Inductive Sensor for Measuring the Speed of Reciprocating Parts of High-Speed Mechanisms
Abstract Generated abstract
This paper describes an inductive sensor intended to measure the time-dependent velocity of reciprocating parts in high-speed mechanisms, where displacement amplitudes may exceed 50 mm and accelerations may reach very high values. The proposed design uses a symmetrical magnetic system with a movable steel rod, thin-walled stationary poles, a direct-current magnetizing coil, and two series-connected measuring coils, so that motion of the magnetic flux induces an emf proportional to velocity while reducing errors from flux pulsations and leakage redistribution. The author reports that the sensor is mechanically robust, easy to install, and highly sensitive, producing about 1.5 V at 1 m/s with a 70 mA magnetizing current, and illustrates its use with an oscillogram obtained after calibration by a magnetoinduction sensor.
Full Text
Proceedings of the Tomsk Polytechnic Institute
named after S. M. Kirov
Vol. 232
1975
UDC 621.317.39:531.767
INDUCTIVE SENSOR FOR MEASURING THE SPEED OF RECIPROCATING PARTS OF HIGH-SPEED MECHANISMS
A. M. Elenkin
(Presented by the seminar of the Power-Supply Systems Section)
In conducting experimental studies of various high-speed mechanisms—for example, pulsed electromechanical energy converters, synchronous switching devices, etc.—as well as for automatic regulation of the parameters of machines engaged in some technological process, it is necessary to measure, as a function of time, the velocity of parts moving in a reciprocating manner. The displacement amplitude of the indicated parts reaches 50 mm and more, with accelerations up to 10000 \(g\). From the standpoint of the mechanical strength of the moving element and reliability of operation, inductive-type sensors prove to be the most suitable for such parameters [1, 2]. However, such sensor designs do not satisfy the requirements for measurement accuracy because of significant pulsations of the main magnetic flux during the motion of the moving element.
The design of the sensor is shown in Fig. 1. The sensor consists of a cylindrical insulating frame 1 (textolite), a magnetizing coil 2 (PETV wire, \(\varnothing 0.2\), 8000 turns), an external magnetic core 3, made split, two poles 4 made in the form of cylinders with a wall thickness of 0.3 mm, a movable magnetic core 5, made in the form of a thin steel rod with two poles, and two measuring coils 6, wound in two layers with PETV wire \(\varnothing 0.05\) mm. The magnetizing and measuring coils are impregnated, and
Fig. 1. Inductive speed sensor.
the poles are attached with epoxy resin. Both parts of the outer magnetic circuit are fitted tightly onto the poles and fastened to each other with screws. The rod is connected to the object under investigation and can move inside the insulating frame as along a guide in the axial direction. The dimensions of the sensor are: length 200 mm, outside diameter 55 mm.
When the magnetizing coil is supplied with direct current, a magnetic flux appears, closing through the outer magnetic circuit, the poles, and the rod. Together with the displacements of the rod poles, the magnetic flux also moves, inducing an emf \(e(t)\) in the measuring coils
\[ e(t)=-\frac{d(W\Phi)}{dt}=-\Phi\frac{dW}{dt}, \]
where \(\Phi\) is the magnetic flux linked with the moving magnetic circuit,
\(W\) is the number of turns of the measuring coil.
Fig. 2. Oscillogram of the velocity of motion of the part and calibration marks.
Horizontal scale: one large division of the coordinate grid corresponds to 2 msec
Since the turns are arranged uniformly along the length of the measuring coil, passing to finite displacements, we obtain
\[ e(t)=BlW_{\text{и}}v, \]
where \(B\) is the magnetic induction in the gap between the poles,
\(l\) is the length of a turn of the measuring coil,
\(W_{\text{и}}\) is the number of turns of the measuring coil simultaneously crossed by the magnetic flux,
\(v\) is the velocity of motion of the moving poles at the given instant of time.
The magnetic system of the sensor is made symmetrical with respect to the magnetizing coil; therefore, the total length of the magnetic circuit remains unchanged when the magnetic flux is displaced. When the poles of the rod move, a redistribution of leakage fluxes inevitably occurs, changing the main magnetic flux in the poles; however, in this design this effect has practically no influence on the accuracy of measurement, owing to the presence of two measuring coils connected in series, i.e., if the main magnetic flux, for example, increases in one movable pole, it decreases to the same extent in the other pole, and vice versa.
The influence of eddy currents induced in the bodies of the cylindrical poles of the stationary magnetic circuit on the sensor readings is very small, since the poles are made thin-walled. Making the internal cavity of the sensor open on both sides facilitates installation of the sensor, monitoring of its condition, and protects the coils from possible mechanical damage in the event of rod breakage.
The sensor has high sensitivity. Thus, for example, at a motion speed of 1 m/s and a magnetizing current of 70 mA, the emf taken from the measuring coils reaches 1.5 V.
A magnetoinduction sensor was used to calibrate the sensor. Figure 2 shows a typical oscillogram of the velocity of motion of a part of a high-speed mechanism with calibration marks.
REFERENCES
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A. M. Turichin. Electrical Measurement of Nonelectrical Quantities. Moscow–Leningrad, Gosenergoizdat, 1959.
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N. P. Potapov, N. P. Ryashentsev, E. M. Timoshenko, P. V. Chaplygin, V. S. Shestakov. Inductive sensor for the speed of linear displacements.—Electrical Pulsed Power Systems. Siberian Branch of the Academy of Sciences of the USSR, Novosibirsk, 1969.