Shaft motion can be tracked by incremental encoders for analysis and use by system controls downstream. Rotational direction is monitored with an encoder disc paired with simple electronics and a dual optical interrupter; with a bit of additional electronics, speed and relative shaft location are tracked as well.
These encoders determine shaft speed by counting signal pulses for a fixed time and then dividing that by the number of periods observed passing a point per revolution. Here, a period is one of two things: A fraction of 360 electrical degrees, or the mechanical width of one arm of the disc's opaque web, plus two halves of adjacent cuts or etchings. Relative location is determined by dividing 360 by this number of periods. Quadrature transitions, four per period, allow tracking of rotational shaft direction by the phase relationship between the interrupter's two channel outputs — normally a 90° shift.
Q&A
How do the slots on an encoder disc affect performance?
The openings must be greater than aperture width, or they degrade output signals. For a 50% duty cycle, large period widths can be separated into two sections; small period widths may require slots smaller than the web.
The number of periods on an encoder disc can be an arbitrary number or referenced to the resolution of the disc's steps. For example, if 0.7° steps are required, the number of periods per revolution is 128:
Turning an off-multiple of periods between sensor aperture center lines — 1/4, 3/4, 1-1/4, 1-3/4, 2-1/4, and so on — is required for a 90° phase shift. Period width is calculated by dividing the centerline spacing by the off-multiple. For example:
0.040 in. /1.25 = 0.032 in.
0.040 in. /0.75 = 0.053 in.
As a rule of thumb, slot length should be at least twice the disc eccentricity plus the sensor aperture length. Maximum slot length depends on the mechanical design; it should fit inside the encoder housing.
What's the best material and size for an encoder disc?
Encoder discs — whether plastic, metal, or glass — should be made from materials compatible with the application, and their openings or applied patterns as close as possible to the sensor side of the encoder. (This allows for encoder endplay.) Minimum width is determined by the material's stiffness, opacity to near-infrared light, and eccentricity. Maximum disc width is determined by the slot width and eccentricity expected.
As for the minimum and maximum disc radius, those values depend on the centerline pitch radius plus the aperture's optical centerline, disc eccentricity, slot length, and a mechanically suitable support distance.
Should logic sequences be considered?
By anticipating encoder logic sequences, designers can confirm disc direction and speed. These digital sequences depend on when A and B apertures pass encoder disc slots — in turn, depending on the design's off-multiples and channel spacing.
Say channel A is on an opening for a 0.212/4.25 configuration with a period width of 0.050 in. Here, channel B is on an opening. But say we have an encoder in which channel A is on an opening for a 0.040/0.75 configuration, where channel B is on a closed part of the encoder disc. This reverses the logic sequence. How?
For the first example, the expected sequence is 1-1, 1-0, 0-0, and 0-1; expected for the second is 1-0, 1-1, 0-1, and 0-0. In short, off-multiples ending in 0.25 output a quadrature-direction sequence opposite to that of off-multiples ending in 0.75.
This month's handy tips provided by OPTEK
Technology Inc., Carrollton, Texas. For more information, call (800) 341-4747 or visit optekinc.com.