For linear encoders, resolution designates the number of measuring units per distance inch or millimeterwhereas for rotary encoders, it refers to the number of measuring units per revolution also called pulses per revolution, or PPR or per degree of angle.
Magnetic encoders use permanent magnets placed on the edge of a rotor, with hall sensors that detect changes in the magnetic field as the alternating poles of the magnet pass by.
The more magnetic poles and sensors an encoder has, the higher its resolution. The number and pattern of opaque and transparent sections determines the resolution. Optical encoders typically have between and segments per revolution, which means they can provide between 3. Incremental encoders produce one or two square wave pulses, termed A and B.
When only one pulse is produced, the encoder can detect position. For detecting both position and direction, encoders use quadrature output, which produces two pulses—A and B—that are 90 degrees out of phase. Direction is determined by which channel is leading and which is following.
Some incremental encoders also produce a third channel often termed Z with a single pulse, which is used as the index or reference position for homing. With X1 encoding, either the leading aka rising or the following aka falling edge of channel A is counted.
If channel A leads channel B, then the rising edge is counted and the movement is forward, or clockwise. If channel A follows channel B, then the falling edge is counted, and the movement is backwards, or counterclockwise. With X2 encoding, both the leading and trailing edges of channel A are counted. This doubles the number of pulses counted for each rotation or linear distance increment, which provides twice the resolution. X4 encoding counts both the leading and following edges of both channels A and B, which quadruples the number of pulses and provides a four-fold increase in resolution.
With X4 encoding, both the leading and following edges of channels A and B are counted. Image credit: National Instruments Corporation.
For rotary encoders, position is calculated by dividing the number of edges counted by the product of the number of pulses per revolution and the encoding type described above 1, 2, or 4and then multiplying the result by in order to get degrees of motion. For linear encoders, position is calculated by dividing the number of edges counted by the product of the number pulses per distance and the encoding type.
This result is then multiplied by the inverse of the pulses per millimeter or per inch. Absolute encoders have multiple concentric rings, or tracks, of opaque and transparent segments on the encoder disk. These tracks start in the middle of the disk, and as they go outward, each track has double the number of segments than the previous track.
The first track has one transparent and one opaque ring, the second has two of each, the third track has four of each, and so forth. Absolute rotary encoders are further distinguished by whether they are single-turn or multi-turn. A single-turn encoder uses one code disk, and the digital values for position are repeated for each revolution of the encoder. When the measurement is conducted over more than one revolution, a single-turn encoder has no way to determine how many turns the encoder has completed.
Multi-turn encoders do not repeat the digital position value until the maximum number of encoder turns—typically —is reached. The most common type of multi-turn encoder is an optical version that uses multiple disks that are geared together. Resolution for this type of encoder is the sum of the output of each disk.
So if the primary disk gives 12 bit output, and two secondary disks give 4 bits of output each, the total encoder resolution will be 20 bits, or 1, unique digital position values. Can you please guide me in this calculation to achieve. You must be logged in to post a comment. Image credit: Dynapar.
Optical encoder disk. Comments Hello, Thanks for your information. Leave a Reply Cancel reply You must be logged in to post a comment.Rotary encoders inherently monitor the displacement or position but can be used to measure linear distance by calculating the number of pulses compared to the known number of pulses per arc length and designed into a system that returns linear feedback. For a detailed discussion of distance measurement using draw-wire encoders, see the article How to Measure Distance with Draw Wire Encoders.
Here, we focus on length-measuring applications, reviewing best practices for specifying and implementing these systems. Length measuring applications are common in industry, from cutting pieces of plastic web for potato-chip bags to sawing rough planks into two by fours, to tracking distance traveled along a curb by road-paving equipment. Each use case provides its own set of requirements for feedback. A measuring wheel or follower-wheel encoder consists of an encoder wheel mounted on the shaft of an encoder or vice versa.
The wheel interfaces directly with the surface of the material being measured. As the material moves relative to the encoder, the wheel turns, rotating the encoder code disc and generating a signal. To calculate the length traveled L inches using the output from an incremental encoderwe start by calculating the number of pulses per 1 in.
Then length L is given by:.
Depending on the application, the encoder measuring wheel may be fixed, as for a paper measuring system in a converting line or a printing line. Alternatively, the follower wheel may be in motion while the material being evaluated remains fixed. Examples of this case include roadways for paving applications or the side of a hoistway in an elevator shaft. The most important aspect of measuring length with a measuring wheel encoder is avoiding slippage. The encoder wheel needs an adequate coefficient of friction relative to the material being evaluated.
