Basic Motion Control - Absolute Optical Encoders

Q: Should accurately determine the position of the shaft for the PLC / motor applications that often stops and starts. Any suggestions?

Reply:. A typical solution would be to install an absolute optical encoder

Ok ... Why use an absolute rather than incremental optical encoder?

Optical encoders are devices connected to the motor can tell us the distance traveled and speed. Absolute Encoders provide the actual shaft position and brzinu.Jeftinije encoder only informs the distance of movement in relation to motor the last position.

the absolute value of

of the absolute values ​​are typically found in applications where:

• position data must be retained after a power loss,
  
• device is inactive for a long time,
  
• device is moving slowly (eg pumps, valves or gates ).

If you recall, an incremental encoder is a counter that should be 'homed' or reference to a certain point. Lose power and it must be re-invited.

Establishment of position after a power loss

Absolute encoders solve this "dove" problem directly by providing a unique "code" for each position, regardless of where the shaft starts and stops. Traditional absolute optical encoder is mounted on a glass shafted disk with concentric rings are alternating transparent and opaque dijelove.Broj share with each ring provides a natural binary weighting. Pulses from each ring routed to the appropriate bit position in the parallel data riječi.Broj rings to determine the size of the data word and therefore the rotational resolution. To increase the resolution -. Increasing the number of rings

For example: 1) disk with eight rows (8 bits = 256 points) in the settlement of 360 degrees / 256 points / rotation = 1.4 degrees. 2) disk with 12 rings (12 bits = 4096 points) in the settlement of 360 degrees / 4096 dots / Rotation = 0.087 degrees.

to count segments of the infrared emitter is mounted on one side of each song and an infrared phototransistor (receiver) on the other strane.Izlaza have passed the module that insulates the rapidly changing segments (bits) from the PLC slower scan rate.

problem with natural binary codes when switching between opaque and transparent segments do not occur simultaneously for accurate position. Consider a hypothetical four bit encoder, a change in the position, all four bits will change stanje.Detektori will see a fake podataka.Promatrač will not be able to tell if this is the right position, or a transitional state between the positions. If the output of food in a sequential system, then the system can store a false value. To avoid this problem reflected binary code (like Gray) are employed. Gray and its derivatives to ensure that the sequence of consecutive positions will differ only one bit. This limits the maximum position error to one LSB (least significant bit). Most PLC has a gray to binary conversion instructions which encoded data to be used anywhere in the program.