A stepped motor which could convert an electric pulse signal into an angular displacement or linear displacement and its driver circuit diagram.
A stepping motor is an open-loop control element stepping motor that converts an electrical pulse signal into an angular displacement or a linear displacement. In the case of non-overload, the speed and stop position of the motor only depend on the frequency of the pulse signal and the number of pulses, and are not affected by the load changes. When the stepper driver receives a pulse signal, it drives the stepper motor to rotate a fixed angle in the set direction, called “step angle”, and its rotation is performed step by step at a fixed angle.
It is one of the most commonly used control electronic components in industrial control and instrumentation. It has the characteristics that the input pulse is proportional to the motor shaft. It is widely used in intelligent robots, floppy disk drives and data machine tools. The most suitable system for microcomputer-controlled stepper motors is the 20BY-0 stepper motor. It uses a +5V DC power supply with a step angle of 18 degrees and the motor coil consists of four phases.
Introduction
Stepper motors are brushless DC electric motors that move in discrete angular steps in response to digital pulse signals. They are precisely controllable and can be used in open loop position control systems without the need for feedback sensors.
Stepper motors are used extensively in industrial automation, robotics, 3D printers, CNC machines, and many other applications requiring precise positioning and speed control. They provide excellent low speed torque and response to digital inputs.
This comprehensive guide covers the operating principle of stepper motors, drive methods, common types and configurations, typical applications, and drive circuit schematic implementations.
How Stepper Motors Work
Stepper motors consist of a rotor containing permanent magnets and a stator containing electromagnets (windings). Applying current pulses to the windings generates magnetic fields which interact with the rotor magnets, causing rotational movement one step at a time.
Stepper Motor Rotation (Image Credit: Bakon/Wikimedia)
Some key operating characteristics of stepper motors:
- Motion occurs in discrete steps measured in degrees (Most common is 1.8°/step or 0.9°/step)
- Accurate positioning and repeatability – errors less than 5% of per step angle
- Open loop control – No feedback sensors required
- Driven by digital pulse trains from controllers
- Torque optimized for low speed high precision applications
- High holding torque when not moving resists external forces
- Can be miniaturized down to tiny step angles enabling precision micro-positioning
Understanding the stepping sequence and drive methods allows proper control of stepper motors.
Drive Methods
Stepper motors are commutated by energizing the phase windings in a sequential pattern to rotate the shaft. Two primary drive methods are used:
Wave Drive (1 Phase ON)
Only one winding is energized at any time. Rotor magnet aligns with magnetic field produced by active winding before next step occurs.
Provides greater torque at lower speeds. Resonances can occur at higher speeds.
Wave Drive Sequence (Image Credit: Festo Didactic)
Full Step Drive (2 Phase ON)
Two windings are energized simultaneously to produce stronger torque. Minimizes resonances.
Cannot achieve very low speeds. Provides higher torque across operating range.
Full Step Drive Sequence (Image Credit: Festo Didactic)
Proper selection of drive mode depends on required speed and torque. Controllers allow selecting different drive modes.
Stepper Motor Types
Stepper motors are classified based on their internal construction:
Variable Reluctance
Contain salient pole stator with concentrated windings. Multi-toothed rotor provides varying magnetic reluctance as it rotates. Simple and inexpensive construction.
Permanent Magnet
Have magnetized rotor with defined poles. Stator has distributed windings. Provides good torque and low resonance issues. Most common type.
Hybrid
Combine permanent magnet rotor with salient pole stator. Provides high torque, low resonance, good speed range. Most advanced and powerful type.
The number of stator teeth determine the step angle. More teeth provide finer resolution but require more switching.
Stepper Motor Configurations
Beyond the internal construction, stepper motors are also classified by their physical size, shaft type, and operating voltages:
Size and Frame
- Very small steppers < 28 mm used in precision instruments. 0.9° step angle.
- Small steppers between NEMA 8 – 17 sizes used in automation. 1.8° step angle.
- Large NEMA 23-34 frame steppers used in high torque applications. 1.8° step angle.
- Pancake or slim profile steppers optimized for tight spaces.
Shaft Type
- Standard round shaft – most common
- D-shaped shafts prevent slipping of couplings
- Geared – have integrated gearheads to increase torque
- Hollow shaft – for applications like driving lead screws
Voltage Rating
- Low voltage steppers – 12 or 24V types common. Higher currents.
- Medium voltage – 36 or 48V rated. Used in automation.
- High voltage – for precision or high power steppers. Up to 300V.
Combine winding configuration, drive mode, frame size, shaft and voltage rating to select optimal stepper.
Stepper Motor Performance Specs
Key specifications to evaluate stepper motor performance:
- Steps per Revolution – Typically range from 24 to 400 steps/revolution. More steps enables greater positioning precision.
- Step Angle – Angle moved per step. Common values are 0.9°, 1.8°, 7.5° and 15°. Smaller step angle provides higher resolution.
- Torque – Rated holding and pull-out torque values. Higher torque allows faster accelerations under load.
- Voltage – Rated voltage and input current determines power input needs.
- Inductance – Higher is better for drivability at higher speeds but reduces torque. Values from 1mH to 50mH typical.
- Resistance – Lower resistance enables higher torque output. Range is typically 1-5Ω per phase.
- Inertia – Lower rotor inertia allows faster acceleration and response.
