- Encoders by application
- Technology overview
Technology overview - encoders
Rotary encoders are electromechanical devices that convert rotational motion into useful electric signals. They monitor:
- Position
- Linear distance
- Speed
- Angular displacement
- Direction of rotation
Encoders are available in two main types – incremental and absolute. Incremental encoders provide an output of one pulse (square wave cycle) for each increment on the encoder. Absolute encoders provide a unique coded value for each increment on the encoder. Multi-turn versions also include the number of full revolutions in the coded value.
Incremental encoders
The number of square waves (pulses) generated in one full rotation of the shaft is referred to as resolution. For example, an encoder with a resolution of 1024 will generate 1024 pulses when the shaft is rotated 360°.
When referenced to a starting point, the number of pulses generated gives feedback on the position of the shaft. When power is cycled, the encoder does not retain the position value.
Basic incremental encoders normally have 3 output channels: A, B and Z. Channels A and B are offset by 90° and can be used to monitor direction of shaft rotation. If channel A leads channel B (top drawing), the shaft is rotating clockwise and the increment is counting up. If channel B leads channel A (bottom drawing), the shaft is rotating counter clockwise and the increment is counting down.
Only one Z pulse is generated for a full 360° rotation of the shaft. This is referred to as the zero point or reference mark. This is a reference point (or index “idx”) where you would begin counting A and B pulses. Often times, based on how the machine is built, it’s difficult to align the Z marker, so a proximity switch will be used as a reference point.
Advanced incremental encoders also include Channels A, B and Z in the inverted (mirror image) state. By monitoring the voltage drop between the inverted and non-inverted states, electrical noise can be filtered out and ignored. This is especially useful for applications with long cable runs.
Incremental encoders can have HTL or TTL signal levels.
- HTL – the output signal level and polarity is the same as the supply voltage. For example, if the supply voltage is +24 VDC, the output signal is +24 VDC.
- TTL – regardless of the supply voltage, the output signal level is 5 VDC and the same polarity as the supply voltage. For example, if the supply voltage is +24 VDC, the output signal is +5 VDC.
Incremental encoders use two different types of technology: optical and magnetic.
Optical encoders use through beam pairs to shine light through gaps etched into a coated glass disc. Pulses are generated as the beam is broken. Optical encoders have very high accuracy and resolution. But, they are complex, difficult and expensive to manufacture. They are also prone to breaking in applications with high shock and vibration.
Magnetic encoders generate pulses by rotating a magnet over a chip and measuring the change in the field. This information is converted into a series of pulses. This is a simple, compact design that can withstand high levels of shock and vibration.
Optical encoders are more accurate and capable of greater resolution and speed than standard magnetic encoders. Magnetic technology has the benefits of being able to fit into smaller packages, great robustness and a simple design that allows for lower manufacturing costs.
Those who are experienced with encoders usually consider magnetic technology inferior to optical technology due to their decreased accuracy and response time. ifm’s line of magnetic encoders is superior to standard magnetic encoders and are equivalent to optical encoders. This is accomplished by using a 32 bit, 66 MHz microcontroller that allows for greater accuracy by tuning out noise. It also has almost no lag time in speed of response.
- Analog sampling
- Noise and latency compensation
- One amplifier
- Fast sampling and data processing
As opposed to incremental encoders, absolute encoders provide a specific numerical value for each angular position of the shaft.
Single turn encoders divide a mechanical revolution (0…360°) into a certain number of measure steps determined by the resolution of the encoder.
Multi-turn encoders provide a unique coded value consisting of the location on the disc (resolution) and the location of the gear train (revolutions).
Common absolute encoder resolutions include 10-bit (1024 steps), 11-bit (2048 steps), 12-bit (4096 steps) and 13-bit (8192 steps). When combining resolution with revolutions, unique steps can be 24-bit or higher.
Absolute encoders are available with parallel, serial or fieldbus outputs to transmit shaft positon data.
- Parallel outputs require an output for each bit of information. For example, a 12-bit parallel output requires 12 separate outputs to transmit the data.
- Synchronous Serial Interface (SSI) is a means to transmit encoder position data serially.
- Common fieldbus outputs include DeviceNet, ProfiNet, Profibus, CANopen, Ethernet IP and EtherCAT. ifm encoders are available with ProfiNet, Profibus and CANopen interfaces.
- ifm offers IO-Link enabled encoders to provide diagnostic information and easy integration into your controls architecture.
Like incremental encoders, absolute encoders use optical and magnetic technology.
Optical absolute encoders have a “ring” of marks for each bit of resolution that will break the light beams. For each additional bit, the number of marks doubles. The inner most ring refers to the MSB (Most Significant Bit) and the outer ring refers to the LSB (Least Significant Bit) of the binary word. The glass disc will generate a unique value for one full rotation. Multi-turn encoders additionally incorporate gears to count the number of rotations.
Hollow shaft encoders mount directly to the rotating shaft and are held in place by a set screw.
ifm offers a variety of couplings, measuring wheels and other accessories that can be found with all our accessories.