What are Special Driver Circuits?

Special driver circuits are drivers that have been developed to overcome the problems described above, as well as improvements of the current build-up. Let’s explore these below:

• L/R (Inductance to Resistance) Drive. Also referred to as “constant voltage” drives, L/R stepper drives supply constant voltage to the motor winding. In the L/R drive, the initial slope of the current waveform is made higher by adding external resistance in each winding and applying a higher voltage proportionally. The current produced in the windings depends on the motor’s time constant, which is the relationship between its inductance and resistance (L/R). While this increases the rate of rise of the current, the maximum value remains unchanged. The circuit time constant is now reduced, and the motor is able to develop normal torque even at high frequencies. There are a few disadvantages associated with L/R drivers. One is the flow of current through external resistance, which causes I²R losses and heating; this denotes a wastage of power as far as the motor is concerned. For very high pulse rates, full current – and thus full-rated torque – may not be reached. This limits the use of L/R drives to primarily low-speed applications.

                 

Figure 2:Equivalent Circuit of L/R Driver                                   Figure 3: Effect of External Resistor

• Chopper/Constant Current Drive. In chopper drive, if a higher voltage of 5-10 times the rated value is applied to the phase winding, the current is allowed to rise very fast. As soon as the current reaches between 2-5% above the rated current, the voltage is cut off, allowing the current to decrease exponentially. When the current reaches 2-5% below the rated value, the voltage is applied again. This process is repeated 5-6 times within the ON period before the phase is switched off. During this period where the current oscillates above the rated value, a minor modification is to chop the applied voltage at a high frequency (around 1 kHz) at the desired duty cycle to obtain the average on-state current equal to the rated value. Chopper drivers are more advanced than L/R drivers, as they regulate the current flowing through the active coil to have better control over the torque produced – and thus improved control over the dynamic behavior of the whole system. These drives are also energy efficient and feature greatly reduced noise and increased speeds, making them particularly suitable for high-torque stepper motors. However, the circuit design and anti-interference requirements are relatively high, as are the performance requirements of the components.

      

Figure 4: Equivalent Circuit of Chopper Driver                              Figure 5: Current Chopping Waveform

Conclusion

The specific special circuit driver utilized in an application is based on the specific application requirements, as well as the torque and speed requirements. Example applications utilizing these drivers include actuators, railway tractions, and battery-operated electric cars. However, keep in mind that a chopper drive requires additional electronics to monitor current in the windings and to control voltage switching, thus allowing a stepper motor to produce higher torque at higher speeds than a traditional L/R drive.

Unsure which driver would be best suited for your application? Reach out to our team to discuss!