Have you performed a good tolerance stack up? What happens to your system performance if you slip beyond the tolerance limits? The costs of holding tighter tolerances often add up to much more than one might expect. Let’s look a little closer:
Here is an accuracy comparison of radial position tolerances for on-axis versus off-axis magnetic sensors:
The first question you should ask is, “What accuracy do I need?” Most of the encoder specifications for servo systems that I see are toward the bottom of the graph. Can you live with errors in excess of this? If you believe you can, then note the sharp increase in error as the on-axis sensors get even slightly off dead center.
The tolerance stack-ups I have done often include 10 or more individual parameters. They include PCB tolerances, chip and die placements, magnetic field errors, temperature effects on magnetic fields, Hall trigger points, electrical tolerances, magnet off-center plus all the normal mechanical tolerances. There is a cost associated with holding these tolerances and it often gets overlooked because it is spread out among different manufacturing groups and suppliers. There are also many hidden long-term costs when suppliers are pushed to hold tighter tolerances. Problems arise over time and the costs to fix problems once in production can often be very big.
We all know the best solution is to design a system with large, forgiving tolerances. But how do you do this when everything is dependent on the sensor’s tolerances? The answer is to choose a sensor and target magnet that can handle large placement tolerances. The acceptable radial tolerances for the off-axis magnetic sensors are large in comparison to the on-axis sensors. When there is a manufacturing issue, the off-axis will handle the problem, but the on-axis will cause large errors. These tolerance problems often can be hidden until your customer discovers them in the field, which is not a good scenario.
When looking at optical encoders, the same holds true in the axial direction. The large axial (air gap) tolerance for magnetic sensors is frequently 5-10 times larger than the tight air gaps required for optical sensors.
When choosing a sensing technology, do your homework to determine your real costs. Consider hidden manufacturing cost, supplier’s costs and the day-to-day cost to manage the issue. Don’t lock yourself into a corner that you can’t get out of. Choose the technology that can handle large tolerances in both radial and axial directions. Off-axis magnetic technology can offer that advantage.
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