Motion Control Systems Information & Resources

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Position Sensing and Motion Control Systems

Position
In the simplest terms, an incremental rotary encoder provides information about the instantaneous position of a rotating shaft. It does this by producing one square wave cycle per increment of shaft movement. This increment is referred to as the resolution of the encoder and is built directly into the internal hardware of the device. A resolution of 360 means that 360 square wave cycles will be produced in one complete rotation of the shaft. By counting the number of cycles, one can tell the position of the shaft, relative to it’s starting position. In our example, 90 cycles means that the shaft is now at a position 90 degrees from where it started.

Velocity and Acceleration
If we add one more measured value, time, to our position information then velocity (position/time) can now be determined. If a shaft rotates 90 degrees in 1/4 second then it is rotating at 360 degrees per second (1 revolution per second) or 60 RPM. Similarly, if we were to measure the changes in velocity over time we could determine an acceleration rate for the shaft as well. Position, velocity and acceleration are all important factors to control in a complex motion control system.

Direction of Rotation
With incremental encoders, it is a standard convention to output a second data channel of position information. For simplicity these two channels are usually called the A and the B channel. These two channels are arranged with a preset offset of 1/4 of a cycle, or “quadrature”. This means that the A channel will go from LO to HI and then, while it is still HI, the B channel will go from LO to HI. Shortly after the A channel goes from HI to LO, then the B channel goes from HI to LO. This relationship is described as A leads B, that is the A channel makes its transition first, followed by the B channel. If the encoder was to reverse direction, then the B channel would make its transitions before the A channel. In this case, then B would lead A. Using this relationship it is possible to determine the direction of rotation of an encoder.

A Point of Reference
With dual channels in quadrature it is possible to determine relative position, velocity and acceleration information of a shaft. However it is not possible to determine the exact rotary position without some sort of internal position reference. This is provided with an index mark. This is an additional data track and is usually referred to as the Z track (other names include “ref” or “0”). The Z data channel usually changes state from LO to HI only once per revolution and only for a short duration (from 1/4 cycle up to about 1 cycle being fairly common). By observing this index track it is possible to create a “zero point” from which to begin counting the A and B channels.

Applying Feedback to Control
Encoder square wave outputs can be easily read by machine controllers. Controllers, as their name suggests interpret the encoder inputs according to their operating instructions and then provide a signal output to adjust the machines that they are controlling.

In a basic example, imagine a conveyor belt that is instructed (via the controller) to move at 100 feet per minute. This conveyor is powered by an electric motor with an encoder on its shaft. Output from the encoder goes into the controller, and as long as the square wave pulses are coming out of the encoder at the right speed, everything is fine.

If this conveyor is carrying dump truck loads of dirt up to a hopper, and a truck has just dumped a load of dirt on the bottom end of the conveyor, the extra heavy load will make the motor slow down. The controller notices that the pulses coming from the encoder have slowed down and so it commands the motor to speed up. Similarly, as the dirt reaches the top of the conveyor and is dumped into the hopper, the conveyor is suddenly lighter and it starts to speed up. The controller, via the encoder, knows what’s going on and commands the motor to slow back down again.

Another Type of Encoder
Let’s take a look at another popular type of encoder, an absolute encoder. Absolute encoders by their very nature have a more complicated signal structure. Optical encoders use photo-detectors internally to tell whether an internal light is ON (unblocked) or OFF (blocked). This ON/OFF character is inherently binary in nature and so it makes sense to produce an absolute position output where a rotation is successively divided into 2 parts, then 4, 8, 16 and so on. The trade-off with this approach is that each subsequent two-fold increase in resolution requires another data channel, and resolutions are only available in factors of two. Whereas the incremental encoder with 4096 cycles per revolution required one data channel and an index to precisely define a position, the absolute encoder requires 12 data lines (2 to the 12th power equals 4096 discrete positions). This means bigger, more expensive cabling and connections.

Absolutes have some clear advantages, however, in situations where the position information is only needed periodically (not continuously) or in instances where a power outage requires a machine to be able to reorient itself without having to “re-zero” any of its settings. In these instances, as soon as power is applied to the encoder and the data lines are read, then the position is known.

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