DESIGN AND CONSTRUCTION OF A PRECISION ROTATING MAGNETOMETER

Leon S. Chao, Richard K. Afoakwa, Thomas A. Zirkle, Xiang Zhang

Stephan Schlamminger, Jon R. Pratt, and Nolan J. Brandenburg Fundamental Electrical Measurements Division National Institute of Standards and Technology

Gaithersburg, MD, USA

ABSTRACT The next generation NIST watt balance will feature a large samarium-cobalt permanent magnet (860 Kg) as the source of its magnetic field. The magnet, currently in fabrication, must generate a field between a pair of pole pieces of 0.55 Tesla uniform along a prescribed axis to 100 ppm (or ± 55 µTesla). Such uniformity is difficult to verify using standard instrumentation. Here we describe the design and construction of a new magnetometer capable of precise measurement of the field strength via monitoring the magnitude of the voltage induced in a rotating coil. We discuss the principle and main elements of this sensor, the methods devised to translate and center it within the air gap between the pole pieces, and an electro-optical interface for digitizing and communicating the raw analog signals from its rotating magnetic and capacitive sensors. Results from a prototype device are presented. BACKGROUND The next generation NIST watt balance (NIST-4) has a magnetic circuit comprised of two SmCo permanent magnets housed in mild steel yokes whose pole faces define an air gap. The radial distribution of the flux density B inside the air gap is strictly governed, and can be manipulated, by the pole face geometries [1]-[2]. The air gap inside the magnet assembly measures 3 cm wide and 8 cm deep, which can be externally accessed through 12

entrance holes located on the top and bottom surfaces of the outer yoke. The magnetic field strength inside the air gap is designed to be 0.55 Tesla and should deviate by less than 100 ppm (or ± 55 µTesla, comparable to the Earth’s surface magnetic field) at all points on the central axis of the air gap, as depicted in Figure 1.

FIGURE 1. Magnetic flux density distribution inside air gap [3]. Field strength must be 0.55 T ± 55 µT at the central axis of the air gap in order to function as the watt balance’s principal component. The magnetic system implemented by the current watt balance has an outstanding field uniformity that deviates in a systematic fashion by as little as 100 ppm about a mean value; a benchmark for what the new magnet should reach or surpass. This existing magnetic field has been functionally characterized using the large moving coil that is part of the apparatus. The voltage induced in this coil is accurately tracked as a function of its motions, in effect mapping

the field profile during each experiment (see, Steiner et al [3]). For the new magnet, the expectation is that adherence to geometric tolerances (approximately 50 µm) in the machining of the components will produce the desired magnetic properties. However, the required geometric precision is likely difficult to achieve in the overall assembly, and it is anticipated that post assembly machining of the yoke, or other such modifications, will be required in order to compensate the magnetic circuit [4]. This requires a device that can measure the field at the absolute center of the air gap. Ideally, this device would be a moving coil, as in the present balance, but such a solution would require a suspension and position sensors that would be too complex to implement at the factory where the magnet is being assembled. Alternatively, magnetic field strength could be mapped using Hall probes. Typical Hall sensors can measure fields on the order of 0.55 Tesla, but readout precision is only on the order of parts per thousand, which is inadequate for this application. In principle, the dynamic range can be extended, but the output is also dependent on orientation with respect to the field, so that errors in orientation might be misinterpreted as variations in the field. Thus, rather than trying to solve the problems associated with Hall probes, a novel sensing strategy based on a different type of moving coil was pursued, as we describe in the remainder of this paper.

FIGURE 2. Cross sectional view of the SmCo permanent magnet assembly. The 2.54 cm diameter rotating magnetometer (without stand) is inserted into the 3 cm wide air gap. PRINCIPLE OF OPERATION To accurately measure the magnetic field, a unique instrument has been designed and constructed, represented in Figure 2. A brushless DC motor spins a 19.9 in. long drive shaft attached to a probe head. Consisting of a 60 turn copper coil, the probe head rotates at a selectable RPM to generate a sinusoidal induced voltage caused by the fixed magnetic field. Equation (1) describes the relationship between the induced voltage V and the magnetic field B.

where N is the number of turns in the coil, A is the area of the coil, and ω is the angular velocity. The radial center of the air gap is significant in that it will be the location where the large moving coil resides. The probe must first locate this position and then

measure the magnetic flux locally. This is done using a set of differential capacitors that sandwich the probe coil and rotate coaxially. Each probe plate couples with the nearest magnetic pole face, creating a differential capacitance reading that indicates probe position. Since the probe head rotates, this signal is modulated at the rotation frequency. The center location is found once the capacitor pairs have a differential reading of zero. MECHANICAL DESIGN The initial iteration of the probe included only one single drive shaft connecting the motor to the probe head. Straightness imperfections and runout of the shaft were amplified as the distance from the motor increased. To remedy this issue, the single shaft was severed into two sections: a main drive shaft and a short segment precision shaft, represented in Figure 3. Two flexures (0.08” dia x 0.59” long) added to the drive shaft compensate for eccentricity and remove the majority of the runout through minimal load bending. This engineering practice ultimately decoupled the vibration source from the precision shaft.