The coefficient of friction can be adjusted by a combination of the structure of the wheel surface and its materials. In the case of measuring lengths of silk fabric in a large industrial cloth-cutting operation, the wheel might be fitted with soft rubber fingers. In a hay baler, the device would use wheels fitted with long metal teeth. The spikes catch in the hay, turning to measure out the length of a bale so that the machine knows when to cut it and band it.
On a machine for applying asphalt sealant to roadways, the follower wheel may simply be finished with a treaded O-ring. Another way to prevent slippage is by adding a preload. The preload needs to be carefully balanced, however. If it is too low, the wheel might skip or slip, resulting in lost motion.
If the preload is too high, it can damage the product being measured. It can also apply excess load to the bearing, resulting in increased wear and, potentially, premature failure.
Overhung loads can be a risk in follower-wheel applications. Excessive overhung loads make the bearing a fulcrum and therefore susceptible to premature failure. A simple example of an overhung load is in a belt and pulley system.
In this use case, the unit would apply a toothed follower wheel that would engage with the belt. The belt tension controls the load on the wheel: If the belt is too loose, the wheel can slip; if it is too tight, it has the potential to damage the encoder.
Another common use case would be tracking the movement of a gantry. The encoder would be mounted on the trolley with the wheel pressed against the frame. Rack and pinion designs require care to avoid damaging the equipment or potentially compromising the measurement. Here, too, excess overhung load can damage the encoder. The problems can be more subtle than that, however.
If the rack is mounted to the machine frame, it can transfer machine vibration to the encoder via the pinion. This could introduce an artifact into the data at the resonant frequency of the machine. Even in the absence of an overhung load, problem applications abound, such as in the railroad industry where maintenance vehicles use follower-wheel encoders to measure track length.When industrial motion applications require detection of direction in addition to speed, quadrature encoders provide a reliable solution.
A quadrature encoder is an incremental encoder with 2 out-of-phase output channels used in many general automation applications where sensing the direction of movement is required. Each channel provides a specific number of equally spaced pulses per revolution PPR and the direction of motion is detected by the phase relationship of one channel leading or trailing the other channel.
The code disk inside a quadrature encoder contains two tracks usually denoted Channel A and Channel B. These tracks or channels are coded ninety electrical degrees out of phase, as indicated in the image below, and this is the key design element that will provide the quadrature encoder its functionality. In applications where direction sensing is required, a controller can determine direction of movement based on the phase relationship between Channels A and B. As illustrated in the figure below, when the quadrature encoder is rotating in a clockwise direction its signal will show Channel A leading Channel B, and the reverse will happen when the quadrature encoder rotates counterclockwise.
Quadrature encoders are used in bidirectional position sensing and length measuring applications. An error in count could occur with a single-channel encoder due to machine vibration inherent in the system. As subsequent mechanical shaft vibration forces the output back and forth across the edge the counter will up-count with each transition, even though the system is virtually stopped.
By utilizing a quadrature encoder, the counter monitors the transition in its relationship to the state of the opposite channel, and can generate reliable position information. Counting both leading and trailing edges of both channels A and B channels of a quadrature encoder will quadruple x4 the number of pulses per revolution.
This technique is known as encoder and will depend on how the signal is decoded through the users drive, PLC or Controller. As a result, 10, pulses per turn can be generated from a 2, PPR quadrature encoder.
Likewise, 40, pulses can be generated from a 10, PPR quadrature encoder. By triggering on the rising and falling edges of the pulse train, we can double or quadruple the counts per revolution from the same quadrature encoder disc. This technique can be an effective way to increase resolution without changing the code disc.
However, it requires a well-behaved square wave output for effective detection. Care should be taken with choice of output driver; particularly over long cable runs or in noisy environments. The accuracy of the quadrature encoder output should also be taken into account as this will also be multiplied by the encoding factor. Like all encoders, choosing a quadrature encoder starts with your application. If your motor has a shaft, a shafted encoder with a coupler can be used.
A hollow shaft encoder is another option mounted with the motor shaft going through the encoder for more accuracy. If your motor is used in a contaminated or dirty environment or you are using a large vector motor, a bearingless or magnetic encoder will provide the most reliable feedback. Rotary Encoders. Condition Monitoring System. Spare Parts and Accessories. Best-In Class Analytics.
calculating ppr for encoder
Learn More. Knowledge Center Technology Quadrature Encoders.Image credit: Encoder Products Company. For incremental encoders, resolution is typically specified in pulses per revolution PPRor, in the case of linear encoders, pulses per inch PPI or pulses per millimeter PPM. These square-wave pulses are very precisely spaced, and the encoder determines its position by counting the number of pulses generated during a movement.
