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.
Exactly why do magnetic encoders have higher inherent reliability, and what do the reliability numbers really look like? Let me explain why they are reliable, and it’s not just because they can handle dirt and debris in the air gap.
There are some other inherent factors:
- Magnetic encoders are a system on a chip. The one sensor chip takes in the magnetic field and does all processing right to the output signals. Other technologies often require multiple chips (optical) and/or extra wires (resolvers) or even separate sensor and processing chips. One chip means less component and interconnects to cause problems.
- One tough magnet. The magnet is robust compared with a delicate optical disk. Timken sensor magnets are being used in the harshest environments, where an optical disk would never perform.
- Works in high temperatures. Both the chip and the magnet are inherently shock and vibration resistant and easily handle temperatures up to 125 C or more. Designed for high temperatures, which means lower temperature operation is no challenge at all.
- Allows larger tolerances without failure. Reliability is determined not just when things go right, but when things go wrong as they often do in the field. This means when tolerances aren’t met: axial, radial or other. Timken sensor-to-magnet tolerances are much larger than those of both other magnetic technologies and optical sensors. The sensor-to-magnet tolerances are forgiving and handle the job much better when things go wrong. This means higher reliability for the entire system.
- Outstanding track record. There are over 12 million Timken sensor chips in the field, most of which are automotive. Timken’s MPS160 sensor has undergone the rigorous automotive AECQ100 qualification. The qualification is severe and thorough, pushing components beyond their specified limits. The actual accepted die failure rates are in the low single-digit PPM range, an impressive track record.
So what are some of the applications that really test the reliability? Here is a sample of some of the harsh applications it is being used in:
- Mining trucks
- Farm equipment
- Off-road transmissions
- Robots – civilian and military
- Box-making machines
When you look at all the factors that make Timken’s magnetic sensor technology reliable, why take a chance? Go with Timken magnetic encoder technology.
Questions? Call me at 603.358.4737 or send me at email at [email protected]. Please join our LinkedIn Group to continue the discussion.
Maybe the benefits of using magnetic encoders over optical encoders have captured your attention. They have greater durability and reliability, an ability to withstand harsh environments, higher vibration and shock tolerance, withstand higher temperatures, come with much larger gap tolerances and come in compact packaging.
But motors and many industrial environments have high magnetic fields. What happens to a magnetic encoder when it is used in a high magnetic field? Are any of the advantages or its performance compromised?
Most magnetic sensing technology is differential. The sensor detects the difference between the magnetic north and south poles. Stray (common mode) magnetic fields are cancelled. At Timken, we’ve tested the quadrature signals to more than 800 gauss common mode field.
Some magnetic encoders do appear to handle stray fields better. Look at the chart below for a comparison of some of the leading magnetic sensors/encoders on the market that we have tested. Only two of them show no effect from the stray magnetic fields – see the green and blue lines. (The Timken encoder is the blue line at the bottom). In fact, Timken sensing technology is successfully buried inside motors on a regular basis where there are large magnetic fields.
Magnetic encoders have a number of advantages and stray magnetic fields won’t compromise the performance if you’ve chosen the right one.
Magnetic encoders from Timken offer the highest accuracy and are resilient in the most challenging environments, including those with high magnetic fields. Questions? Reach me at 603.358.4760 or send me at email at [email protected]
In industrial automation and motion control applications, selecting the proper encoder technology is key to maximizing performance. Optical or magnetic, which to choose?
Optical encoders perform well in clean environments, but what about those that are dirty, hot, humid or subject to high levels of vibration? Are magnetic encoders a better option in those cases? Let’s take a closer look at each.
Optical Encoder Technology