Encoder Basics

Basically an encoder is a device that emits X amount of pulsed signals per resolution of a motor or shaft. This can be used as was stated to determine speed, location (or distance traveled), and acceleration/deceleration.

This article is a good primer for common questions we get from customers using our Encoder Simulators.

It is an article that was written by Ron Doran.

Question: What is the basic definition of an encoder?

  • Answer: An encoder comes in two architectures. The first architecture is linear. The second architecture is rotary. Both types sense mechanical motion and translates the information (velocity, position, acceleration) into useful electrical data.

Question: What is the difference between an absolute and an incremental encoder?

  • Answer: There are a few subtle differences between absolute and incremental rotary encoders. Incremental encoders have output signals that repeat over the full range of motion. It is important to understand that each mechanical position is not uniquely defined. When the incremental encoder is turned on, the position of an incremental encoder is not known since the output signals are not unique to any singular position. Absolute encoders have a unique value (voltage, binary count, etc) for each mechanical position. When an absolute encoder is turned on, the position of an absolute encoder is known (this function resembles a resolver, although the principles of operation have no similarity.) The similarities of both absolute and incremental encoders are form factor and the issues of count and directional information. They can be obtained from both absolute and incremental encoders equally.

Question: What is a channel?

  • Answer: A channel is an electrical output signal from an encoder. Typical incremental encoders have either two or three channels. An example would be: A, A not, B, B not, and Z, Z not (also referred to I or I not.).

Question: What is quadrature and 4X (4 times) quadrature encoding?

  • Answer: quadrature is a description of the 90 degrees out of phase between channel A and B. The rising edge to rising edge (referred as the Cycle) on channel A or B indicates that one set of bars (on the internal encoder disk) have passed by the optical sensor. The quadrature state of channels A and B create four unique logic states. When these four unique logic states are decoded, the resolution obtained is 4 times (4X) the resolution of the encoder disk. So with this in mind 250 sets of bars would yield 1000 quadrature states.

Question: What is the Index pulse (strobe, Z, and I.)?

  • Answer: It is the absolute reference added to an incremental encoder. It is also known as home position. It signifies a full rotation of the encoder disk

Question: How can I reduce the effect of noise when using encoders?

  • Answer: There are several ways to improve noise immunity. Be careful to separate motor and all power wires away from encoder wires. Always separate the channels from each other. All encoder wire should be shielded and carefully terminated. The pigtail that connects to the control unit using the encoder should be as short as possible. The leads will act as antennas for any stray RF in the vicinity. The addition of other noise suppression devices may be required in high noise environments.

Question: What are "Push Pull" and "Totem Pole" outputs? Why don't they have a differential output?

  • Answer: Looking at the A and A not channel, the optical sensor internal to the encoder is comparing the two. If A’s light amount is greater than A not, then A is high. When A is less than A not, then A is low. The same reasoning defines channel B. This process is called “Push Pull.” A “Totem Pole” output is the same as a “Push Pull” output. The big difference is the ability of the “Totem Pole” to handle sinking or sourcing less current than the “Push Pull.” It is confusing because the other factor is the amount of voltage usually used to define the difference between the two. “Totem Pole” usually uses 5vdc, where as the “Push Pull” follows the input voltage.

Question: Do I need to shut down my machine to change an encoder?

  • Answer: The best method is to power down the unit. If the supply voltage is accidentally shorted or grounded, it will damage the encoder.

Question: Q: What equipment do I need to troubleshoot an encoder?

  • Answer: A: An oscilloscope or specially designed encoder tester. A basic multimeter will do for basic diagnostics such as power supply and low speed pulse outputs. Any advanced pulse detection and diagnostics needs to be performed by a trained technician and an oscilloscope.

Question: What is gray code?

  • Answer: Gray code is a form of binary. The difference is how it is incremented to the next number. Only one number can increment at a time. Gray code: 0, 1, 3, 2, 6, and 7 Binary: 0, 1, 2, 3, 4, and 5

An example of use: We have a flexographic printing press that moves the printing plates toward a drum, at the same time it moves an anilox roller (inking roller) towards the printing plate. This is an automated system so the travel distance from "home" to the "print" postion is preset/determined, therefore when you send it to "print" it knows the precise position to go to via the count given from the encoder. In this application the encoders are incremental which means they are step by step device and have no memory. Absolute encoders are designed to remember position even if powered off.