l MTS Systems Corporation Sensors Division 3001 Sheldon Drive Cary, NC 27513 Phone 919-677-0100, Fax 919-677-0200 TECHNICAL PAPER Part Number: 08-02 M1167 Revision A Smarter Sensors reduce costs for Motion Control Integrators David Edeal Introduction While sensors with traditional analog outputs are still a primary choice for many users requiring displacement feedback, the demand for digitally based, "smart" sensors continues to increase in the motion control industry. The world of systems development is being shaped by the continually growing emphasis on productivity enhancements and reduced installation and maintenance costs. At the same time, there appears to be a diminishing number of resources and expertise available to develop the next generation of higher complexity control systems. As a result, the need for smart "plug and play" components and subsystems is undeniable. This is particularly visible in the migration toward systems utilizing industrial fieldbus networks as a basis for machine control. Many manufacturers have stepped up to the challenge of incorporating the necessary controllers and software into their newer products to meet this need. A recent example of this type of integration can be found in magnetostrictive displacement sensors that include a direct quadrature type interface. In the past, enhanced functionality such as programmable parameters, encoding of outputs and uploading program "recipes" could only be accomplished at the control location, using a discrete converter interface. Borrowing advances in digital microelectronics from the telecommunications industry, sensor manufacturers are able to provide more sophisticated "smart", digitally based products that are fully integrated thereby eliminating the cost and complexity of separate converter interfaces. At the same time, lower costs and significant gains in functionality often make the digital products a more attractive design option. In the case of magnetostrictive displacement and level sensors, where the fundamental physical measurement is inherently digital, the addition of digital signal conditioning or signal translation makes for a highly optimized sensor package. Surprisingly, this type of higher level integration can be accomplished without increasing the size of the standard magnetostrictive sensor electronics housing. State-of-the-Art Magnetostrictive Position Sensing The fundamental principles behind magnetostrictive position sensing have not changed since they were brought to the industrial sensor marketplace by Jack Tellerman in 1975. However, a great deal of refinement of the technology implementation has taken place through continual product enhancements. These improvements have resulted in 50 fold improvement in raw position resolution and a factor of 5 improvement in output linearity versus those of the first generation of magnetostrictive sensors pioneered by Temposonics (now MTS 1
Sensors Division). The impressive performance gains are partly due to manufacturing and materials processing improvements, and partly due to theoretical and signal processing enhancements afforded by more capable digital electronics components. The basis of the magnetostrictive measurement is time ("temporal"). What the sensor actually measures is the time it takes for a sonic pulse to travel a measured distance (hence the name Temposonics). This type of measurement takes advantage of the effect in magnetostrictive materials, where magnetic fields can produce mechanical strains and vice versa. As illustrated in Figure A, when an magnetic field produced in the sensor s "waveguide" (made from magnetostrictive material) interacts with the perpendicular magnetic field from a position sensing permanent magnet, a torsional strain pulse is produced (Wiedemann effect). The strain ("return") pulse travels mechanically away from the position magnet location toward an electromagnetic pickup at the speed of sound in the waveguide material. The measured position is simply the time it takes for the sonic wave to travel from the position magnet to the pickup, multiplied by the speed of sound in the waveguide material. The average speed of sound in the waveguide is accurately measured for each sensor on laser test beds at the factory in order to achieve extremely precise sensor position outputs. Figure A: Magnetostrictive Position Sensor Design The true basis of the high resolution found in magnetostrictive sensors is the ability to sense extremely small slices of sonic wave travel time. This is accomplished using a high speed counter (clock), which is enabled as the waveguide is energized ("interrogated") with its magnetizing current. In the past, the use of slower counters (28 MHz) resulted in raw position resolutions of ~ 0.1 mm. The only way to achieve higher resolution values was to perform repeated back-to-back interrogations (recirculations). Each recirculation improves the sensor resolution by the reciprocal of the number of repeated interrogations. The problem with this approach is that each recirculation adds to the update time. Today, the use of higher speed counters (4 GHz) results in a resolution of 2 microns, a factor of 50 improvement with no increase in update time or concern for tracking errors due to position magnet motion during recirculations. This is a key requirement for sensors that produce both position and velocity or multiple magnet position outputs. Refer to Figure B for the basic magnetostrictive position sensing relationships. 2
