Encoder resolution and accuracy

Sensor resolution and accuracy on the understanding that we can use the machinery used three pointer watches play such an analogy: the resolution of the hour is the hour, minute resolution is minutes, seconds resolution is second, his eyes quick response through the spaces between the second hand in seconds, we can even tell to about 0.3 seconds, which is three pointer needle mechanical watches can be done; while precision what each watch is the accuracy of the standard time, which is each watch is different, or at different times using all different (faster or farther and farther and slow), roughly between 1-30 seconds.

Similarly, in using of rotary encoder, the resolution and accuracy are two totally different concepts.  Encoder resolution of the encoder is read and output to a minimum angle variation, the corresponding parameters are: number of grooves per turn (line), the number of pulses per revolution (PPR), the minimum step (Step), bit (Bit) and so on.

The accuracy of the encoder, the encoder output is the signal data of the real angle of the measurement accuracy of a corner of the corresponding parameter are arcmin (‘) , arcsec (“).

Resolution: cable, encoder optical code disc engraved lines, if the encoder is a direct square wave output, it is the number of pulses per revolution (PPR) (Fig. 1), but if it is positive cosine (sin / cos) signal output , the analog signal can be achieved through changes in electronic subdivided get more output square wave pulse PPR (Fig. 2), the square-wave output encoder with A and phase B, the equivalent of people left and right eye, A phase B difference of 1/4 pulse period, you can determine the direction of rotation can also be rising and falling edges of the judgment, you can get 1/4 pulse cycle changes step (4-fold), which is the minimum measurement step (step), so, strictly speaking, the smallest measuring step is the resolution of the encoder.

For example, Germany’s ROD426 Heidenhain 3600 line encoder, square wave output is 3600ppr, pulse period 0.1 degrees, through the A phase B phase four octave, the availability of 0.025 degrees measuring step; And it provides precision parameters 18 arc seconds (0.005 degrees).

Cosine signal sincos, by changing the amount of phase angle subdivision

If ROD486 3600 lines cosine signal output, can be 25 times the electronics segment, access to 90000 pulses (ppr), 0.004-degree pulse period, through the A / B phase four octave, minimum 0.001 degrees obtained measuring step resolution, while its original encoder accuracy of 18 arc seconds (0.005 degrees, excluding subdivision error).

Communication data output type encoder or absolute encoder, the output resolution is based on the number of “bits” to express, that is a power of 2 dividing the circumference of degree.

Rotary enocder accuracy and resolution has a little relationship, but not at all. For example, Heidenhain’s ROD400 series, the 5000 line, the engraved lines provided by HEIDENHAIN accuracy is engraved line width of 1/20 (with the resolution related),6000-10000 line with an accuracy of 12 arc seconds (regardless of resolution). The HEIDENHAIN angle encoders RON, the same is 9000 lines -36,000 lines, its RON200 series of precision is 2.5 to 5 arcsec, RON700 series is 2 arc seconds, RON800 series is an arc second, RON900 series 0.4 arcsec, are not determined by the resolution. In fact, the impact of the accuracy of the encoder has the following four sections:

A: optical portion

B: mechanical parts

C: electrical part

D: Using the installation and transmission receiving portion, the accuracy decreases after use, the mechanical part of the deviation itself.

A: Optical part impact on accuracy

Optical code disc – the main thing is the motherboard precision engraved lines per revolution, engraved lines accuracy, consistency groove width, edge finishing and so on.

Light-emitting source – parallel and consistency of light, light attenuation.

Light-receiving unit – read angle, read the response.

After use of the optical system – pollution attenuation.

Such as an optical encoder, the first motherboard reticle precision, Heidenhain’s motherboard is said to be several tens of meters in the ground processing of dual suspension working chamber, for a variety of external factors, reduced to a minimum, even taking into account the wave the sub-sonic vibrations and distant car engines, for many encoder manufacturers even purchase their motherboards. Secondly, the process of processing, optical imaging time, temperature, physical and chemical change, pollution, etc., will affect the code disk engraved line width and marginality. Therefore, even if the code is the same as the number plate engraved lines, each accuracy can be also different.

B: Encoder mechanical part on the accuracy:

Axis machining accuracy and mounting accuracy.

The accuracy and precision of the bearing structure.

Code disk installed concentricity precision optical formation installed.

Install anchor points and shaft concentricity.

For example, the bearing structure, the single bearing support structure bearing the deviation can not be eliminated, but the deviation will be greater after use, while the double bearing structure or supporting structure can effectively reduce the deviation of a single bearing.

C : Encoder electrical part on the accuracy:

Power stability and accuracy – the light emission of the source and receiver unit.

Reads the response and the error caused by the electrical processing circuit;

Electrical noise impact, depending on the encoder electrical system anti-jamming capability;

For example, if the electronic segment, will bring the error, in accordance with the description provided by the German HEIDENHAIN HEIDENHAIN encoders electrical breakdown cosine curve errors and errors in the original engraved line width of about 1% or so.

D: Encoder uses to bring the Accuracy of:

When installing the measuring concentricity of the shaft coupling;

Interference with the signal output cable delay (longer distances or faster frequency);

The response receiving device the receiving device may be internal processing errors.

Speed ​​of rotation of the encoder dynamic response bias.

The most common is to use our own installation methods and results of the installation bias.



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