Why are optical encoders so expensive

The system monitors which channel goes high first, which allows it to differentiate between the two directions. The most common implementation of a quadrature encoder involves a separate concentric track for each channel, clocked to introduce the phase lag. Each channel has its own detector.

The output of a quadrature encoder can be decoded in multiple ways. The exact technology and terminology differ depending on the type of encoder and circuitry being used. The simplest form triggers off of the rising edge of the A channel, which gives it a resolution equal to the PPR of the code disk see figure 4.

This is also known as 1X decoding. We can increase the resolution by triggering off of both the rising and falling edges of channel A 2X decoding. This effectively doubles the resolution, giving pulses per revolution from a PPR encoder, for example. To further enhance the performance, the encoder also can be set up to trigger off the rising and falling edges of both channel A and channel B 4X decoding.

Now, we can get pulses per revolution from that PPR disk. Optical mask and phased-array encoders As resolution rises in quadrature encoders, a problem emerges: The detectors tend to be large compared to the width of each slot, which can lead to spillover between channels. Adding a mask that matches the pattern of the channels helps clean up the signal see figure 5. Such tight spacing imposes strict specifications on disk parameters such as flatness, eccentricity, and alignment, and makes the devices more vulnerable to shock and vibration.

A mask can also be used to create a quadrature encoder from a single-track disk. This approach can be used to compensate for thermal variation by removing common-mode signal variation.

At low resolutions, encoder disks can be cut out of metal, with the gaps forming the transparent slots. Metal disks can be a particularly good solution for high shock and vibration environments, but their resolutions are limited--if the slots are spaced too closely, the disk will lack mechanical strength. Glass substrates patterned with metal deliver higher resolutions, but their fragility makes them a bad fit for high shock and vibration applications.

Mylar substrates are tougher than glass in many ways. Even they have limitations, however--improperly mounted, the disks can sag and they can oscillate at very high RPMs. Phased-array incremental encoders use solid state technology to provide a more robust solution. Instead of a discrete detector for each channel, a phased-array encoder features an array of detectors integrated at the chip level with an application-specific integrated circuit so that each channel is covered by multiple detectors see figure 6.

This approach averages the optical signal, minimizing variations introduced by manufacturing errors like disk eccentricity and misalignment, improving performance while relaxing fabrication tolerances. The approach also increases the air gap allowed between the disk and the phased-array chip, making the devices easier to build and install.

Absolute optical encoders An incremental encoder generates an output of pulse streams that can be converted to position or speed. The incremental method and the absolute method were explained in detail in part 3. The resolution of an optical encoder is basically determined by the number of slits in the code wheel.

Therefore, it is necessary to increase the number of slits in the code wheel in order to achieve high resolution, but it is necessary to reduce the area of each slit in order to be compatible with the miniaturization of the encoder. As a result, high precision is required for assembling the components, and physical limits are reached somewhere. In order to further improve the resolution, there is a method of "electrical interpolation" the A phase and B phase of the output signal by using a pseudo sine wave signal instead of a pulse signal.

In this way, the optical encoder can realize high resolution and high accuracy by optimizing the structure of LED, code wheel, photo sensor, etc. The optical encoder has the advantage that it is easy to improve accuracy and resolution by devising the shape of the slit becasue it has a mechanism that detects whether light passes through the slit or not. Therefore, it is used for servo control and hollow through shaft type motor control that require high precision.

In addition, since it is not affected by the surrounding magnetic field, it can be used in applications where a strong magnetic field is generated. Therefore, it is used in devices that use large-diameter motors. In this part, we introduced the principle and advantages of optical encoder.

I hope you understand the principle and advantages, and the applications in which optical encoders are used. In the next part, we will explain the principle and advantages of the magnetic encoder.

Principle and advantages of optical encoder. Structure of optical encoder. Principle of optical encoder. Advantages and applications of optical encoder. These discs were about mm in diameter. When I was looking at accurate position sensors an intriguing one used a rotating disc with lines and measured the time taken to move from sensor to sensor.

Time, or frequency, is the most easily measured physical phenomena of course. CD ROM? Thanks for all the input guys, I do appreciate that. The question of accuracy versus resolution is an important one and part of the issue is how you use the encoder. My understanding of how survey crews operate there 1sec total stations to get the most accuracy out of them is to do field calibration every time they do a new setup.

First they level to within 15 seconds or so using the tribrach knobs but then, to get better than that they shoot an object then rotate the scope degrees in elevation and azimuth and re-sight the same object. Do that again at a target about 90 degrees different in az and the smarts in the total station now knows where level is within less than an arcsec.

So, although pretty much all things are less accurate then the resolution would indicate but with appropriate operation you can claw back a good bit of the difference. Dubbie Supporter Posts: Country:. Are you absolutely sure you need an absolute encoder? For the price difference, you could afford to keep a zeroed incremental encoder powered on permanently.

Quote from: branadic on May 07, , pm. Quote from: Dubbie on May 07, , pm. Quote from: raptor on May 07, , pm. That's a very hard spec for any encoder to meet, much less a "low cost" one. But, that speaks only resolution and nor accuracy or repeatability. I just built a DRO for my mill rotary table using a 80, count encoder and I thought that was high. Pages: [ 1 ] 2 Next All Go Up. There was an error while thanking.

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