What’s the Difference Between Single- and Multi-Turn Encoders?
The position of a shaft within a machine can affect several concerns including safety, quality, volume and other variables. To monitor and control information that can be obtained from the shaft position and rotation, encoders were devised to measure and control the data and motion of the machine along the shaft.
There are two types of rotary encoders used today – incremental and absolute. The most common, the incremental encoder, provides position information in real-time. These encoders can measure up to 10,000 counts per revolution and transmit position changes quickly. As a result, they are used in applications that require highly accurate position and velocity measurements.
Incremental encoders are available in a variety of technologies depending on applications. These may include mechanical, optical or magnetic sensors which help determine precise positions. They may act as a potentiometer, volume control, and other applications where fine tuning is required.
Absolute Encoder Technology
Absolute rotary encoders are most likely found in industrial or factory automation systems where shaft position is a required variable. Absolute rotary encoders can use a range of technologies from low tech to high tech to measure position of the shaft. These technologies Include:
- Mechanical – A mechanical rotary encoder can be relatively low tech and consists of etched tracks that signal a code or “word” to the encoder to specify the exact position of the shaft. These encoders are very cost effective, acting as a potentiometer for simple applications. However, they may be subject to wear as there is regular friction applied between the resistive element and the brush when changing positions.
- Optical – Optical Encoders use light sensors and a light source to detect position. Light is sent through a slit in a rotating disc attached to the shaft of the motor. This “code wheel” allows the light to pass through the slit to the optical sensor to count the number of pulses to determine position.
- Magnetic – Magnetic encoders use a magnet attached to the motor shaft. The change in the magnetic field distribution can be read with a magnetic sensor such as a microprocessor to determine the position of the motor.
- Electromagnetic Induction – An electromagnetic encoder also uses magnets to determine position. However, here, the magnetic field generated between an induction coil and a detection coil attached to the shaft that reads position. These encoders can be used in harsh environments where mechanical and optical encoders are not practical.
- Laser- For applications which require very precise positioning, such as aerospace and military applications, laser encoders can be used. Here, the encoder uses a displacement interferometer to determine price position and speed.
Advantages of Absolute Rotary Encoders
There are several advantages for using an absolute rotary encoder. These advantages include:
- Reliability – Absolute rotary encoders can remember position when depowered. This is useful in power outages as the position is maintained during the stop. The user/operator does not have to wait on homing or calibration cycles to restart.
- Control – Because they can take advantage of non-volatile memory, they do not lose track of positioning in a loss of power or in the off position, allowing you to determine and maintain the exact position of the equipment. This helps maintain safety, equipment damage and other variables to reduce risk of starting in the wrong position. They also allow for the electronic data storage to be maintained and controlled.
- Resolution – Absolute encoders have a resolution of up to 16 bits. This means they can read as many as 65,536 revolutions per pulse.
- Automation Friendly – As modern automation and industrial control systems grow, using an absolute controller to determine position can allow the system communication bus to quickly find position in real-time.
- Less Programming – Especially in the use of robotics, absolute controllers can be combined in a single system with each reporting data individually, a benefit not possible with incremental encoders.
- Background Noise – Modern manufacturing can be noisy in terms of electrical interference. Absolute rotary encoders are generally quieter because unlike incremental encoders, they do not need to read pulses to gather data. Instead, they can determine position by use of a code from a binary encoder or even use a serial bus for positioning data.
Single Turn Encoders
Single Turn encoders are absolute rotary encoders that measure position within one rotation. This can be in the form of pivot position, speed or linear motion of the shaft. Because they are only rated to measure position for a single revolution, any rotation beyond a 360-degree spin will return the counter back and begin forward again. For example, a 361-degree turn would be read as 1 degree.
A single turn encoder uses a defined starting position. As the shaft returns to 360 degrees, the position is repeated for each turn depending on the stop position. It does not record the number of revolutions or cycles; it only reports the position within the 360-degree range.
One example of a single turn encoder is the 845G series from Allen Bradley. This family of encoders offers versatile connection with options ranging from different code output types (gray and binary) to different power supply options ranging from as low as 5V DC+/- 5% as well as 8-24V DC and 10-30V DC. They also offer several connections including axial 12, 19 and 17 pins and radial 19 pin. These single turn encoders can count to 4095 positions on a single shaft within the single revolution. After 4096, the count returns to zero. Similar encoders are found among other control system makers as well.
