When you’re choosing an encoder for a motion control system, you’ll be faced with numerous technical terms. Which ones should you focus on first and which can be deferred? This article looks at three important concepts that deserve your attention: resolution, accuracy, and precision.
At first glance, it may seem that all three mean roughly the same thing and that they are interchangeable; indeed, many people speak of them as if they are. After all, if an encoder has high resolution, doesn’t that mean it’s accurate? And if it’s accurate, then it has to be precise, right? (The answer to the last two questions is a firm no.)
In fact, the terms are independent of each other. Each refers to a specific encoder characteristic and they are not interchangeable. To clear up any confusion, we’ll first explain what resolution means for incremental encoders, then note any differences for linear and absolute encoders. We’ll move on to accuracy and finish with precision. Along the way, we’ll give tips on how to use knowledge about each term to make the best encoder selection and how to calibrate a system once the encoder is in place.
In math, science, and engineering, the word resolution specifies the smallest distance that can be measured or observed. To make an incremental encoder, a manufacturer creates a disk with a pattern on it (Figure 1). The pattern divides the disk into distinct regions; for example, one common pattern consists of lines and windows printed on a transparent disk. When an LED projects light at the disk, the light strikes either a window or a line. Windows allow light to pass through the disk to a photo sensor on the other side. Lines block the light. As the disk rotates, output from the encoder module — Channel A — is a series of high and low signals; their value depends on whether the photo sensor receives light (high) or not (low).
Resolution, when applied to optical encoders, specifies the number of times the output signal goes high per revolution. This number can match the number of lines on a disk or especially with higher resolutions, it can be a multiple of the number of lines. (We’ll talk about this more in the section on Scalability.) The number of lines on a disk is always related to the resolution. Typical values range from low numbers like 32 or 64 to much higher resolutions of 5,000 or 10,000 and beyond.
Encoder resolution is measured in units of Cycles Per Revolution (CPR). The word cycle has both a physical and an electrical meaning. Physically, on the disk, a cycle is composed of a line/window pair; therefore, in its most basic form, CPR is the same as the number of lines, the number of windows, or the number of line/window pairs. Electrically, a cycle refers to one full cycle of the encoder’s output waveform: one high pulse and one low pulse. One cycle is equal to 360 electrical degrees.
CPR, then, can refer to the number of lines and windows on the disk or the number of electrical cycles in one rotation. Native CPR will be the same number in either case because each line/window pair is exactly what generates each electrical cycle. CPR also gives us the smallest distance that can be measured. Divide the total distance of 360 mechanical degrees by the number of cycles per revolution and the answer will be mechanical degrees per cycle; for example, with an encoder resolution of 3,600 CPR:
While cycles per revolution is a common term to specify resolution for incremental encoders, some manufacturers use terms like “counts per revolution” (also abbreviated CPR), “pulses per revolution” or “positions per revolution” (both abbreviated PPR), and other phrases. To avoid confusion, in this article, we’ll use cycles per revolution and CPR. In the next section, we will use PPR to mean pulses per revolution but in a different context: resolution multiplication.
The resolution of a disk is tied to physical reality — physical lines on a physical disk. The number of lines, in its most basic form, is the resolution; however, a motion controller can interpret the output waveforms resulting from those lines and produce higher resolutions from the same disk.
Incremental encoders commonly use quadrature. Manufacturers add another LED and photo sensor, displaced from the first LED by 90 electrical degrees (Figure 2). Note that 90 electrical degrees is 1/4 phase or quadrant, which is the origin of the name quadrature. This yields a second output waveform, Channel B, shifted in phase from Channel A by 90 electrical degrees. Two important results emerge from adding Channel B: Direction can now be determined (“A leads B” can indicate clockwise rotation, for example). And more importantly, resolution can be multiplied by a factor of 2 or 4.
This is called resolution multiplication. System designers can implement it by using an encoder to counter interface chip. As an example, let’s look at an encoder with 100 lines and windows on its disk. The encoder’s resolution is 100 CPR.
×1 – If we count the rising edge of each Channel A pulse as the disk rotates, we’ll get 100 pulses per revolution (100 PPR). This is the same number as the resolution of 100 CPR, as expected for multiplication by 1.
×2 – If we count each rising edge and each falling edge of Channel A, we’ll get 2 pulses per cycle, which adds up to 200 pulses per revolution (200 PPR).
×4 – If we count each rising edge and falling edge of both Channel A and Channel B, we’ll get 4 pulses per cycle, for a total of 400 pulses per revolution (400 PPR).
Notice we’re not changing the resolution of the disk; it remains set, as determined by the number of cycles per revolution. But by decoding the output waveforms in different ways, we are able get up to four times as many pulses per revolution as there are lines on the disk.
Everything we have said so far about resolution also applies to incremental linear encoders. This makes sense; linear encoders use a linear strip that is equivalent to a circular disk that has been cut along a radius and straightened out. The term Cycles Per Inch (CPI) is used for resolution with linear encoders, although Lines Per Inch (LPI) is also sometimes used.
Absolute Encoders and Resolution
Thus far, we’ve discussed incremental optical encoders, whose lines and windows represent relative positions on the disk; each line/window pair looks like every other line/window pair. What matters are the high/low output transitions as each line and window goes past the sensor.
Absolute encoders operate differently. They output a unique code for each position on the disk — each code is absolute, which means that since it is unlike any other code on the disk, it specifies a unique, absolute position on the disk. Figure 3 shows a disk for a traditional absolute encoder. It has four tracks and an LED array with sensors that read the pattern from each track.
Resolution for absolute encoders is defined as the number of positions per revolution as the disk rotates through 360°. Sometimes the equivalent term “codes per revolution” is used. You will often see the resolution of an absolute encoder specified in bits; for example, the disk in Figure 3 has 4-bit resolution, one bit being produced from each of the four tracks at each position. Higher resolutions would have more tracks; 10-bit resolution would require 10 tracks, for example.
With some designs, each absolute encoder is set at one specific resolution. Some manufacturers, however, take a different approach and make disks with a single band that contains a unique bar code for each position.
An absolute encoder with a bar code can offer programmable resolution; for example, a 12-bit encoder (4,096 positions per resolution) can be programmed to output from 2 to 4,096 codes per revolution. Table 1 shows how resolution in bits is related to positions per revolution and degrees of rotation per position. For a 12-bit absolute encoder, notice that each unique position occupies less than 1/10 of one degree of the disk’s circumference, which is less than 6 arcminutes. Absolute encoders don’t use quadrature, so there is no equivalent to resolution multiplication that’s available with incremental encoders.