by Steve Meyer, Contributing Editor
Improved position sensors can reduce motion system cost and increase performance.
Motion control is, first and foremost, about control. The old engineering adage applies; you cannot control what you cannot measure. Rotary and linear position sensors play the crucial role of providing the mechanism to measure motion. Understanding how feedback works as part of the motion system is important in applying the right technology at the right price and getting the best results.
Like most devices based on electronics, feedback devices for the motion control industry have improved accuracy and reduced cost in the last few years. Despite the importance of feedback to motion control systems, feedback technology is sometimes mis-applied.
A brief tutorial on the optical encoder
An optical encoder is simply a rotating wheel that breaks a light source. The flashing light is picked up by a phototransistor and converted to square wave pulses, usually 0 to 5 Vdc. When the encoder is attached to a motor that is moving a load, each square wave is proportional to a specific amount of rotary motion.
Optical encoders are, by far, the most common sensor for measuring motion. When the motor is attached to a leadscrew or belt drive, the linear motion of the payload can be measured as a function of the same signals. One of the benefits of the two channel, or quadrature approach, is that the direction of motion is directly contained in the phase relationship between the two channels. The Z channel is usually a 1-pulse-per-revolution “marker” pulse that provides extra information for correction of cumulative error.
Every technology has its weaknesses, and optical encoders are no exception. Optical encoders can be sensitive to vibration because the disks are low in mass. If the disk vibrates sufficiently it can cause read errors. Mechanical assembly can be complex since the disk must be centered in between the light source and sensors. On balance, the low cost and reliability have made the optical encoder incredibly popular and the simplicity of quadrature output makes it easy to integrate in control systems.
Lots of options
Because of the importance of feedback, manufacturers have developed an array of feedback options over the years. Rotary feedback on servomotors has improved in recent years from 13 bit or 8192 pulses per revolution to 20 bit, more than 1 million pulses per revolution, and higher in some product offerings.
Too much of a good thing?
There are lots of high-resolution options available to the user, but too much feedback can create some problems that are difficult to diagnose; for a position loop, start with the mechanical accuracy required. Digital control requires a minimum of two samples to measure a real value, so two times the mechanical resolution should be the minimum feedback required. Five to 10 samples may be required depending on the profile of the motion and the degree of control required. Much more than that and the control system will start to “hunt” in between mechanical errors in the rest of the equipment
Rotary versus linear
Since positioning is a property of something mechanical, it is always preferable to measure directly. Linear measurement systems have been around for a long time, but there has been a significant gap between sub-micron linear position sensors and general-purpose position sensors like limit switches or proximity devices. High accuracy sensors found in CNCs and semiconductor equipment tend to be expensive. For expensive capital equipment, this is not an issue, but for the mainstream motion control user, something else is needed.
Enter magnetic feedback
The Hall effect transistor has been around for a while as a sensor of magnetic fields. By passing a permanently magnetized strip next to a Hall effect transistor, a sinusoidal wave is generated. Using two parallel rows of staggered magnetic “poles” and digitizing the analog waveforms results in quadrature output just like an optical encoder. Magnetic encoders have a number of interesting properties that make them ideal for many motion control applications.
Since magnetic encoders do not use a light source, they can operate in contaminated environments. Dust, dirt and oil do not affect accuracy. Encapsulating compounds or epoxy do not alter its sensing abilities, so the entire printed circuit can be sealed for washdown environments where optical encoders would be impractical. The magnetic sensing approach is not sensitive to minor differences in the air gap, which also makes location of the magnet ring very easy.
Timken linear magnetic feedback technology
Linear actuators generally try to regulate position indirectly from a rotary sensor on a motor. This approach has inherent limitations due to torsional compliance and backlash in most drivetrains. Direct measurement of linear position offers the simplicity of controlling position without consideration for the mechanical error in the drivetrain. Position loop stability is much easier to control since the position is being controlled by direct measurement. In addition, “book keeping” to manage error is eliminated and the control system tuning is simplified since the load is being controlled directly from the feedback source.
The Timken Hall Effect sensing array chip works equally well with linear magnet strips. In the same way small magnetic poles are magnetized on a ring of material, poles are magnetized on linear strips of magnetic material. Currently available magnetic linear position sensors can provide position feedback of 0.0004 in. per pulse at price points ranging between $150 and $200 per system.
This makes it economical to add feedback to many applications that were not previously suitable. At these price levels, it becomes economical to use feedback with lower cost mechanical components and reduce the overall cost of actuators generally. Since the system is based on Hall effect sensors, it is immune to all forms of contamination except ferrous particles. This environmental immunity makes it possible to package actuators for harsh atmospheres with relative ease.
The perfect match
An Austin, Texas-based startup company is currently developing a range of low-cost, high-performance actuators in both open-loop and closed-loop versions. The guiding principle is to use commonly available components and materials strategically in order to make inexpensive actuators that do the work of systems that are much more costly. Lower cost actuators lead to lower cost products, which allows the introduction of new products that were previously not economical to build. The company is in final development of several large format Cartesian systems including a 4 x 4 ft system, currently under test, that can be used for any process, additive or subtractive.
According to Robert Stiffler, business development manager at Timken, the company continues to extend the performance of its magnetic sensing technology. Available magnet strips from Timken are limited to sizes ranging from of 18 to 24 in. They can be assembled “heel and toe” to create larger scales, but longer scales would be more convenient for larger equipment. Timken is currently evaluating a number of strategies to increase scale length. Accuracy is also expected to improve to ± 0.0002 in. in upcoming versions of the product.
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