Encoder Resolution, Encoder Accuracy, and System Repeatability

Encoder Resolution, Encoder Accuracy, and System Repeatability

Everything You Need to Know

 Are you looking to buy magnetic encoders in UAE and do not know what to look for in one? Or, are you are unsure of how to go about the whole procedure? This guide looks to give you a soft landing. It is set to discuss some of the most important factors you must be knowledgeable about to help you get the most appropriate encoder for your needs. These factors are;

  • Encoder resolution
  • Encoder accuracy
  • System repeatability

Encoder Resolution

Encoder resolution is the distance by which a single encoder counter occurs. It’s the least distance an encoder can record. It is measured in Pulse per Revolution (PPR) or pulses for rotary encoders. It is measured in nanometres(nm) or macrons in Linear encoders. In absolute encoders, it is identified in bits, as absolute encoders display the words in respect to the encoder's position.

How to Calculate Resolution

An increase by one set pulse measures position only without direction. It is therefore why most incremental encoders produce two channels or sets of pulses phased at an angle of 90 degrees from each other. The technique is called quadrature encoding. It allows for up to three types of encoding;

a)X1 (either the falling or rising edge of channel A)

b) X2 (both rising and falling edges of channel A, making it twice X1)

c) X4 (both rising and falling of channels A & B)

To measure linear distance in a rotary encoder, you divide the distance traveled per revolution by the required linear resolution of the application.

PPR = Lead/Linear Resolution

Note; Remember to divide the PPR by a factor of 4 or 2 when using a lead resolution of X4 or x2 respectively.

Factors That Affect Encoder Resolution

Encoders are based on patterns on the encoder scale or disk, for absolute magnetic encoders and number of line, for incremental encoders. This means that the resolution is physically fixed. Therefore, there is no way to increase the patterns or lines after an encoder is manufactured.

Encoders with a lot of optical discs or slots tend to have higher resolution. This is because the many slots decrease the light on the system as the discs get filled up. Little light has few effects which ensure better resolution.

There is an option though, for incremental encoder by increasing signal decoding through the quadrature encoding. This increases the resolution by a four-factor.

Encoder Accuracy

Encoder accuracy is a measure of the difference between the readout speed/position of the device measured and the actual speed/position of the device being measured. Accuracy is measured in arcminutes in linear encoders and arcseconds for rotary encoders.

Accuracy is determined by how the whole application interacts with the system and can be traced on the encoding disk.

The relationship between encoder resolution and accuracy is such that a higher resolution does not automatically mean a higher accuracy, even though too low-resolution levels may affect the encoder's speed and positional control which reduces the accuracy.

Factors That Affect Encoder Accuracy

Both internal and external factors affect the magnetic encoders’ accuracy. The spacing and width of the units or lines are important in determining accuracy. Inconsistencies in the spacing and width cause errors on the timing of the pulses. The precision by which the pattern is set on the disk or code also ensures more accuracy.

A known amount of internal error can be corrected using the encoder controller. In case of unknown errors, it is advisable to use an external high powered encoder to establish and correct the error such that it gets back to functioning normally.

Some of the external factors that affect encoder accuracy are mounting errors like irregular mounting surfaces, and wrong assembly. On the other hand, irregular expansion of the mounting surface and scale reduces accuracy in linear encoders. To ensure the encoders works as required, it is important that you remove any mechanical errors in the work environment.

Types of Encoder Accuracy Errors

Slope error – this is the highest expected incremental error. It is an error that is easily corrected using the system controller.

Linearity error – this is an error experienced after correction of the slope error. It covers the range of the distance and the accumulating positional error.

Angular error – an error between the driving shaft and the system caused by certain alignment conditions. This can be avoided by ensuring that the encoder angle is firmly mounted on a similar shaft and as the system. Make sure that they rotate on the same axis.

System Repeatability

Repeatability is the ability of the system to return to the exact prompted position. Most encoders provide for 2 to 10 times better repeatability compared to accuracy.

Factors that affect system repeatability

System repeatability errors originate from different sources like the positioning of the equipment. Even though a few encoders might experience cumulative errors after some time of use, most are designed with automated systems that prevent the accumulative error.

Mechanical system errors like hysteresis, gearing issues, and poor tension belts, can reduce the system repeatability.

Even in as much as most users believe that the best way to deal with the encoder errors is by purchasing a system with high repeatability, the truth is that no system can correct the errors caused by poor repeatability. This is because repeatability does not determine accuracy.

An ideal machine should have a way of providing accurate figures, with the ability to repeatedly hit those figures. Looking for systems that have both high accuracy and repeatability might be costly without guaranteed performance. Low accuracy coupled with low repeatability will also not function properly in any setup.

A Finishing Note

Are you ready to optimize performance by choosing an encoder that offers the right balance in repeatability, resolution, and accuracy? Well, you can never go wrong when you contact Dubai Sensors anytime you want to buy magnetic encoders in Dubai. We are a reputable supplier of all system machines with some of the most trusted, affordable encoders around. Our encoders are tried and tested for reliable performance. We also have a team of experts to help you in case of accuracy, resolution or repeatability errors. Contact us now!

1st Aug 2019 S. Abd

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