Position Resolution = 1/(G x F) Output Update Time ~ G x L G = Gradient = 1/Speed of sound in waveguide (nominally 0.35 msec/mm) F = Counter Frequency (Hz), L = Stroke length (mm) Figure B: Magnetostrictive Position Sensing Equations Today s high-end magnetostrictive sensors employ additional design and signal processing enhancements in order to provide more precise measurements under a wider range of noise and vibration conditions. One particular patented enhancement is the use of a Vallari transformer (referred to as a "tape") as part of the sonic pulse pickup (see Figure A). This small flat piece of magnetostrictive material is attached to the waveguide perpendicular to its axis. The tape passes through a coil and is magnetized by a small permanent bias magnet. As the torsional strain pulse acts on the tape, its magnetic field is altered and a resulting voltage pulse is produced in the pickup coil. This voltage pulse is then used to trigger the counter to stop at which time the position output can be computed. By utilizing the Vallari transformer, the raw sensor output signal is increased by a factor of 13 over designs that utilize the conventional concentric waveguide and pickup coil configuration. The Vallari transformer design, along with double EMI shielding, increases the sensor s signal-to-noise ratio from values on the order of 3 to more than 250. Not only does the Vallari transformer significantly improve the sensor s noise immunity, but the fact that the pickup is perpendicular to the waveguide essentially decouples it from longitudinal motion. This makes the sensor highly immune to external shock and vibration. Achieving precision feedback with magnetostrictive sensors requires accurate counter triggering from the pickup coil signal. A more accurate time slice measurement results in superior output repeatability and linearity. It is not hard to envision that triggering off of a signal with relatively significant noise and vibration components will result increased measurement uncertainty. Sensors that utilize a concentric pickup coil design must therefore rely on amplification and filtering of the raw signal in an effort to improve noise and vibration immunity. The negative impact of this type of conditioning is that it typically adds phase lag to the sensor output, resulting in larger position errors, especially at higher velocities. Quadrature Interface Smart Sensor Whereas traditional magnetostrictive sensors utilize numerous integrated electronics designs to provide various stroke ranges and signal outputs, the newer modular design provides much greater design flexibility at a minimal cost penalty. The idea is to utilize a single electronics design for raw position sensing (the "base" board) and a separate "personality" module for each desired output (the options board). Cost benefits are derived from the increased volume of base boards along with the use of a proprietary position sensing ASIC. For example, the newly introduced Temposonics R Series AQB quadrature interface sensor utilizes this approach to eliminate the need for external converter modules such as the AEC-100 made by CMC SENCON, Inc. (see "Converter Multiplies Position Sensor Accuracy", Machine Design, 11/20/97). Through a cooperation between CMC SEN- 3
CON and MTS Sensors Division, the performance and functionality found in the AEC-100 in an enclosure roughly 2" x 5" x 7" have been incorporated into an output module roughly 0.5" x 1.5" x 2", designed to fit inside the standard sensor electronics housing (See Figure C). Figure C: New AQB Sensor vs AEC- 100 and sensor Quadrature is a very common means of feedback in industrial control applications. It is most often generated by encoders for both linear and rotary motion. The signal is a squarewave pulse train that is communicated at a TTL level signal (5 Vdc). As is typical of quadrature outputs, the AQB interface provides both polarities for each format (A, A, B, B, Z, Z) in order to reduce the susceptibility to external electrical noise. For encoders, the resolution and pulse width are typically defined by the fixed physical characteristics of its hardware. For example, the number of physical lines per inch etched in a glass scale defines the resolution or number of pulses transmitted in a defined displacement increment. This is usually represented in counts per inch (CPI). As the speed varies in an encoder, the width of the quadrature pulses change accordingly. Slower speeds result in longer pulse widths while higher speeds result in shorter pulse widths so that the sensor speed can be computed directly by the controller. Direction of motion is indicated by which channel (A or B) leads the other by one half pulse width. One problem with this method of sensing is that at very low speeds (near zero), the pulse widths will increase significantly so that updating the speed to the correct value takes longer, thus producing a "choppy" speed output. While similar to that of an encoder, the AQB interface differs in a number of ways. The fundamental difference is that the quadrature pulse outputs are not a result of a fixed hardware configuration such as an encoder grid. The AQB sensor provides a pulse output with a fixed width (and therefore frequency, See Figure D), independent of position magnet speed. While speed may not be a direct result of the AQB output, it can be easily computed as a function of counts (position change) per a given time interval. An important difference between the AQB interface and encoder outputs is that even the slowest speeds, position and velocity information is regularly updated at the user specified pulse frequency. 4
