Rotary encoders, also known as shaft encoders, are electromechanical devices used to convert the angular position or motion of a shaft into digital signals. They are used in a wide range of applications that require precise shaft unlimited rotation including industrial controls, robotics, consumer electronics, automotive, aviation, and medical equipment.
How rotary encoders work?
The working principle of rotary encoders is relatively simple. They consist of a rotating disk coupled with a shaft. The disk has patterns of opaque and transparent sections etched into it. There is an LED light source on one side of the disk and a light sensor on the other.
As the shaft and disk rotate, the patterns interrupt the light beam. The sensor detects these interruptions and converts them into digital pulses. The rate at which the pulses are generated corresponds to the speed of rotation. And the total number of pulses indicates the angular position.
Optical rotary encoders
Optical rotary encoders can be further divided into incremental and absolute types.
Incremental encoders:
These produce digital pulses to indicate motion and direction of rotation. But they cannot provide any information about the absolute angular position.
They use optical sensors to detect rotation andconvert movement into digital pulses. Common sensor arrangements are:
- Transmissive sensors – Have LED source on one side and phototransistor on the other. Interruption in light beam causes change in transistor output.
- Reflective sensors – Have LED source and phototransistor on same side. Light is reflected off a coded pattern on the disk.Interruption in reflected light triggers phototransistor.
- Quadrature encoders – Use two optical sensors with outputs offset by 90°. This enables incremental counting as well as detecting direction of rotation.
Absolute encoders:
These encoders provide a unique digital code for each angular position over a single revolution. This allows determining the absolute position of shaft at any given point.
Some common types of absolute encoders are:
- Gray Code encoders – Use gray code pattern on disk. Each position has a unique binary code.
- Multi-turn absolute encoders – Use gears to track number of revolutions. Codes position over multiple revolutions.
Magnetic rotary encoders
Instead of optical sensors, these encoders use Hall Effect sensors to detect magnetic pole positions on a diametrically magnetized rotating magnet. Rotation of magnet generates a digital pulse.
Key specifications of rotary encoders
Some key specifications and parameters to consider while selecting a rotary encoder are:
- Resolution – Pulses generated per revolution of shaft. Higher resolution provides more precision.
- Accuracy – Deviation from specified resolution. Depends on manufacturing quality.
- Direction sensing – Ability to detect rotational direction. Requires quadrature encoder.
- Voltage output – Voltage levels of the digital output signals. Common levels are 5V, 12V, 24V.
- Output interface – Digital interfaces like TTL, CMOS, RS422, Open collector.
- Index pulse – Additional pulse to indicate complete revolution. Used for synchronization.
- Operating temperature – Temperature range over which encoder can operate.
- Ingress protection – IP rating for dust and water resistance.
- Shock and vibration – Ability to withstand vibration, shock and impacts.
Main applications and uses of rotary encoders
Some of the most common applications that use rotary encoders are:
1. Motors and motion control
Rotary encoders are extensively used in various types of electric motors and motion control systems that need to track motor shaft position and speed. Some examples:
- Industrial motors – Encoder provides speed control feedback and position data for accurate motor control.
- Robotics – Encoders measure joint rotation to determine robot arm/gripper position.
- CNC machines – High resolution encoders provide precision position data for milling heads.
- Elevators – Encoders track motor rotation to control elevator car position and speed.
- Conveyor systems – Used to monitor conveyor belt motion and speed.
2. Automotive systems
Encoders play a vital role in automotive systems such as:
- Transmission – Measures gear shaft position for automatic gear shifting.
- ABS – Monitors wheel speed for anti-lock braking.
- Throttle – Tracks pedal position to control fuel injection and engine speed.
- Steering – Assists electric power steering by sensing steering shaft position.
- Odometer – Calculates distance travelled based on speedometercable rotation.
3. Consumer electronics
Rotary encoders are ubiquitous in many consumer electronic devices. For example:
- Digital cameras – Used in dials to provide input for aperture, shutter speed settings.
- Cell phones – Tracks scrolling and selection in volume, menu dials.
- MP3 players – Used in jog dials for music navigation and selection.
- Drones – Encoder feedback enables stable flight control and position hold.
4. Test and measurement equipment
High resolution encoders are indispensable for precision position sensing in:
- Laser scanners – Precisely tracks angular position of scanning mirror.
- Magnetic scanners – Determines angle of rotating coil in MRI machines.
- Spectrometers – Measures diffraction grating angle during wavelength scanning.
- Telescopes – Pointing and tracking of telescopes depend on encoder feedback.
5. Others
Other common applications include:
- Printers – Tracks paper feed roller position.
- Photocopiers – Monitors motor speed and scan belt motion.
- Fax machines – Counts roller motion for precision paper feeding.
- Angle grinders – Controls and limits rotation speed.
How to choose the right rotary encoder?
Factors to consider when selecting an appropriate rotary encoder for an application:
1. Operating environment
Consider temperature, humidity, vibration, shocks, water exposure the encoder will experience. Get suitable IP rating and ruggedized encoders if required.
2. Resolution and accuracy
Determine the smallest rotational increment that needs to be detected and the precision required. This decides the resolution.
3. Electrical outputs
Select suitable voltage levels, interface types (TTL, CMOS etc.) and connector styles as required by the interfacing circuits.
4. Power requirements
Consider the supply voltage, current rating, output loading and power dissipation while matching with other system components.
5. Size constraints
Evaluate physical size limitations. Get miniature encoders for space constrained applications.
