by Eric Mueller, Sensor Magnets Product Manager, Dexter Magnetic Technologies, Elk Grove Village, Ill.
Non-contact linear and rotary position measurement is attainable with robust multi-pole magnets
Today’s machinery and motion systems have an increased need for accurate, repeatable and reliable measurement of linear and rotary position. The evolving history of position sensing ranges from simple electromechanical devices like potentiometers to inductive techniques to optical encoders. The latest strategies include a variety of non-contact methods.
In particular, some new magnet strip technologies now allow for cost-effective linear and rotary positioning measurement systems with all the benefits of traditional non-contact systems, including optimal accuracy.
Contact versus Non-Contact
The options that system engineers have to measure linear and rotary motion are quite varied. There are two distinct types of measurement; contact and non-contact. Contact systems are exactly that – they require contact to measure the motion. Conversely, non-contact systems do not require contact. Examples of contact systems include potentiometers and mechanical proximity switches. Non-contact means of measurement include optics and lasers, magnetostrictive, capacitive, inductive and ultrasonic methods.
There are benefits to both methods which depend on the application requirements. For applications demanding long life and high reliability, non-contact sensing is generally a good option. This is due in part to the limited component wear and degradation. Non-contact options limit damage from contact with a work piece and offer the added security of allowing for measuring systems to be encapsulated or potted, protecting them from harsh environmental conditions.
The overarching benefit of a non-contact measuring system is the potential for high degrees of accuracy. When selecting a system for accuracy, engineers come to expect that the higher the system accuracy, the higher the price of the system. The positive correlation of pricing and accuracy is a legacy for measurement systems that has become the leading disadvantage for using a non-contact system. Weighing cost versus benefit is needed to determine whether the system will fit the application and what level of accuracy is needed in the overall design.
The Hall Effect
For a deep-dive into an example of a non-contact positioning system, consider Hall cell technology. The Hall-Effect principle was discovered by physicist Edwin Hall in 1879. He discovered that when an electrical conductor or semiconductor with current flowing through it in one direction was subjected to a magnetic field perpendicular to the direction of the current flow, a voltage could be measured across the conductor at a right angle to the current path.
This induced voltage has three key characteristics. First, its strength is proportional to the strength of the current in the conductor and to the strength of the magnetic field passing through the conductor. Second, the change in the induced voltage with changes in the current and magnetic field strength is repeatable. Lastly, this phenomenon is measurable. This basic principle can be applied to the development of a low-cost non-contact sensor to measure linear and rotary motion. To measure the movement of one location, surface or component relative to another, a Hall sensor is attached to one location and a magnet is attached to the other. As the two change position relative to each other (either linear or rotary), this change is directly measured by the change in the induced voltage across the Hall sensor. This measurement is achieved in a completely non-contact environment.
Unlike traditional non-contact systems, one of the limitations of measuring linear position with Hall sensors is the limited range and accuracy of measurement due to limitations in the magnetic field of a “traditional” bipole axially or diametrically magnetized magnet with a single north and south pole. The strength of the magnetic field of this sized bipole magnet drops below the required minimum for sensor manufacturers. For example, a standard for Austriamicrosystems (AMS), a leading manufacturer of multi-pole Hall sensors, states that magnetic strength needs to be maintained within 0.3 to 0.5 mm from the surface of the magnet, depending upon the specific magnet used. To increase linear measurement distance and accuracy when using a Hall sensor, a magnet with multiple north/south poles with very precise pole lengths is needed.
This knowledge has led to the development of a multi-pole magnet which would fulfill the market requirements for accuracy, cost and field strength in a continuous pattern of repeating north and south poles. The inherent design of the multipole magnet makes the length of possible linear measurement limited only by the length of magnet that can be physically made.