Perhaps the most significant difference between the AQB interface and an encoder is that the sensor resolution and fixed pulse frequency can be independently user-programmed, giving this type of sensor a wide range of applicability for a given stroke length. The resolution of the AQB sensor can be varied from 50 to 12500 CPI (0.02 to 0.00008 inches/count), regardless of stroke. The fixed pulse frequency has a user selected range of 8 khz to 1 MHz. Other adjustable parameters include sensing polarity, zero reference (Z channel) pulse location and width and "burst mode" enable (see below). A A Channels A A, 90º phase lag (1/2 pulse width) B B Channels B, B Pulse width (seconds) Z Z Channels Z, Z Above sample signal represents counts of sensor displacement. Quadrature frequency (Hz) = 1/(2 x Pulse width) Figure D: AQB Interface Quadrature Output The basic function of the AQB output module is to keep track of absolute position changes and convert this information into quadrature signals. Each new absolute position count subtracts from the previous one, thereby resulting in an incremental output. When the result is a positive number, the channel A signal will lead channel B by one half pulse width and vice versa when the result is negative. The discrete processes of sensor interrogation, signal conversion and output generation execute simultaneously, reducing the update time thereby providing a faster, more accurate sensor. As a result, typical output update times for the AQB sensor are less than 1.5 milliseconds for stroke lengths up to 100 inches. Because all magnetostrictive sensors produce inherently absolute position information, the AQB sensor is capable of producing such an output at any time. One situation where the absolute position information is required is after a loss of power to the axis controller or quadrature interface. Once power is restored, the controller will not be able to determine displaced position without an absolute reading or some sort of re-homing algorithm. With the incremental quadrature output produced by encoders, this type of absolute output is not possible. The AQB sensor solves this problem by producing what is known as a "send-all" or "burst" output. This is simply a continuous stream of quadrature pulses at the prescribed pulse frequency corresponding to the absolute position of the position magnet. The AQB sensor can provide this type of output either at power-up after a user 5
specified delay, or at any time during normal operation using a switched power input signal. The burst at startup feature can be disabled or programmed with a delay from immediate to 30 seconds. This feature should not be confused with position updating. It is intended for the controller to reestablish absolute position at critical instances. Advantages of Other Smart Magnetostrictive Sensors Fieldbus, SSI & ServoSensor TM position controller The Sensors Division of MTS Systems Corporation has applied the modular digital electronics approach toward a number of other products including Fieldbus sensors with CANbus, DeviceNet, Profibus DP, Interbus-S and Modbus protocols. Among the advantages of these sensors is the ability to provide diagnostics, upload sensor parameters or download process information corresponding to various setups within a given application. These capabilities enable users to minimize startup times as well as maintenance and retrofit efforts. Additionally, since the above protocols allow varying degrees of programming flexibility, configurations or "recipes" within a given standard can be customized for a specific application. For example, MTS Sensors has provided a variation of CANbus that provides an output format for the simultaneous use of 30 position magnets on a single R Series sensor. Other recently introduced smart sensors include the Synchronous Serial Interface (SSI) and the ServoSensor TM position controller. The programmable SSI sensor generates the state-of-the-art absolute position encoder output. This interface is rapidly becoming the industry standard for providing the highest resolution (2 micron) and serial data throughput (at least 7500 measurements per second) available for servo system feedback sensors. The ServoSensor position controller is a high performance sensor and position controller in a single package. Because this product provides for external serial parameter programming and controller inputs as well as direct control outputs suitable for most servovalves, the need for typical external controller and driver interfaces is eliminated. Thus, the ServoSensor position controller provides high performance closed-loop PID control while at the same time reducing system complexity and costs. All of these sensors utilize the same base electronics for the raw position sensing. As system and sensor requirements change, this modular design approach will enable relatively simple integration of newer interfaces such as Ethernet or ControlNet into a wider variety of sensor application housings For further information contact: MTS Systems Corporation Sensors Division 3001 Sheldon Drive Cary, NC 27513 Tel: 919-677-0100, Fax: 919-677-0200 displacement@mtssensors.com www.mtssensors.com 6 Part Number: 08-02 M1167 Revision A