FIGURE 3. Cross sectional view of the probe assembly (bottom half). Only one flexure is shown. The ceramic coated precision shaft is radially bound by two bearings and connects the drive shaft to the probe head, allowing the probe head to rotate true to its axis, depicted in Figure 3. This design reduced the runout by five orders of magnitude and produced a final runout reading of only 2.5 microns. To position the probe in the air gap, a 5 DOF motion system is necessary. A fine

pitch translation stage moves the system horizontally along the radial axis, necessary to adjust the capacitor feedback. A vertical translation stage allows the probe to shift in the up/down direction to scan the whole depth of the air gap. Rotation of the probe is generated by a brushless DC motor while the tip and tilt of the frame is adjusted by stand screws arranged in a three point contact formation. ELECTRICAL DESIGN Four copper wires (two for the rotating coil and two for the capacitors) deliver the analog signals to the electronics and are read in an alternating fashion. These wires feed directly through the hollow precision shaft to avoid interference with the bearings and are guided along the surface of the drive shaft. A thin burrow on the drive shaft located at the top bearing allows the wires to bypass the bearing bore with minimal intrusion. The wires relay the dual channel coil and capacitor data to the signal conditioning unit, located on the rotating shaft below the motor. This location was chosen to avoid interference of the magnetic field with the integrated circuits. The signal conditioning unit is a printed circuit board consisting of a Microchip* PIC18F648 microcontroller, an Analog Devices AD7746 24-bit differential capacitance-to-digital convertor chip, an operational amplifier, and passive components that encode the data from the two channels into serial data. The signal conditioning unit is fixed on the shaft and rotates in sync with the probe head. Data from the rotating system is transmitted via infrared LEDs to a stationary receiver using phototransistors. The signals from the receiver are converted to TTL signals and recorded via a National Instruments data acquisition board, NI BNC-2090A.

FIGURE 4. Transmission sequence encoding for data transfer to stationary receiver. The microcontroller reads each channel and encodes the analog data into a sequence of 13 pulses with a delay of 1 µs, as shown in Figure 4. The encoding scheme consists of a 155 µs start-bit pulse which indicates data transmission. Subsequently following the start-bit are 2-bit pulses indicating channel (coil or capacitor) and 12-bit pulses indicating the actual data for the specified channel. The channel and data pulses consist of 80 µs and 30 µs pulses, with an 80 µs pulse indicating a zero-bit and the 30 µs pulse indicating a 1-bit. In the first iteration of the probe, a slip ring was used to carry the analog signals from the probe head to the data acquisition system. The slip ring was rated for a maximum rotation rate of 300 rpm. In addition, the slip ring introduced noise to the voltage sinusoid due to unreliable connections. The new digital-infrared interface improved both limitations. With the infrared interface, reliable communication would be possible even with angular velocities that would surpass the mechanical failure point of the system. The noise, caused by resistance variations, is entirely avoided. Figure 5 depicts the

difference between the slip ring and wireless waveforms.

Waveform Received Through Slip Ring

Waveform Received Through Optical Interface

FIGURE 5. High noise slip ring data vs. clean optical relay data. Noise is a result of poor internal connections in the slip ring. CONCLUSIONS Whitworth’s adage, “we can only make as well as we can measure”, still rings true. Through good engineering practices, such as vibrational decoupling and wireless interfacing, a functioning magnetometer with centering capabilities has been designed and constructed. We have started testing the probe to verify its design specifications. The instrument will be used to measure the magnetic field generated once the permanent magnet is fully manufactured. REFERENCES [1] F. Villar et al., Conference on Precision

Electromagnetic Measurements Digest, pp. 128-129, June 2008.

[2] P. Gournay et al., IEEE Trans. Instrum. Meas., Vol. 54, no.2, pp. 742-745, April 2005.

[3] Steiner, et al, Metrologia, Vol. 42, pp. 431-441, 2005.

[4] J. Liu et al., Magnetic Design of Watt Balance, Electron Energy Corporation, February 24, 2012.

* Certain commercial equipment, instruments, or materials are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the materials or equipment identified are necessarily the best available for the purpose