Multipole Magnet Evolution
The development of accurate, low-cost, linear and rotary position measurement systems not only required development of the multi-pole magnets but also an analysis and development of the Hall sensors that use the magnets. Looking at the needed research and development effort, Dexter Magnetic Technologies partnered with AMS for the Hall sensors. Dexter’s expertise is grounded in magnetic materials and the science behind magnetic attraction. AMS was selected as a partner for the Hall sensors because the company has developed a family of non-contact high-resolution magnetic encoders for accurate linear and off-axis rotary sensing. The AMS sensors offer a measurement resolution of <0.5 micron. A multi-pole strip magnet or ring magnet with a pole length of 1.0 mm, 1.2 mm, or 2.0 mm (length is dependent upon sensor selection) is required for the sensor to operate properly.
There are several requirements of these magnets for an overall measurement system to be successful. First, the magnets must be made of a material that can be magnetized to sufficient field strength to meet the requirements of the Hall sensors. Secondly, they must also be made of a material that can be tailored to different lengths. The length of linear measurement is limited by the length of the magnet, so a magnet that can be easily modified for different lengths is needed. Lastly, and perhaps most importantly, is that the magnetizing process must result in a highly accurate and repeatable pole length.
To initiate development, the specifications for a multi-pole strip magnet were finalized with pole lengths of 1.0 mm, 1.2 mm, or 2.0 mm.
The next step was to investigate the different materials that could be used to produce the magnets according to the unique specification mentioned above. Analysis of several materials was made with a final selection being a flexible ferrite material with an Energy Grade (a measure of the stored energy in a magnet) of 1.4 MGOe (megagauss-oersteds) that exhibited the required characteristics for proper magnetizing as well as the ability to alter the length options. Using a flexible ferrite material to produce the magnets enabled the production of very long magnets, up to 5,000 mm or longer.
During the testing phase, two standard sizes were developed and tested. Looking at the popularity of the AMS Hall sensor options on the market, two multi-pole strips were tested. The first multi-pole unit was 1.5 mm thick by 9.5 mm wide. The second multi-pole unit was 0.76 mm thick by 3.18 mm wide. Both magnets were magnetized to produce a 10 mT field at 1.0 mm. They met the AMS sensor requirement of a magnetic field strength range at the surface of the sensor of 5 mT to 60 mT. The operating temperature of the magnet ranged from -40° to 125°C with a linear coefficient of thermal expansion of 1.8 x 10-4 °C-1.
Analysis into the proper means of mounting the magnets resulted in both magnets being able to perform to the required technical specifications while being mounted with a pressure sensitive adhesive applied to the non-magnetized side. This mounting configuration allowed mounting of the magnet on a range of surface materials and textures.
The targeted test parameters of the multi-pole magnet were accuracies of better than 40 microns. Testing of multi-pole magnet strips to such stringent standards had never before been performed. It became apparent during the testing phase that a new test fixture would have to be developed to ensure accurate results. So a custom test fixture was designed and fabricated to begin initial validation of the 1.0 mm pole length magnet design.
The magnet to be tested was mounted on a small sample platform at a designated distance below a Hall sensor, which was attached to a carriage that moved linearly by means of a stepper motor. An optical encoder was used to measure the linear movement of the magnet along the test fixture during testing. This included a linear scale with graduations attached to the base of the testing fixture. A read-head, which provided required resolution, was used to precisely measure movement of the test platform and the magnet being tested along the linear scale. An AMS NSE5310 Hall sensor was used to measure the magnetic field of the test magnet and its linear position as the sample platform moved along the testing fixture. The NSE5310 is a high-resolution magnetic linear encoder that provides instant indication of the magnet position with a resolution of 0.488 microns per step (12-bit, or 4096 bits, over a 2.0 mm pole pair).
Initial testing results showed measurements of greater than 2% accuracy, which were higher than expected. Analysis into the test fixture highlighted improvements that needed to be made to the device. After needed tooling improvements were made, standard 2% accuracy results were achieved.
Testing also showed the need for proper placement of end effects to ensure appropriate travel distance during application. To achieve optimal accuracy in desired travel distance, the end effects needed to be removed from the overall length of the multi-pole magnet strip. Adding the additional 2 mm needed for end effects to the overall length increased the length needed when specifying overall motion.
Dexter Magnetic Technologies