- Accuracy – Deviation from intended step angle measured in arcminutes. Sub-arcminute accuracy is achievable.
Review rated specifications against the application’s resolution, torque, speed, and accuracy requirements when selecting an optimal stepper motor.
Stepper Motor Drives
Stepper motors require properly designed drive circuits to energize their phases in coordination with input step and direction signals. Here are the key functions stepper motor drivers perform:
Power Amplification
Increase current and voltage levels of control signals to adequately drive motor windings. Typically chopper or linear drives used.
Stepping Sequence Generation
Translate step and direction inputs into a sequence for energizing the phase windings. Microstepping capability is desirable.
Current Control
Vary winding currents to optimize torque output. Lower current at high speeds to overcome inductive impedance.
Protection Circuits
Detect error conditions like missed steps or overtemperature and respond appropriately to prevent damage.
Feedback Interfaces
Provide connections to external encoders or sensors when closed loop operation is needed.
Command Inputs
Step and direction signal inputs dictate motor motion. Additional control inputs like enables may be included.
Properly designed drives ensure smooth and reliable stepper motor operation and protect against malfunctions.
Basic Unipolar Driver
Here is a basic unipolar driver for a variable reluctance stepper:
- ULN2003 Darlington transistor array provides 500mA current rating per channel. 8 channels total.
- Diodes protect against back EMF when winding currents are switched.
- Resistors limit current through windings. Can alter values to change maximum current levels.
- Inputs must be energized in proper sequence to step motor.
- Only one winding active at any time. Limited torque capability.
Low component count makes this a simple and inexpensive drive choice but performance is limited.
Basic Bipolar Driver
A basic bipolar driver energizes two windings simultaneously:
- Dual H-Bridge (L298N) allows 4 quadrant operation of windings.
- Allows current in both directions through windings enabling bipolar drive.
- Higher performance but increased component count over unipolar design.
- Provides up to 2A per winding. Higher than unipolar drives.
- Fast switching time < 100ns.
Bipolar drive with H-bridges provides good performance for small to mid-sized steppers.
Microstep Driver
Microstepping divides steps into smaller increments. Typical microstep resolutions are 1/8, 1/16, 1/32, 1/64 step:
- Sinusoidal microstepping currents smooth motor operation and provide position interpolation.
- Microstep table maps control input state to sine wave DAC value.
- On-chip DAC and PWM generate sinusoidal current profiles in windings.
- Chopper circuits allow microstepping and current control capability.
- External potentiometer sets maximum current level.
Microstepping allows very fine resolution motion and smoothing with stepper motors.
Integrated Stepper Controller
Integrated stepper controllers contain drive electronics, sequencing logic, and microcontroller:
- Microcontroller generates control signals and sequences drive waveform based on incoming step and direction inputs.
- Integrated motor drivers with current control. May include thermal shutdown.
- Onboard supply generates motor voltages from main DC input.
- Status indicators and I/O allow monitoring and interfacing.
- Parameters and drive modes configurable through communication interfaces.
All-in-one integrated controllers simplify implementation with advanced features and protection.
Stepper Motor Applications
Stepper motors provide many advantages for precise positioning in automation, robotics, instruments, and other applications:
- CNC Machines – For precisely controlling cutting tools and workpiece position.
- 3D Printers – High precision positioning of print heads over multiple axes.
- Scanners & Plotters – Provide linear motion and positioning for optics.
- Robots – Allow precise joint control and position feedbackless operation.
- Pick-and-Place – X-Y positioning for PCB assembly and packaging machines.
- Laser Cutters – For accurate motion and focusing of optics.
- Textiles – Provide incremental stitching motion in sewing machines.
- Displays – Used in meter movements, gauge mechanisms, advertising displays.
- Valves – Precise control over fluid flow without complex feedback.
Stepper’s excellent response to digital inputs makes them ideal for computer controlled automation tasks requiring high precision.
Summary
- Stepper motors provide precise digital control of rotational position without feedback sensors.
- Operation based on magnetic field interactions between stator windings and rotor magnets.
- Driven by pulse train inputs that sequence the winding energization to step the motor.
- Major types include variable reluctance, permanent magnet, and hybrid steppers.
- Driver circuits amplify signals and provide sequencing and protection. Microstepping enables fine increments.
- Widely used in automation, robotics, CNC machines, 3D printers and other applications requiring high precision motion under digital control.
FAQ
What are the typical step angles for stepper motors?
Common step angles include 0.9°, 1.8°, 7.5°, 15°, and 30°. Smaller step angles allow higher precision but require more switching. 0.9° and 1.8° are most common for fine control.
What is the difference between unipolar and bipolar stepper motors?
Unipolar steppers only have current flowing in one direction through their coils, while bipolar motors reverse current direction providing higher torque. Bipolar motors require full H-bridge drivers.
Can stepper motors be used in closed loop systems?
Yes, stepper motors can incorporate rotary encoders or resolvers to provide position feedback. The controllers can use PID control in a servo loop to achieve even better precision and compensation.
Why are stepper motors operated at lower speeds?
The construction of stepper motors makes them best suited for operation below 1000 RPM. At higher speeds, resonances, switching delays, and inductance effects impair torque and control.
What causes lost steps in a stepper motor?
Exceeding torque limits, accelerating too quickly, too high a speed, or electrical noise can all cause missed steps and loss of synchronism. Proper drive design and torque margins prevent lost steps in most cases.
