FEATURE Drives, Controls & Motors
Figure 3: Resolver
pulses as the disc rotates. In one rotation of the encoder, an incremental encoder delivers a specifi c number of pulses, which enables the movement value to be determined along with the speed. Common voltage levels are 5V (TTL) and 24V (HTL). The signals consist of three tracks: A, B and Z (zero signal); see Figure 5. The A and B tracks have a 90-degree phase shift to indicate the rotation direction, whilst the zero signal (Z) track gives the number of revolutions, useful for homing routines. Its resolution is the maximum number of pulses that it sends per revolution. Here are some useful points to consider about incremental encoders: • Incremental encoders provide position feedback but the absolute position is not retained when the drive/encoder is powered down.
• VFDs can evaluate the rising and falling edges of the signals to determine the direction of rotation. Monitoring the rising and falling edges eff ectively doubles the position information for each of the two tracks, hence their name “quadrature encoders”. • Incremental encoders are typically used on induction motors with indexing applications, cut to feed applications, as well as any speed and position-type control. • The glass disc can be a failure point in applications with vibration. Steel disc options are commonly available as an alternative.
Serial feedback devices Serial feedback devices measure shaft position with analogue or incremental means but then transmit the information to the VFD via a serial connection. This can reduce conductor wires in the encoder cable and susceptibility to electrical noise. Furthermore, serial feedback devices can transmit much more information including OEM encoder/ motor parameters, error codes and encoder diagnostic information. BiSS encoders
A BiSS encoder is an open-source feedback interface for digital absolute encoders that
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Figure 4: Sine and cosine waves
was designed and developed by IC-Haus in Germany. It is capable of transmitting the position value of the encoder as well as reading and updating the information stored in the encoder. Like other digital feedback encoders (Hiperface, EnDat), BiSS comms link can be used to carry information other than position value. Additonal information such as encoder resolution, manufacturer information and temperature can be stored in a non-volatile memory area in the encoder. KEB drives, for example, can read and write to the encoder memory without interrupting real-time operations. Due to their high transfer rate, BiSS encoders can be limited on the permissible encoder cable length. BiSS encoders do not require additional license fees and so provide good value in terms of price and performance. Hiperface encoders
A Hiperface encoder is a proprietary hybrid feedback interface developed by SICK Stegmann in 1996. It consists of a bi- directional interface for absolute encoders that combines a digital channel for absolute position information and an analogue channel for incremental position and speed feedback. The encoder also features a memory area, which is read or written to by drives through a communication channel. With asynchronous serial transmission, only two lines are needed to transmit the encoder position data. The serial link requires terminal resistors to operate, along with pull-up and pull-down resistors to increase interference immunity. EnDat encoders
An EnDat encoder is a proprietary feedback interface developed by the Heidenhain Company in the 1990s. It consists of a bi-directional digital interface for absolute encoders. It can transmit position values from incremental and absolute feedback, transmit or update information stored in the encoder, and save new data. EnDat 2.2 off ers serial transmission; only four lines are necessary for transmitting the encoder position data synchronously with the
Figure 5: Incremental encoder
clock delivered by the electronics. The type of transmission (position values, parameters, diagnostics, etc.) is determined by mode commands sent to the encoder (by KEB VFDs for example).
BiSS, Hiperface and EnDat encoders are all available in single- and multi-turn absolute formats, making them ideal for applications that require absolute positioning. Typical uses for them include robotic manufacturing, motion control with multiple axes, lifts and CNC machines.
Absolute position feedback Some feedback devices off er absolute position feedback in the form of single or multi-turn variants.
Absolute encoders provide a unique position value at every point of rotation, representing the ‘absolute’ position of the encoder. Immediately as the drive is switched on, the absolute encoder can provide the exact position of the shaft that it is measuring. If the machine were to lose power, absolute feedback would tell the system where it is at, without having to re-home the machine – this is particularly important in critical applications where re-homing procedures might take too long or lead to costly scrap material.
Single-turn absolute feedback measures displacement over 360 degrees of shaft rotation, with the output being repeated for each revolution of the encoder’s shaft. Multi-turn encoders use a special term for each position and number of revolutions to measure the degree of rotation (within 360o
and track the number of total revolutions of the encoder’s shaft. Absolute types are options within the encoder family. For example, a user could specify a single-turn BiSS absolute encoder, or a multi-turn absolute Hiperface encoder.
CONTACT:
KEB Automation
www.kebamerica.com
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Automation | April 2022
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