To measure distances, angles, positions or revolutions of machines, numerous sensor solutions are offered to the user, among them a variety of encoders. Encoders are divided into two types: incremental and absolute. Turck has expanded its sensor portfolio with encoders of both types.
The most cost effective solution is an incremental encoder, which produces a number of pulses per revolution and can be used to measure rotation, length or position. Incremental encoders are typically used for monitoring revolution and speed.
Absolute encoders are normally used to monitor positions or angles. Here a code pattern is assigned to each position; the dedicated assignment of a code for each and every position ensures that an absolute encoder will give accurate positional output under the most trying of conditions. After a power disruption, or in an extremely noisy environment, the absolute encoder will correct an error as soon as it moves to the next position, as the new position will have its own unique code. An incremental encoder in the same situation will carry the error until it reaches a ‘home’ position or zero point when pulse counts can be corrected.
Scanning technologies
Turck makes use of two established scanning technologies in its industrial encoders: optical and magnetic scanning. Optical scanning provides high resolution and accuracy, as well as high resistance to electromagnetic interference. Magnetic technology is especially shock resistant and can have a high protection rating up to IP69K. Furthermore, the devices are compact and suitable for application where there are high temperature fluctuations. Often, magnetic encoders are cheaper but have less resolution and therefore less precision. Turck offers both scanning technologies for all categories of encoders (incremental, absolute, single-turn, multiturn and analogue) and has a standard comprehensive portfolio from which the user can select the best technology for the application.
Mechanical design
Linear position sensors and encoders are usually placed in close proximity to the action and therefore need to resist environmental influences and mechanical stress. Technical attributes, like protection category, temperature zone, resistance to shock and vibration as well as load capacity of the shaft are important selection criteria for the user.
When used outdoors, encoders have to withstand temperatures from -40 to 55°C and maintain the IP67 protection category. Often outdoor devices are exposed to large temperature fluctuations that allow the formation of condensate inside the housing of the encoder. Turck avoids this by using a stable and compact die-cast housing and multiple seals between the encoder flange and the housing. If the encoders are exposed to high-pressure water or steam jets as often occurs in the food and beverage industries, an IP69K rated encoder is needed.
The deciding factor for the lifespan of encoders is the quality of the bearing assembly. The encoders from Turck are robust in this respect with two large ball bearings that are mechanically interlocked and have a maximum distance between each other.
Interfaces
For connection to the control system, incremental encoders use either an RS422 interface for long cable lengths or a push-pull configuration. The common push-pull interface is used for the connection to counter cards, electronic counters and SPS inputs. The synchronous serial SSI-Interface has prevailed as the standard for absolute encoders, but Turck also supplies absolute multiturn encoders that can be connected to the various fieldbus systems like Profibus-DP, CANopen, DeviceNet, EtherCAT and Profinet.
Speed
Encoders with precise integral bearings and powerful electronics can also be used for applications with high operational speeds. The incremental encoders from Turck are equipped with fast electronics operating up to 300 kHz. The absolute encoders provide very fast SSI-interfaces with a frequency of 2 MHz.
Whether incremental, absolute, single or multiturn, SSI or fieldbus interface, Turck offers a comprehensive standard portfolio of encoders for a wide variety of applications.
For more information contact Rodney Topham, RET Automation, +27 (0)11 453 2468, [email protected], www.retautomation.com
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