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.
Questions? Call me at 603.358.4760 or send me at email at [email protected]. And please join the discussion on our LinkedIn Group.
When it comes to saving money on inventory while assuring that customers get exactly what they need for motor feedback, off-axis magnetic encoders can help both the motor OEM and their customers win. This type of magnetic encoder is capable of handling up to 80 – 90% of servomotor and closed loop stepper motor applications while providing the durability and reliability that they are well known for.
How do they work?
There are several good reasons to choose off-axis magnetic encoders for standardization.
First, whether the application is in a challenging environment or not, off-axis magnetic encoders can handle both environments, without expensive packaging options that limit encoder choices in most applications. Magnetic encoder technology is resistant to contamination, including dirt and moisture. This allows for an economical choice across a broad range of applications that may not even be known by the motor manufacturer.
Second, some magnetic encoders can offer on-site programmability at the motor manufacturer. There is no need to purchase numerous encoder configurations to satisfy a multitude of customer needs. This technology features multiple resolution programming in combination with just a few magnetic target disk configurations to achieve nearly all common resolutions. Modular components can be purchased, avoiding the headache of stocking expensive, prepackaged fully assembled encoders.
Third, many motor OEM’s choose to customize their encoder solutions by building the encoder chip and target directly into the motor itself. This type of chip and target solution is great for reducing the overall motor size, saving material cost and allowing compact motor envelopes to fit in applications restrained by size.
Finally, many of these OEM’s don’t like to tie money up in costly inventory, particularly for expensive, environmentally protective encoder packaging. While this does save money, it can create prohibitively long lead times in getting finished motor product to the customer. Standard off-axis modular encoders and components that work in tough environments can reduce lead times and provide the reliability customers expect from more expensive, fully packaged encoders.
For more information on how to cover more applications, reduce your production costs, inventory expense and lead time with Timken off-axis magnetic encoders, reach me at 603.355.4502 or send me an email at [email protected].