The Ultimate Guide to Stepper Motor Driver

Published:2024-02-21

Prof. David Reynolds stands as a luminary in the field of electrical engineering, renowned for his expertise in integrated circuits. Holding a distinguished position as a Professor of Electrical Engineering, Prof. Reynolds earned his acclaim through decades of research, teaching, and industry collaboration.

A motor driver is a crucial component that supplies the necessary voltage and current to a stepper motor, ensuring its smooth operation. Stepper motors, which operate in discrete steps, require a careful selection of power supply, microcontroller, and motor driver for their design. While microcontrollers can control the motor rotation, the design of the driver must consider voltage and current requirements. Typically, a single motor driver board can manage the necessary currents and voltages for a motor. The stepper motor is precisely controlled using a controller that synchronizes pulse signals with the help of a driver. This motor driver receives pulse signals from a microcontroller and translates them into the motion of the stepper motor.

 

 

What is a Stepper Motor Driver?

A stepper motor driver is tailored to continuously rotate a motor, like a stepper motor, with precise position control and without needing a feedback system. These drivers offer variable current control and support various step resolutions. They include fixed translators for easy motor control through step and direction inputs.

These drivers feature a range of integrated circuits (ICs) that operate at supply voltages below 20 V. ICs with low voltage and low saturation voltage are ideal for use in two-phase stepper motor drivers commonly found in portable devices like cameras and printers.

Stepper motor drivers are available in different voltage and current ratings, allowing for selection based on the motor's requirements. Most of these drivers are compact, typically measuring 0.6" x 0.8".

 

How does the Stepper Motor Work?

The driver circuit operates by controlling a stepper motor's movement through the sequential application of current in different phases. Wave driving, a technique where only one phase is used at a time, is not commonly employed due to its inefficiency and low torque output.

Key components for driving a stepper motor include controllers (such as microprocessors or microcontrollers), a driver IC, a power supply unit (PSU), as well as switches, potentiometers, heat sinks, and connecting wires.

 

Controller

The initial step involves choosing a microcontroller for driver design. For stepper motor applications, the selected microcontroller should have at least four output pins, as well as ADC, timers, and a serial port, depending on the driver's intended use.

 

Motor Driver

Motor driver ICs are cost-effective and simplify circuit design, reducing overall design time. Driver selection depends on motor ratings such as voltage and current. The ULN2003, a popular choice for non-H-Bridge applications, is suitable for driving stepper motors. This driver features a Darlington pair capable of handling currents up to 500mA and voltages up to 50VDC. The stepper motor driver circuit is depicted below.

 

 

Power Supply

The stepper motor operates within a voltage range of 5 volts to 12 volts, drawing a current supply ranging from 100 mA to 400 mA. The power supply design should align with the motor's specifications and be regulated to maintain consistent torque and speed, minimizing fluctuations.

 

Types of Stepper Motor

 

Permanent Magnet Stepper Motor

This stepper motor variant features a rotor with permanent magnets. The stator design can resemble that of a conventional 2- or 3-phase induction motor or be constructed like a stamped motor, with the latter being the more prevalent type.

PM stepper motors utilize rotors made of permanent magnets that interact with the stator's electromagnets, generating rotation and torque. These motors typically have lower power demands and can produce more torque per unit of input power.

 

Variable Reluctance Stepper Motor

This type of stepper motor features an electromagnetic stator paired with a rotor made of magnetically soft iron, which has teeth and slots similar to those found in the rotor of an inductor alternator. While PM motors are typically 2-phase machines, VR motors require a minimum of 3 phases. Most VR stepper motors have either 3 or 4 phases, although 5-phase VR motors are also available.

In a VR stepper motor, the magnetic field moves at a different rate than the rotor.

It's important to note that the Phase A coil has two south poles and no north poles for a flux return path. The flux will return through the path of least reluctance, which is through the pole pairs nearest to two rotor teeth. This path varies with the rotor's position. The flux induces a voltage in the coil wound on the pole, which in turn induces a current in the winding, slowing the rotor. The amount of current is determined by the voltage across the coil. A diode-clamped coil will have more current than a coil clamped with a resistor diode or zener diode.

