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].

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
• Tractors
• 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.

A. John Santos, Chief Engineer for Sensors

Let’s say your motor needs a rotary encoder to provide feedback to a controller. Which would you choose: a magnetic or an optical encoder?

In the past, an optical encoder might have been your only choice. Today, you have a second option. Magnetic encoders – once reserved for high-end process-industry applications – are now cost effective for virtually all encoder applications.

Magnetic encoders have benefited from overall advancements in integrated circuit technology. There is a wide range of Hall effect-based high-resolution encoder chips, modular encoders and kit encoders on the market.

Magnetic Encoder Advancements Outpace Optical Technology

The decision to use optical or magnetic technology for your next sensor will likely depend on the environmental and mechanical characteristics of your application. Magnetic encoder technology advancement has been outpacing optical technology for years. This has led to significant improvements in performance and pricing. While optics used to be the only choice for resolutions greater than 1,000 PPR, there are now 1.5 in. modular magnetic encoders and encoder kits available with up to 8,192 PPR.

Optical Encoders: How They Work

Traditional optical encoders have a light source, two or three light sensors, and a glass, metal or plastic code wheel placed between the light source and the sensors. These are the only functional components of an optical encoder. The other components you might find are used to keep contamination like dirt, dust and condensation out of the optical path or used to precisely position the optical disk in the working range between the light source and light sensors. A typical recommended tolerance range for an optical disk is +/- .003 in.

Traditional optical encoders have the optical components packaged in a sealed can with the code wheel precisely positioned using two ball bearings.  The packaged encoder is mounted using a shaft coupling or, in the case of a hollow shaft encoder, using a flexible tether. The housing, bearings and couplings are often the largest and most expensive parts of the assembly.

Magnetic Encoders: How They Work

Modular magnetic encoders can offer the same standard resolutions as optical encoders with greatly improved resistance to environmental conditions such as dirt, dust and humidity. Magnetic encoders consist of a system-on-a-chip encoder chip on a PCB and magnetic target wheel that is set to run with an air gap of between .020 in. and .060 in. above the PCB.

Magnetic encoders typically have much more lax assembly and shaft end play requirements. The ability of magnetic encoders to perform well over a wide range of assembly tolerances have eliminated the need for bearings in most magnetic encoders. This in turn has resulted in eliminating failure modes, lowering component cost and reduced overall axial length may applications.

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

• Optical encoders pass a beam of light through a series of windows in a rotating disk. Any contamination that impedes the light source will cause encoder failure. This includes condensation from fluctuating humidity and temperature.
• Optical encoders require a precise, small air gap, often less than 0.25 mm (0.010 in.). As a result, they not only require very small shaft end play but they are vulnerable to shock and vibration.
• Because of their need for a clean, protected environment, they often are protected by an expensive, bulky package to help isolate them from the elements.

Magnetic Encoder Technology

• Magnetic sensing does not need a clean, transparent gap. Dirt, dust, oil, condensation and other non-ferrous contaminants do not affect their reliability.
• Magnetic encoders also are inherently shock- and vibration-resistant. The sensor-to-target gap is large compared to that of an optical encoder, and can be as large as 4 mm (0.157 in.). They can handle shaft end play as high as 2.5 mm (0.98 in.) total.
• Modular magnet encoders do not require an expensive, ruggedized package because the magnetic technology can handle dirty and harsh environments up to 135° C (275° F). Digital quadrature output signals are the same as in an optical encoder and their accuracy is sufficient for most applications where optical encoders are typically used.

The Most Attractive Choice

Magnetic encoders offer a number of clear advantages over optical encoders including:

• Greater durability
• Higher reliability
• Compact packaging
• Much larger gap tolerances
• Higher temperature range
• Harsh environment operation
• Higher vibration and shock tolerance

When selecting the proper encoder technology for an application, engineers should consider environmental, packaging, resolution and cost requirements. In cases where performance requirements can’t be compromised and the operating environment could be challenging, magnetic encoder technology can meet a customer’s needs at a relatively low cost.

Learn more:

Read our white paper, “The Advantages of Magnetic Encoder Technology in Harsh Operating Environments.”

Ask me about our success in harsh duty automotive, off highway and industrial applications.  Reach me at 603.358.4760 or send me an email at [email protected].

On Axis
As the names suggest, the on axis design has the sensor located directly over the center line of the shaft.

On Axis encoders

• Sensor and magnet must be aligned with the center line of the rotating shaft.
• Button-shaped rare earth magnet mounted directly on the end the shaft.
• Can offer absolute position information and/or quadrature signals

Off Axis
The off axis design has the sensor sitting off to the side of the shaft with the magnetic target in the form of a ring.

Off Axis encoders   (Timken M15 shown)

• Do not block access to the end of the shaft.
• Shorter installed height; more tolerant of sensor and magnet position at installation.
• Higher accuracy and less latency than on axis designs.
• One chip works with a wide range of pole sizes and diameters.

For a more in-depth comparison see my paper on the same subject.

The charts below offer additional guidelines that may make your choice clearer.

If you still have questions and would like to discuss whether an off axis or an on axis encoder is the right choice for your application, call me at 603.358.4760 or send me at email at [email protected].

• New advancements in magnetic encoding technology have resulted in the development of compact, low-cost encoders that are more tolerant of harsh, dirty operating environments. However, on-axis magnetic encoders have poor accuracy. It’s not unusual for them to have much lower accuracy than their resolution. For example, a 14-bit on-axis sensor may provide transitions every 0.02 degrees, but the error could be 50 times larger than this.
• Conversely, off-axis magnetic encoders provide much higher accuracy than on-axis magnetic encoders. Timken engineers have patented magnetic encoder designs that use Hall effect technology to obtain high resolution from a durable magnetic target disc.

Rotary encoders provide position feedback in all sorts of systems. The demand for encoders continues to grow as customers demand higher efficiency and smarter motion control.

In the beginning
In the old days, encoders were baseball-sized metal cans containing a glass disk with opaque metal lines. Other parts included a set of bearings and a shaft used to mount the glass disk. One side of the disk had an incandescent or LED light source and lens assembly; the other reverse had a metal grating mounted in front of two or three light sensors.  More...