Not every rotary encoder disc is a tidy set of concentric rings. Some carry a single irregular band that looks more like a barcode; others have an extremely fine pattern around the rim with something much coarser tucked inside it. With concentric rings you can at least guess at the reading by counting which track is light, but these discs give away very little at a glance.
What they have in common is a motive. Adding one track per bit will certainly represent absolute position, but the track count feeds straight back into how wide the disc has to be and how hard it is to manufacture. All three styles on this page try to get absolute position, or high resolution, either without adding tracks or by mixing patterns that do different jobs.
If you would rather start with the multitrack styles — binary, Gray and incremental — begin with (1) Multitrack codes. That page also explains the conventions these figures follow: white is 1, black is 0, and the outermost track is the LSB.
| Question | Figure that answers it |
|---|---|
| How can one track alone carry absolute position? | Single track discs with a PRBS code (Figure 1) |
| What is a disc doing when a fine pattern and a coarse one sit side by side? | Hybrid discs: coarse absolute plus a fine cycle (Figure 2) |
| What do you learn by overlaying two tracks whose periods differ slightly? | Vernier discs: a difference in period (Figure 3) |
Every disc below can be dragged. These three reveal much more in motion than they do standing still, so watch how the lamps change rather than studying the static pattern.
Single track discs with a PRBS code (Figure 1)
How the encoding works
Instead of adding a concentric track for every bit, this design writes absolute position into one irregular binary sequence running around the circumference. The sequence is pseudo-random (PRBS), and when a maximal-length one is chosen it is called an m-sequence.
The property that makes it work is this: every window of n consecutive bits is unique, wherever on the ring you cut it. Read any n adjacent bits and there is exactly one place on the circumference they could have come from. One track is enough, which lets the disc be narrow, and the resulting pattern looks much like a barcode bent into a ring.
That raises the question of how you read a window at all. A sensor looking at a single point only knows whether that point is light or dark. Since fixing the position needs n consecutive bits at once, the read side has to either place a head with n elements side by side, or take one bit at a time as the disc moves and hold the last n in a shift register.
This is why real hardware often adds a synchronization track to keep the sampling aligned. Think of it as a trade: the disc drops to a single track, and the read head grows more complicated in exchange.
In the figure on this page
There is one code track only, and no synchronization band is drawn. An m-sequence of period 2n−1 runs around the circumference, and the fixed head from x0 onward reads a consecutive segment of it as the window.
The lamps show the bits sitting in that window, in order. They are not weighted bits, so there is no MSB or LSB among them. That meaning differs from the absolute figures in part (1), so it is worth not reading them the same way. The resolution slider sets the number of bits in the window, which is also the exponent that fixes the period.
Reading the lamp pattern as a decimal number gets you nothing that corresponds to an angle. On a single-track code the lamps show the bits currently inside the window, not a position value. Turning that pattern into a position takes a separate step that matches it against the sequence.
Hybrid discs: coarse absolute plus a fine cycle (Figure 2)
How the encoding works
This pairing turns up constantly in high-resolution absolute encoders. A coarse absolute code — a few bits of track, or a barcode band — establishes which interval the shaft is currently in, and a fine periodic band, usually near the rim, is interpolated to pin down the precise position inside that interval.
Splitting the work across two stages is the whole point. With absolute position and resolution assigned to separate tracks, you can reach a fine position without inflating the bit count of the absolute code. On photographs of real discs this family tends to look busy: something fine around the outside, something coarse further in or on a separate band.
In the figure on this page
The drawing places a fine light-and-dark periodic band on the outside with a coarse multitrack Gray code inside it. The lamps show the coarse bits, running MSB to LSB, followed by a “Fine” lamp for the periodic band.
What the figure does not do is the interpolation itself. It shows the sensor under the fine band blinking between 0 and 1, and stops short of calculating an angle within the interval from it. Treat it as a way to watch the division of labor: the coarse code settles the interval while the fine cycle streams past much faster on top of it.
Vernier discs: a difference in period (Figure 3)
How the encoding works
Borrowing from the vernier scale on a pair of calipers, this design overlays periodic tracks whose periods differ by exactly one cycle. The phase difference between the main and secondary tracks yields coarse absolute information, so the position can be narrowed down from two periodic signals rather than from a stack of digital absolute tracks.
The aim is economy of tracks. Instead of dedicating one track to each bit of absolute position, the drift between two periodic tracks does the work, which keeps high resolution available in a small package.
In the figure on this page
The outer band is the main period and the inner one is the secondary period, carrying exactly one cycle fewer than the main. There are only two lamps, “Main” and “Sub”, and the slight mismatch in how fast they blink is the phase difference made visible.
