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 M1160 Revision A Magnetostrictive Position Transducers in Medical Applications David S. Nyce Introduction Magnetostrictive position transducers are non-contact, absolute reading, and have essentially infinite resolution. The non-contact nature of this technology means that there are no rubbing or sliding parts to wear out, and assures an unlimited service life for the position transducer. High reliability and unlimited service life make this technology an ideal match for incorporation into products for the medical equipment industry. The transducers are inherently absolute reading and have infinite resolution, meaning that periodic re-zeroing or calibration is not required, and the measurement increments can be as fine as the electronic interface will support. A typical magnetostrictive linear position transducer is shown in figure 1. It comprises an electronics head, a sensing rod, and a position magnet. The sensing rod is normally stationary, and the position magnet is attached to the moving part which is to be measured. As the position magnet moves along the length of the sensing probe, the distance between the position magnet and the electronics head is constantly reported. Readings typically take place at a rate of approximately 1,000 to 4,000 times per second. The position magnet does not touch the sensing rod, and is coupled to the rod only through it's magnetic field. Figure 1: A Magnetostrictive Linear Position Transducer comprises an electronics head, sensing rod, and position magnet. 1
How it works Magnetostriction is a property of ferromagnetic materials (like nickel, iron, and cobalt) whereby application of a magnetic field causes a physical change in size or shape of the material, as shown in figure 2. This change is due to the alignment of the magnetic domains in the material with the externally applied magnetic field. Magnetic domains (represented by arrows) can be considered as tiny permanent magnets which are randomly arranged in a non-magnetized material, as in the upper part of figure 2. When magnetized, they line up as shown in the lower part of figure 2. Figure 2: An unmagnetized (H=0) magnetostrictive material has randomly arranged magnetic domains as shown in the upper figure. A material with positive magnetostriction [1] undergoes an increase in size due to the alignment of the domains with the magnetic field (H), as shown in the lower figure. Just as the application of a magnetic field causes a physical change (strain) in ferromagnetic materials, the reverse is also true: application of strain causes a change in the material's magnetic properties (permeability). This "reverse magnetostriction" is called the Villari Effect, and is utilized within the electronics head to detect the ultrasonic pulse on the waveguide, which is described next. The waveguide is a length of wire, made of a magnetostrictive material, and housed within the sensing rod portion of the transducer. When a current pulse (called the interrogation pulse) is applied to a magnetostrictive wire, and an axial magnetic field is applied at some point along the wire, a torsional strain is generated at the location of the axial magnetic field. This is called the Wiedemann Effect, and is depicted in figure 3. The torsional strain travels as an ultrasonic pulse in both directions on the waveguide. The pulse going away from the electronics head is removed by a damping element (called the damp). Figure 3: The Wiedemann Effect - application of an electric current through a magnetostrictive wire causes a torsional force (waveguide twist) at the location of an axial magnetic field. A permanent magnet, called the position magnet, supplies the axial magnetic field. 2
The pulse going toward the electronics head is detected when it arrives. The ultrasonic pulse is a strain wave which is accompanied by a change in permeability (the Villari Effect), as compared with the remainder of the waveguide. When this area of different permeability passes through a pickup coil within the field of a bias magnet, a voltage pulse is produced (the Faraday Effect) [2]. The electronic circuitry detects this voltage pulse (called the return pulse). In operation, a timer is started when the interrogation pulse is applied. The timer is stopped when the return pulse is detected. The elapsed time represents the distance between the position magnet and the electronics head. Additional electronic circuits convert the time period into the desired analog or digital output signal. Physical and electrical interface The sensor housing is normally available in several styles. The style shown in figure 1 is mounted by screwing the male threaded flange of the transducer into a female thread along the sensing axis in the application. The position magnet is a separate component which is mounted by the user onto the moving part to be measured. An automotive style housing, however, has an overmolded plastic housing for low cost and is held in position by use of a bolt hole which is molded into the housing. Alternatively, clevis mounting and profile styles are available which include the position magnet mounted into a carrier which maintains alignment with the waveguide. Further details on housing styles and dimensions are available on the manufacturer's websites. The electronics head includes circuitry which provides the interrogation pulse to the waveguide, amplifies, filters, and detects the voltage produced in response to the ultrasonic pulse, measures the time between the interrogation and return pulses, and generates the desired output. Outputs normally available include analog voltage, analog current, voltage pulse (called start-stop), CANbus, Profibus, SSI, and others. Power supply requirements range from 5 volts to 24 volts DC, depending on the model. Application Examples Figure 4 shows how a magnetostrictive linear position transducer can be mounted into a hospital bed for controlling the bed adjustments. Previous versions have used contact or Hall Effect (magnetic) switches. When using switches, manual adjustment of each switch point is required when assembling the bed frame. An assortment of brackets is needed to hold the switches in each possible position for various bed models. Individual calibration is required to make sure that each switch activates at the desired position. Conversely, using a magnetostrictive linear position transducer provides a full range of possible switch points along the complete stroke length. The controller module, which interfaces to the transducer, can be programmed to automatically calibrate the "switch" point when the bed is placed in the desired position. Only one mounting bracket is needed, and any possible switch point is easily programmed. The magnetostrictive transducer costs more than a single contact or magnetic switch, but it replaces several switches, brackets, and some manual labor. 3
Figure 4: Using a magnetostrictive linear position transducer provides programmability to the adjustment of a hospital bed. Similarly, multiple contacts or magnetic switches often used in positioning a dental chair can be replaced by a single magnetostrictive sensor by the use of more than one position magnet. In addition, since the system can be fully programmable, a microprocessor and memory can be used to set and remember any number of predetermined "standard" positions. The dentist can select one of these preset positions, or program a new position. Magnetostrictive linear position transducers are also useful in improving the durability of a wheelchair lift, increasing the dose accuracy of positive displacement dispensing equipment, and general replacement of contact and magnetic switches where full-range programmable setpoints would offer an advantage to the manufacturer and/or user. A significant feature, mentioned above, that can lower cost in some applications is the use of multiple position magnets. Two magnets, for example, can be positioned along the sensing rod at the same time. When the interrogation pulse is applied, an ultrasonic pulse is launched from the location of each of the two position mag- 4
nets. Both pulses are detected in turn, and the timer stores data for each of the two elapsed times. In this way, two measurements can be made while using (and paying for) only one transducer. Up to 16 magnets can be used on one transducer. Conclusion Substantial advances in magnetostrictive position transducers have recently been made. Some of these are due to the automated manufacturing processes implemented to meet the reliability and low cost requirements of the automotive industry. In addition to the smaller physical size, lower cost, and high volume production capability, failure rates have been reported which are far below the 50 ppm required by the automotive industry. This high reliability, coupled with the inherent unlimited lifetime of magnetostrictive sensors, is an ideal match for the dependable products needed in the medical industry. References 1. Richard M. Bozorth, Ferromagnetism, 1951, D. Van Nostrand Co., Inc, New York, p. 11 2. David S. Nyce, Magnetostriction-Based Linear Position Sensors, SENSORS, April 1994, vol. 11,#4,p. 22 For more information contact: MTS Systems Corporation Sensors Division Tel: 919-677-0100, Fax: 919-677-0200 displacement@mtssensors.com www.mtssensors.com 5 Part Number: 08-02 M1160 Revision A