The resolution of a linear encoder can also be specified in terms of microns, which refers to the distance between pulses. When an incremental encoder outputs just one set of pulses, only position can be determined — not direction.
With two sets of pulses, the encoder can determine both position and direction, based on which channel is leading and which is trailing. And, with quadrature output, any one of three types of encoding can be employed: X1, X2, or X4.
X4 encoding counts both the rising and falling edges of both channels A and B. This provides a four-fold increase in resolution, since now, four edges are counted. With X4 encoding, both the rising and falling edges of channels A and B are counted. Image credit: National Instruments Corporation. When a rotary encoder is used to measure linear distance, the required encoder resolution PPR can be found by dividing the lead of the screw or pulley distance traveled per revolution by the linear resolution required by the application.
For example, if the required linear resolution is 10 microns 0. For example, if a PPR encoder has a maximum mechanical speed of rpm, a frequency response of kHz, and is used with X1 encoding, its maximum electrical speed will be rpm. In this case, the maximum electrical speed rpm is less than the maximum mechanical speed rpmso electrical speed is the limiting factor. In the example above, if X2 encoding is used, then the PPR will beand the maximum electrical speed will drop to rpm.
You must be logged in to post a comment. You may also like: Defining the compliance of electromechanical linear actuators Fighting cancer with precision motion control What is quadrature encoding? Servo motor or stepper motor? How to choose How encoder resolution is determined.Incremental encoders determine rotary position by generating a specific number of pulses per revolution PPR and counting those pulses as the encoder spins.
But how do you determine what PPR is needed for a specific application? Conversely, for an encoder with a given PPR, the resulting linear resolution is calculated by dividing the screw lead by the PPR. If the travel is being measured by use of a wheel or roller, a calibration constant might be necessary, depending on the required display resolution.
Using a calibration constant or scaling factor has the drawback of introducing a rounding error that will accumulate over many cycles of the encoder.
To avoid this, choose an encoder whose PPR is an even multiple of the value that is being measured. For example, if one revolution of the encoder equals 12 inches, choose a PPR encoder. The mechanical speed limit is based on the maximum speed that can be obtained without causing potential damage to the encoder. The electrical speed is 60, rpm, so the mechanical speed, at rpm, is the limiting factor.
You must be logged in to post a comment. Leave a Reply Cancel reply You must be logged in to post a comment.Please Log in or Create an account to join the conversation. Toggle Navigation. Index Recent Topics Search www. Encoder ppr? Start Prev 1 2 Next End. My first question concerns the encoder.
How many pulses can you recomend? My plan is to reuse my Contraves DC servon. Is there anybody who has tips on 6mm axle pulse encoder?
How encoder resolution is determined
Attaches images with dimensions and mounting on current sensors with wrong signal. Have a look at these encorders: www. Just chose one for example. How many ppr are usually?
Have searched but not a good answer. Hej du. I think most common are ppr or there about, the ones I have on my AC servos are ppr. How fast do the servos rotate? If you use the 7i77 then I think you can count pulses at 10MHz.
Also calculate what spatial resolution your encoders give you. Much less than the width of an atom and you are wasting money.
I am using a photoelectric switch to detect number of pulses per revolution. I know I have to look at the response time of the switch as well as the max. Is this right so far? If I choose 8 ppr because for my application's current ppr is 6 I am not sure if this is right but any pointers in the right direction welcome!
Scroll to continue with content. Hi, I think you're a bit lost here. By the way, your equations look right to me until the last one or two. If you want revolutions per minute, you would need 3. It isn't rocket science. I think you may be getting lost when it comes to mating the feedback portion rotary encoder to the motor drive portion, and there are lots of way's to make this work.
If you want constant speed, you'll need to have a stable system, requiring compensation in the feedback loop. This is getting way beyond basics here and into 3rd and 4th year BSEE courses in control systems, analog and digital. If you have more specific questions, reply and I'll answer them.
Regards, Kamran Kazem. Thank you for your reply! I get the idea. I just need to put them in equations and finding the value. The max. My photoelectric switch has a response time of 1ms getting a faster us response time one in a few weeks. Which is within my response time of the switch to detect consequent pulses at max speed. So for slower speeds I need not be too concerned by choosing 8ppr. Last edited: Apr 30, Thanks rjenkins!
Always appreciate your thoughts. Currently, we use 6 ppr and we are able to make it work. Obviously, with slower speeds, the error is higher than we want but the encoder works nonetheless. So, my question is do you think if a photoelectric switch has a response time of us, I can add more notches without losing pulses or the pulse wave turning into a ripple wave?