MR-based digital tachometers are gaining acceptance with motor and drive manufacturers. The combination of magnetoresistive (MR) sensors with a magnetic pulse wheel, Figure 1, enables the practical development of a bearing-less, high resolution, “pancake” style magnetoresistive digital tachometer. The benefits include:
• An increase in sensitivity (up to 5X) in detecting magnetic fields over previous magnetic technologies, such as those that use Hall effect sensors and gear tooth wheels.
• High resolution compared to other magnetic technologies. Biased Hall effect or single-point MR devices typically have a sensor and a gear tooth wheel. They often require a small permanent magnet that is usually placed in the body of the sensor. The magnet provides the necessary magnetic bias field. However, this approach is often too bulky or coarse for the high resolutions required today. In an MR-based tachometer, layers of magnetoresistive film replace the permanent magnet. The thickness of these film layers can range from 250 angstroms to 2 microns, enabling the sensors to be fabricated to count to over 2,500 pulses per revolution.
• Resistance to environmental contamination, making it applicable in such harsh environments as steel mills or pulp and paper production.
A typical configuration has the magnetic pulse wheel mounted on a motor shaft, Figure 2. One to four sensor modules are positioned at the proper distance from the circumference of the pulse wheel.
MR sensors
MR sensors are classified by the material that is active in the sensing process. Recent MR sensors use Permalloy thin film as the active magnetoresistive layer. The film’s resistance decreases with applied magnetic field and is independent of field polarity.
A typical Permalloy resistor operates in a narrow magnetic field intensity range. If the magnetic field is approximately 4 gauss, the resistor is “off.” If the magnetic field is approximately 20 gauss, it is “on.” Thus, the device is suitable for switching rather than linear applications. Once the resistor film is magnetically saturated, there is little change in resistance. Sensors based on this material are unipolar. The resistance versus applied curve is identical for north and south magnetic poles.
Frequency response to an applied magnetic field ranges from dc to the megahertz frequencies. In rotary-speed sensing applications they can sense motor speeds exceeding 20,000 rpm while still responding as a zero-speed sensor.
In digital tachometers, MR sensors use four resistors connected in the Wheatstone- bridge configuration to detect a resistance change from an applied magnetic field. A dc bias voltage is connected across opposite nodes, Figure 3A, with the output across the alternate nodes. There are few temperature effects because the four resistors have identical temperature coefficients.
Depending on the control application, each of the four sensor modules can have different or identical pulse counts, using the same magnetic pulse wheel, as long as those counts are in the same pulse group. For example, in a system with a resolution of 1,024 pulses/revolution, the sensors can have 512, 256, 128, and 64 pulses/revolution.
Magnetic pulse wheel
An aluminum or stainless steel pulse wheel replaces the gear tooth wheel. The pulse wheel is imprinted with a repeated north-south pattern on its circumference. This pattern is magnetized using a thickfilm, permanent-magnet coating. The wheel mounts on a shaft or other rotating device. As it rotates with the shaft, it provides an alternating magnetic pattern. The pattern matches that of the sensor and eliminates the need for sensor biasing. The sensor and its electronics turn that sensed magnetic pattern into a square wave pulse signal that corresponds to the number of sensed north-south poles. This signal is transmitted to the motor control drive by high-current line drivers on the output of the sensor electronics.
The magnetized pulse eliminates the expense and difficulty of machining fine gear teeth, resolving the teeth, and the need for separate sensing heads for A, B, and Index channels, Figure 3B.
The magnetic pulse does not require an external energy source to produce its small magnetic fields after initial magnetization.
The field strength produced by the pulse wheel is typically less than 100 gauss at the surface. It is not easily erased or distorted because of the high coercivity of the coating material. It takes a magnetic field with a strength of 1,000 to 2,000 gauss at the wheel’s surface to permanently change the magnetized pattern.
The permanent magnet coating material is ferrite (FE3O4) based. It enables higher resolutions than devices using gear teeth. For example, this coating can provide magnetized wavelengths as small as 100 microns. Presently, it is difficult to manufacturer a gear tooth with a pitch as small as 2 mm (2,000 microns). The material is already in an oxidized form, and therefore inert and chemically stable.
Operation
For an output to appear, an imbalance must occur within the bridge. This happens when, as the magnetic wheel turns, one pair of opposite legs of the Wheatstone bridge detects the magnetic fields. This produces a small ac ripple signal that rides on a large common mode dc signal. This signal is nearly sinusoidal and has an amplitude on the order of 100 mV peak to peak, Figure 3B.
Additional signal processing is needed before the signal is in a format compatible with the motor control drive or a digital speed readout. A differential line driver provides the low impedance output necessary to drive the signal over long cables while maintaining sharp waveform edges. A MOSFET line drive is often chosen because it is fast (typically less than 35 nsec rise time, no load), has an internal impedance of only 3 to 7 ohms, can drive long cable lines, and offers differential outputs for good noise immunity.
Digital tachometers produce two signals in quadrature to differentiate direction of shaft rotation. Thus, the simplest MR sensor in a digital tachometer needs a minimum of eight interconnected magnetoresistors.
MR-based digital tachometers can also detect index pulses. This requires an additional sensor bridge, axially spaced from the incremental sensor pattern, and a second magnetic track on the rotating wheel.
Mechanical tolerances
The MR system’s allowable mechanical tolerances are relatively large compared to alternative technologies of the same resolution. There are three main measurements that describe the working envelope: the air gap displacement, the axial displacement, and the longitudinal displacement, Figure 4.
The air gap (radial displacement) between the wheel and the sensor face can limit the resolution in modular, bearingless digital tachometer systems. It directly affects the allowable tolerances for shaft runout and motor mounting surface eccentricity. It also affects the ease of installation of a digital tachometer system. In patterned MR-based tachometers, the sensor can operate to near contact with the pulse wheel, or the distance between its surface and the sensor face can be larger because the pulse wheel generates its own field. Nominal air gap is 0.018 in. 60.008 in. The signal amplitude decreases with increasing air gap.
The ball or roller bearings used in large electric motors have significant axial travel, especially compared to their tight radial runouts. Axial travel for patterned MR tachometers, however, is up to ±0.050 in. Acceptable longitudinal translation of the MR sensor is so large that it can almost be neglected as an alignment factor.
Environmental concerns
Tachometers, because of their location at or near the process being controlled, are susceptible to contamination. With analog tachometers, carbon brushes may become contaminated with motor greases. Oil, water, and condensation affect optical encoders. Magnetic sensing systems, however, are not affected by these factors. Operational temperature ranges of Permalloy based MR sensors are 240 C to 180 C, however, the accompanying electronics may limit the system operating range. Digital tachometer systems are limited more by the temperature sensitivity of the signal processing electronics than the sensor.
Ken Dickinson, is senior engineer, Industrial Div., Lake Shore Cryotronics Inc., Westerville, Ohio.