VR stepper rotors do not contain permanent magnets; instead, they are made of plain iron and resemble a gear with protrusions or "teeth" around the circumference. This design provides VR steppers with a high degree of angular resolution, but this precision often comes at the expense of torque.

 

Hybrid stepper motor

This type of motor is often called a permanent magnet motor, utilizing a combination of permanent magnet and variable reluctance structures. Its construction closely resembles that of an induction motor.

 

The rotor consists of two end pieces (yokes) with salient poles, equally spaced but radially offset from each other by half a tooth pitch. A circular permanent magnet separates them. The yokes exhibit a uniform flux of opposite polarity. The stator is made from laminated steel. Some motors feature 4 coils arranged in two sets of 2 coils in series, with one coil pair labelled Phase A and the other Phase B.

The number of full steps per revolution can be calculated using the formula:

SPR = NR x Ø

Where:

SPR = number of steps per revolution

NR = total number of rotor teeth (combined for both yokes)

Ø = number of motor phases

or: NR = SPR/Ø

These motors are designed with multi-toothed stator poles and a permanent magnet rotor. Standard hybrid motors typically have 200 rotor teeth and operate at 1.8º step angles. Due to their high static and dynamic torque capabilities and ability to run at very high step rates, hybrid stepper motors find use in a wide array of commercial applications such as computer disk drives, printers/plotters, and CD players.

HS stepper rotors combine the key features of both PM and VR steppers. The rotor core in an HS motor is made of a permanent magnet, while the circumference is constructed from plain iron and features teeth. This design gives a hybrid synchronous motor high angular resolution and torque.

 

Stepper Motor Driving Techniques

There are four different driving techniques for a stepper motor:

  • In wave mode, the stepper motor energizes only one phase at a time. For clarity, we'll define the current direction as positive when flowing from the + lead to the - lead of a phase (e.g., from A+ to A-); otherwise, it's negative. Initially, the current flows solely in phase A in the positive direction, aligning the rotor (depicted as a magnet) with its magnetic field. Subsequently, it switches to phase B in the positive direction, causing the rotor to rotate 90° clockwise to align with phase B's magnetic field. Phase A is then energized again, but with current flowing in the negative direction, resulting in another 90° rotation. Finally, the current flows negatively in phase B, spinning the rotor another 90°.

 

  • In full-step mode, two phases are energized simultaneously. The steps in this mode, illustrated in the following figure, are akin to those in wave mode. However, the notable difference is that the full-step mode allows the motor to generate higher torque because more current flows through the motor, resulting in a stronger magnetic field.

 

  • The half-step mode combines aspects of both wave and full-step modes. This hybrid approach reduces the step size by half (in this case, to 45° instead of 90°). However, the trade-off is that the motor's torque output is not constant. The torque is higher when both phases are energized but weaker when only one phase is energized.

 

  • Microstepping represents a further development of the half-step mode, allowing for a finer reduction in step size while maintaining a consistent torque output. This is achieved by controlling the current intensity in each phase. However, implementing microstepping requires a more complex motor driver than previous methods. Figure 14 illustrates the operation of microstepping. Assuming IMAX is the maximum current that can flow in a phase, in the initial step, IA = IMAX and IB = 0. In the next step, the currents are adjusted to achieve IA = 0.92 x IMAX and IB = 0.38 x IMAX, resulting in a 22.5° clockwise rotation of the magnetic field compared to the previous position. This process is repeated with different current values to reach the 45°, 67.5°, and 90° positions, allowing for a halving of the step size compared to half-step mode. It is possible to achieve even smaller steps. While microstepping offers high position resolution, it comes at the cost of a more complex motor control device and reduced torque output per step. Torque is proportional to the sine of the angle between the stator and rotor magnetic fields; thus, smaller steps result in lower torque. This can sometimes lead to missed steps, where the rotor position remains unchanged despite the current flowing in the stator winding.

