USE OF INDUCTIVE SENSORS FOR MEASURING FORCES IN AN OVERHEAD CONVEYOR CHAIN

A. V. MURIN

(Presented by the scientific seminar of the Department of Applied Mechanics)

In connection with the need to measure forces in the chain of an overhead conveyor directly under factory conditions, it was necessary to create a reliable and simple device that would make it possible to carry out such measurements and to perform oscillographic recording without the use of amplifying equipment. Usually, wire strain gauges bonded to a chain link are used for such purposes, but in that case, when recording on the film of an oscillograph, it is necessary to use an amplifier. In this way, for example, oscillographic recording of forces in the chain of a scraper conveyor was carried out under laboratory conditions by A. G. Sosnin [1]. It is also possible to obtain a sufficiently powerful electrical signal that can be recorded directly on the film by a type VIII vibrator with the aid of wire transducers, but in this case it is necessary to create special electrical dynamometers of rather complex design [2]. Attempts to solve the above problem by using the so-called high-power wire sensors that have appeared recently, and also by increasing the number of ordinary wire sensors connected into one arm of the measuring bridge, were unsuccessful. The problem posed was solved rather simply by us with the aid of inductive sensors.

For the manufacture of the electric dynamometer, a standard link of the detachable conveyor chain was used. It was established experimentally that a single-element link of a detachable conveyor chain with a pitch equal to 1600 mm is considerably deformed under the action of a longitudinal force. At a force of 5000 kg, the transverse deformation of the link is approximately 0.3 mm. These deformations, proportional to the longitudinal forces acting on the chain link, were detected with the aid of an inductive sensor. The layout of the strain link with an inductive sensor is shown in Fig. 1. The inductive sensor is a coil wound with wire of diameter \(0.14\) mm, grade PEV-2. The number of turns of the coil is \(W = 2130\), and its ohmic resistance is \(R = 110\) ohms. The core of the coil is made of sheet transformer steel. It is fastened by scre—

Fig. 1. Schematic arrangement of the strain link.

to the bracket, which is soldered with soft solder to the chain link. The cross section of the chain link on which the inductive sensor was installed was practically not weakened as a result of the machining of its surfaces, owing to the small thickness of the layer removed, which was necessary for planing its surfaces. The strain link is connected in a differential measuring circuit (Fig. 2), which has a number of advantages in comparison with a bridge circuit [3].

To obtain a linear characteristic of a nondifferential inductive sensor, it is necessary to have only a slight change in the air gap relative to its initial value, i.e., it must be

Fig. 2. Differential circuit for connecting inductive sensors.

$\Delta l \ll l$ [4]. However, in this case the sensitivity of the sensor to the measured parameter is considerably reduced [5]. The influence of the magnitude of the initial gap on the sensitivity of our sensor, connected in a differential measuring circuit, can be seen from Fig. 3, which shows the change in the magnitude of the output signal as the air gap decreases for different initial values. The curves were obtained at a constant supply voltage of the measuring circuit. Because of the large transverse deformations of the chain link and the need to increase the sensitivity of the strain link at small loads, it was not possible to satisfy the condition that the change in the air gap be small in comparison with its initial value; in the strain-link design under consideration, the value of $\Delta l$ differed only slightly from $l$. This circumstance, in our opinion, is not of essential importance. It merely increases to some extent the labor involved in interpreting the oscillograms.

Along with this shortcoming, for the reason indicated above, this strain link also had a very important advantage over other electric dynamometers intended for similar purposes, which consisted in a considerable increase in the power of the output signal. The power of the output signal increased so much that recording the measured parameter on the oscillograph film became possible without the use of an amplifier, not only with a type VIII vibrator, but even with types V and IV. We manufactured a series of strain links for various ranges of measured forces: for measuring forces in the ranges $P = (0—3000)$ kg, $P = (0—5000)$ kg, and $P = (0—7500)$ kg. The initial value of the air gap was, respectively: in the first case—about 0.30 mm, in the second—about 0.35 mm, and in the third—about 0.70 mm. Fig. 4 gives a calibration curve for a strain link designed to measure forces up to 5000 kg. The curve was obtained at a supply voltage of the measuring circuit $U = 20$ V, using a current with a frequency of 50 Hz. Along the abscissa axis in this graph

the magnitude of the deflection of the beam of the V-type vibrator is plotted on the screen of the MPO-2 oscillograph. Curve 1 was obtained under loading, curve 2 under unloading. The dependence obtained for this strain element, as for any elastic element, has a clearly pronounced hysteresis loop.

Fig. 3. Change in the magnitude of the output signal as the air gap decreases for various initial values of it.

Figure 5 shows a sample oscillogram obtained by the author when measuring forces in two branches of the chain of a two-motor overhead conveyor with safety fluid couplings at the Altai Tractor Plant. In accordance with the recommendations in [3] and with the low frequency of variation of the measured quantity, an industrial frequency of 50 Hz was used to supply the sensors. The output signal was converted by means of a bridge assembled from germanium diodes and providing full-wave rectification into a pulsating current of frequency 100 Hz, which was recorded on the oscillograph film (see the circuit in Fig. 2). Thus, the envelope of the maximum current values on the oscillogram gives an indication of the magnitude and nature of the change in the forces in the conveyor chain. The zero lines for the recorded quantities are located one at the lower edge of the oscillogram, the other at the upper edge. The film-pull speed during recording of this oscillogram was 50 mm/sec.

When using alternating current of frequency 50 Hz to supply inductive sensors, it is practically difficult to eliminate the carrier frequency by applying filters at the output. In addition, the oscillating

Fig. 4. Calibration graph of the load cell:
1 — loading; 2 — unloading

Fig. 5. Oscillogram of forces in the chain of an overhead conveyor, recorded using load cells with inductive sensors

with a frequency of 100 Hz, the beam of the mirror of the vibrator used to record the measured parameter simultaneously also serves as a time marker. As can be seen from the oscillogram shown, such records are readily legible even when one is superimposed on another. The reasons indicated above compelled us to abandon the use of filters at the output of the measuring circuit.

The use of the developed strain-gauge links for investigating an overhead conveyor under plant conditions demonstrated their high reliability. This circumstance, as well as the advantages discussed above, allows us to recommend them for use not only in measuring forces in conveyor chains, but also simply as dynamometers.

REFERENCES

1. A. G. Sosnin. Experimental study of a two-drive scraper conveyor. Problems of Mine Transport. Collection of articles. Issue 2, Ugletekhizdat, 1957.

2. A. A. Renggevich and others. Mine dynamometric car and its equipment. Problems of Mine Transport. Collection of articles. Issue 2, Ugletekhizdat, 1957.

3. A. M. Turichin. Electrical Measurements of Nonelectrical Quantities. Gosenergoizdat, 1959.

4. N. P. Raevskii. Sensors of Mechanical Parameters of Machines. Publishing House of the Academy of Sciences of the USSR, 1959.

5. Stresses and Deformations in Machine Parts and Assemblies. Edited by N. I. Prigorovskii. Mashgiz, 1961.