As discussed, single turn encoders are used for measuring potion within a single rotation. This makes them ideal for applications for many applications. Some application examples include:
- Wind Turbine Generators
- Automated Guided Vehicle (AGV) Drive Wheels
- Gates and Doors
If a motor within an application makes more than one turn, a single turn encoder can still be used if it is not required to count revolutions. This use of a single turn encoder helps provide improved torque control on startup in permanent servo motors.
Multi Turn Encoders
Multi turn encoders also measure the absolute position of a shaft. However, they also count revolutions for applications that require this information. A multi turn encoder can count to 4096 revolutions. This is not to be confused with 4095 steps mentioned above that can be counted within a single turn encoder for shaft position.
A multi turn encoder can count to 4096 steps on a shaft rotation and then up to a total of 4096 revolutions. Once the encoder reaches 4096 revolutions, the values begin to repeat. However, some controllers offer “modulo positioning”, a feature whereby overflow turn data can be stored for both full and partial rotations after 4096 revolutions. This can be used to determine correct positioning beyond the 4096 limit.
There are three technology types used for multi turn encoders.
- Geared – Geared multi turn encoders are the most common and have several mechanical gears housed inside the casing. These gears track the number of revolutions made by the shaft. One of the gears is attached to the rotor of the encoder to transmit the turn data. Geared encoders are larger than other types and are subject to wear and tear due to their mechanical parts.
- Battery Back Up – This type of multi turn encoder uses an electronic counter. This eliminates the need for gearing and other mechanical components. However, it requires a battery backup if motion occurs during the off and on period. In this case, the data is saved and transmitted as needed to determine position. Battery backups may also need to be replaced during the lifecycle of the encoder depending on the application and amount of use.
- Wiegand Sensor – The Wiegand sensor encoder uses a specially designed wire that has been hardened and annealed. This gives the wire magnetic properties that can be used to harvest energy within the encoder and power it. The wire is wrapped near the shaft and uses a magnetic pulse to trigger a change in polarity. This reverse polarity then switches back and forth, serving as the counter for the revolutions. Wiegand sensor encoders are not recommended
Multi turn encoder application examples include:
- Robotic Joints
- Moving Stages and Platforms
- Servo Motors
- Wind Turbine Pitch Controls
- Circumferential Motion of Satellite Dishes
- Coordinated Offset Axes
- When a Rotation Pivot Point is Not Accessible
One example of a multi turn absolute encoder is the Allen Bradley 842 family encoder. This encoder is a 25-bit encoder with only 5 transmission wires for data within the encoder. It is offered in low end models with as few as 256 revolution capacity but also goes to higher end capacity with 8192 revolutions. This feature builds in modulo positioning, as mentioned earlier, doubling both the overall steps per turn as well as the total number of turns.
Both single turn and multi-turn encoders can use two types of logic to encode a shaft. One is simple binary code where each contact on the encoder responds to either 1 or 0. One downside of this type of logic is that in the event of a stoppage where the rotary disc stops between two contact points, the angle of the shaft cannot be determined.
To address this issue, many controllers use Gray encoding logic as well. In Gray coding, two sections of code occupying the space next to each other only vary by one bit. This keeps the issue caused by binary coding from happening. Most single and multi-turn encoders, such as the Allen Bradley 845 single turn encoder and Allen Bradley 842 multi turn encoder as well as other manufacturers offer both binary and Gray code options
Encoder Communication Protocols
Both single turn and multi turn encoders use multi-bit word as their output. This can be used to transmit data such as speed, precise position, and more. Absolute encoders can interface with several standard communication protocols.
These include parallel output, binary signals using both current and voltage, SSI, serial and fieldbus protocols such as Profibus, DeviceNet, Modbus and Interbus. They can also operate on a CAN Network and can use EtherNet protocols such as EtherCat as well. This provides reliable communication and means that encoders can function and be monitored and managed through an industrial automation control or IIoT system.
Incorporating all sensors, controllers, HMIs and encoders into a single DataStream has had an enormous effect on efficiency as IIoT and Industry 4.0 have launched across the globe. As the growth of IIoT has increased, many manufacturers are even including diagnostic capability into absolute single and multi-turn rotary encoders. This helps provide both overall system data for holistic monitoring of automation systems as well as spot specific monitoring to understand whether the encoder is performing properly.
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