 

Advantages and Disadvantages

 

Advantages

  • Suitable for battery operation
  • Robust design
  • Spark protection
  • Thermal protection
  • Compact mounting space requirements
  • This motor driver is designed for driving Unipolar Stepper Motors, eliminating the need for expensive driver boards.
  • Stepper motors do not require position sensors due to their internal structure. They move in steps, and by counting these steps, the motor's position can be determined at any given time.
  • Stepper motor control is straightforward. While a driver is necessary, complex calculations or tuning are not required for proper operation. Generally, less control effort is needed compared to other motors. With microstepping, a high position accuracy of approximately 0.007° can be achieved.
  • Stepper motors provide good torque at low speeds, are ideal for maintaining position, and typically have a long lifespan.

 

Disadvantages

  • The design of this driver is inefficient.
  • Requires extensive wiring for small-scale applications.
  • Susceptible to missing steps under high load torque, leading to loss of control as the actual motor position is unknown. Microstepping exacerbates this issue.
  • Stepper motors always draw maximum current even when stationary, reducing efficiency and potentially causing overheating.
  • Stepper motors exhibit low torque and become noisy at high speeds.
  • Stepper motors have low power density and a low torque-to-inertia ratio.

 

Applications

Stepper motors find extensive applications across various industries and fields, including:

  • Industrial sector
  • Brush DC/Stepper motors
  • Computing
  • Robotics
  • Cameras
  • Printing and scanning, including in 3D printers
  • Process automation and packaging machinery
  • Positioning of valve pilot stages for fluid control systems
  • Precision positioning equipment

 

 

 

How to Choose the Right Stepper Motor Driver?

 

Step One: Identify the Drive Mechanism Component

Begin by determining the mechanism and its specifications. Define key design features such as the mechanism type, approximate dimensions, distances to be traversed, and required positioning frequency.

 

Step Two: Determine the Required Resolution

Calculate the motor's resolution needs. Based on the required resolution, decide whether a standard motor or a geared motor is suitable. The use of microstepping technology can greatly simplify meeting resolution requirements.

 

Step Three: Define the Operating Pattern

Establish an operating pattern that meets the specified requirements. Determine the acceleration (and deceleration) period and the operating pulse speed to calculate the required acceleration torque.

 

Step Four: Calculate the Required Torque

Compute the load torque and acceleration torque to determine the total torque required by the motor.

 

Step Five: Select the Motor

Preliminarily select a motor based on the required torque. Use the speed-torque characteristics to finalize the motor selection.

 

Step Six: Verify the Selected Motor

Confirm that the motor meets the required acceleration/deceleration rate and inertia ratio.

 

Popular Stepper Motor Drivers: TMC2208, A4988, DRV8825, TB6600, TMC2130 and L298N.

 

Conclusion

Stepper motors are highly versatile, reliable, cost-effective, and accurate for controlling precise motor movements. They enhance the dexterity and efficiency of programmed movements in various applications and industries, making them an essential subset of automation and control gear.

Given the multitude of stepper motor brands, sizes, torque ratings, design styles, and intended applications available globally, it is crucial to determine the ideal configuration for specific user environments when considering a purchase.

 

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FAQ

  • What is the use of a stepper motor driver?

    Stepper motor drivers translate pulse signals from the controller into precise motor movements for accurate positioning.

  • Can you run a stepper motor without a driver?

    Yes, this can be achieved by manually controlling the current and timing of the signals sent to the motor coils.

  • What is an easy driver for a stepper motor?

    The EasyDriver is an easy-to-use stepper motor driver that is compatible with any device capable of outputting a digital 0 to 5V pulse (or 0 to 3.3V pulse if you solder SJ2 closed on the EasyDriver).

  • Is a stepper motor AC/DC?

    The stepper motor is an AC motor.

  • What is the difference between a stepper motor driver and a DC motor driver?

    Stepper motors are known for their high precision, operating in distinct, reliable steps, while DC motors are controlled by varying voltage and current, converting electrical energy into mechanical motion. While stepper motors offer high accuracy, they are generally less efficient than DC motors.

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