6. Shaft loading
Factor in axial and radial loads on encoder shaft from gears, pulleys or couplings to determine suitability.
7. Cost requirements
Weigh costs vs performance. Striking the right balance for the particular application.
8. Incremental vs absolute
Assess whether absolute position sensing needed or incremental feedback sufficient for the task.
By carefully considering these factors, the most appropriate rotary encoder type can be selected for the specific application requirements.
Typical output waveforms of rotary encoders
Rotary encoders generate digital output waveforms that correspond to the shaft angle and speed. Below are some typical output signals:
1. Simple square wave
A single channel digital square wave with rising and falling edges indicating incremental motion. Provides position change data.
2. Quadrature square waves
Two square waves (Channel A and B) offset by 90°. Enables incremental counting and direction sensing.
3. Pulse + Direction
Additional Direction signal indicates rotational direction. Used when direction needs to be determined externally.
4. Sinusoidal wave
Approximates a sine wave pattern. Can provide higher resolution than square waves.
5. Absolute position binary code
Each angle corresponds to a unique binary code value in absolute encoders.
Interfacing rotary encoders with microcontrollers
Rotary encoders provide digital output signals that can be easily interfaced with microcontrollers like Arduino. Here are some basic techniques:
1. Connecting outputs to digital I/O pins
The encoder outputs can be connected directly to digital input pins. The encoder signals can trigger pin state changes to generate interrupts.
2. Using external pull-up/pull-down resistors
Add pull-up or pull-down resistors between encoder outputs and supply to convert bidirectional signals to unidirectional logic levels for microcontroller.
3. Connecting to external interrupt pins
Many microcontrollers have pins that can trigger interrupts on any change in logic state. Useful for encoders.
4. Using dedicated encoder counter ICs
Encoder signals can be fed into counter chips like LS7366 that convert pulses into binary counter values for microcontrollers.
5. AddingSchmitt trigger circuit
Schmitt trigger buffers clean up and reshape noisy encoder waveforms before sending signals to microcontroller.
Programming techniques for rotary encoders
Here are some common techniques used for programming microcontrollers to interface with rotary encoders:
1. Interrupt service routines (ISR)
Encoders can trigger ISR on signal edges. ISR increments/decrements counter variables to track position.
2. Timer capture mode
Capture timer values on encoder signal edges. Timer values correspond to position.
3. State machine coding
Define states in code for each encoder signal combination. State transitions determine direction and increments.
4. Encoder counter ICs
Use external encoder counter chips and simply poll their counter register from microcontroller.
5. Quadrature decoding
Use both encoder channels to decode direction and enable x4 counting resolution.
6. Error handling routines
Handle encoder faults like missing pulses or noisy signals to improve reliability.
Common problems with rotary encoders
Some typical problems encountered with rotary encoders include:
1. Noisy output
- Causes: Bad connection, loose contacts, defective optical sensors
- Solutions: Check wiring, replace encoder, use Schmitt trigger circuit
2. Missed counts
- Causes: Inadequate resolution, defective sensor, loose shaft coupling
- Solutions: Use higher resolution encoder, tighten couplings, replace encoder
3. Encoder outputs stuck
- Causes: Faulty sensor, loose or broken disk, defective electronics
- Solutions: Check and replace disc, test sensor, replace encoder
4. Reduced accuracy
- Causes: Excessive shaft loading, high vibration, worn out brushes
- Solutions: Correct loading, provide isolation, replace worn parts
5. Intermittent rotation
- Causes: Poor contact in signals or power supply
- Solutions: Check connections and contacts, use proper connectors
Frequently Asked Questions
Q1. What are some common types of rotary encoder?
Some common types of rotary encoders are:
- Optical incremental encoder
- Optical absolute encoder
- Magnetic rotary encoder
- Mechanical encoder
- Capacitive encoder
- Resolvers
Optical incremental encoders are the most widely used.
Q2. How does an incremental encoder produce pulses?
Incremental encoders use optical sensors to produce pulses as shaft rotates. Common arrangements are:
- Transmissive sensor – Detects interruption in LED light beam
- Reflective sensor – Detects breaks in reflected LED light
- Quadrature encoder – Uses 2 out-of-phase sensors for direction sensing
As disk rotates, pattern on disk interrupts the optical beams to generate digital pulses.
Q3. What are the typical voltage outputs from encoders?
Typical voltage output levels are:
- 5V – Common TTL and CMOS logic compatible levels
- 12V – Used when interacting with higher voltage circuits
- 24V – Suitable for industrial controls and motors
- 1-2V – Low voltage analog sine wave signals
Square wave, pulse signals as well as analog sine wave outputs are possible.
Q4. How to determine the resolution of rotary encoder?
Resolution is specified by PPR – Pulses generated Per Revolution of shaft.
Higher resolution encoders have more PPR. For example:
- 100 PPR encoder – Generates 100 pulses per full rotation
- 1024 PPR encoder – Generates 1024 pulses per full rotation
So for high precision, choose encoder with higher PPR.
Q5. What are some rotary encoder applications?
Some common applications are:
- Motors – Closed loop motion control
- Robotics – Joint position and speed sensing
- Automotive – Throttle position, gear stick, steering
- Consumer electronics – Knobs, dials, joysticks
- Industrial – Conveyors, CNC machines, presses
- Medical – Scanners, infusion pumps
Rotary encoders provide position and speed sensing in a wide range of automation systems.