Here too, the figure does not combine the phase difference into an angle. It exists to show what a difference in period actually looks like on a disc.
The “Fine” lamp in Figure 2 and the “Sub” lamp in Figure 3 look alike but do different jobs. The first is a fine cycle used to interpolate within an interval; the second is a secondary period whose offset from the main one yields coarse absolute information. Neither lamp represents a position on its own.
Where each of the three types actually turns up
These three tend to come up alongside a constraint: there is very little room, or the disc cannot be made any thicker. What follows describes general tendencies, and the type used in any particular unit varies by model.
Single track (PRBS / m-sequence)
With only one code track, the disc can be made thin in the radial direction. That pays off in hollow-shaft designs, where cabling or tubing passes through the center, and in small absolute sensors that have to be squeezed into whatever gap the mechanism leaves.
As covered above, though, the read side still needs machinery to capture a window. The effort saved on the disc moves to the head and the processing behind it, so this is not a style chosen as a drop-in replacement for a multitrack disc.
Hybrid (coarse absolute plus fine cycle)
Among high-resolution absolute encoders this combination is close to standard practice, because reaching the same resolution with a coarse absolute code alone would demand an unrealistic number of tracks.
It shows up in the position feedback built into servo motors and in the angle sensing on precision stages — places asked for absolute position and fine resolution at the same time.
Vernier (difference in period)
Because two periodic tracks cover the coarse absolute information, this style suits small, thin designs with no room to line up many tracks. The same reasoning appears in magnetic encoders as well as optical ones, using pole counts that differ by one.
The same idea extends to multi-turn sensing, where the goal is counting how many revolutions the shaft has made. Gears with slightly different tooth counts are meshed together and the revolution count is recovered from the phase difference between them.
This is a family where naming the style from the pattern alone is genuinely hard. A disc with a fine outer rim is not necessarily hybrid, and a ring that looks irregular is not necessarily PRBS. When you need to know, look for a description of how the device derives absolute position rather than trying to read it off the artwork.
What each of the three types saves
All three start from the same motive — avoid adding tracks — but they economize in different places.
| Type | Where absolute position comes from | Code tracks | Processing left out of the figure |
|---|---|---|---|
| Single track (PRBS) | Every window of consecutive bits is unique | One (no synchronization band drawn) | Matching a window to a position |
| Hybrid | A coarse absolute code fixes the interval | One per coarse bit, plus the fine band | Interpolation from the fine cycle |
| Vernier | Phase difference between main and secondary periods | Two (main and secondary) | Deriving an angle from the phase difference |
The fewer tracks a disc carries, the simpler the disc itself becomes — and the more of the job of extracting a position moves into the processing behind the sensor. What these figures leave out is precisely that displaced work. Read them as a way of seeing where the pattern on the disc stops and the arithmetic begins.
The classic styles, where one track corresponds to one bit — binary, Gray and incremental — are collected in (1) Multitrack codes.
Frequently asked questions
Q1. If one track is enough, why do multitrack discs still exist?
A. A single track keeps the disc narrow, but it requires a step that matches the window of bits you read against a position in the sequence. On a multitrack absolute disc the combination you read is the address, so the read side stays simple. It comes down to which you would rather keep simple, the disc or the electronics.
Q2. Is a PRBS disc the same thing as a barcode on a product?
A. They look similar but they come from different places. A barcode encodes a fixed string of characters. A PRBS disc represents position itself, using the property that any window of n bits taken anywhere on the ring is unique. Spin the disc in the figure and you can watch the bit pattern inside the window differ at every position.
Q3. What does the “Fine” lamp in the hybrid figure represent?
A. Whether the sensor sitting under the fine periodic band at the rim is currently seeing light or dark. Real hardware interpolates that signal to obtain a precise position within the interval; the figure stops at displaying the 0 and 1 and does not perform the interpolation.
Q4. Is a vernier encoder the same principle as the vernier scale on calipers?
A. The underlying idea is shared: overlay two scales with slightly different periods and read a fine value from the offset between them. The figure borrows that intuition by placing a main period against a secondary one. It stops short of combining the phase difference into an angle, showing only how the two periods drift against each other.
Q5. Can you list everything these figures leave out?
A. The single-track figure does not match the window of bits to a position, the hybrid figure does not interpolate from the fine cycle, and the vernier figure does not derive an angle from the phase difference. The single-track figure also omits the synchronization track that real hardware often carries. What you see is the pattern on the disc and the signals that can be read directly from it.
Leave a Reply