I often see some confusion in various customers' minds regarding encoder performance with automation systems which have servo drives or variable frequency drives (VFD). Some customers feel that when they are providing best in class encoders, the system must be highly accurate.
Sounds logical? … Yes, but there are some other points to consider also.
There are 3 commonly used terms associated with encoder based systems- Resolution, Accuracy, and Repeatability.
When it comes to judging performance of encoder based systems, all three are important and the weakest of all three determines the actual performance of the entire system.
- Resolution - The smallest position increment that an automation system can detect. Each encoder has a resolution parameter generally measured as 'ppr' (pulses per revolution)
Desirably the encoder should have maximum possible resolution.
- Accuracy - The maximum expected difference between the actual and the set (desired) position/speed for a given input.
Desirably the system should have maximum possible accuracy (minimum difference between set & actual value).
- Repeatability - The ability of a motion system to reliably achieve the 'Set / desired' position over many attempts.
Desirably the system should have maximum possible Repeatability.
So even if the user has the best in class encoder with the highest resolution available and best in class drive with the highest accuracy, there still can be a chance of 'not-so-good' system performance due to these two factors:
- Noise: Wiring of encoder to PLC or electrical drive is not as per standard EMC guidelines (e.g. one good practice is to provide adequate shielding to the encoder cable at both the ends). Such wiring can be susceptible to noise or EMI related issues.
- Mounting: Mechanical mounting of the encoder is not done properly with the motor (e.g. bad coupling can lead to encoder "wobbulation"). This can lead to poor repeatability of the output.
Thankfully, Kollmorgen's encoder-based servo motors come with factory fitted encoders, so user need not worry at all about their performance.
But when it comes to installing machine encoders, or encoders with induction motor system, utmost care needs to be taken to ensure that the encoder's mechanical mounting is done properly on the machine & wiring is done to the electrical drive or PLC in accordance with EMC guidelines.
That is the best way to get maximum performance from your encoder system.