INSTRUMENT FOR STUDYING THE MECHANICS OF SLIDING CONTACT

L. Ya. Zinner, A. I. Skorospeshkin, I. A. Levsky

(Recommended by the scientific seminar of the Departments of Electrical Machines and General Electrical Engineering)

The profilometer is intended for measuring the profiles of commutators of electrical machines under static and dynamic conditions, with a lamella width of not less than 1.5 mm, and for measuring radial and tangential vibrations of brushes.

As the small-displacement sensor in the profilometer, the authors use a double-plate capacitive sensor they developed (Fig. 1). Both plates are arranged along one straight line along the commutator plate and are placed in a massive copper shield. At the end face of the shield

Fig. 1. Construction of a capacitive sensor:
1—sensor shield, 2—insulator, 3—measuring plate of the sensor, 4—zero plate of the sensor

a narrow slot 6 mm long and 0.1 mm wide is left. The sensor plates are placed in the slot of the shield. One of the plates is connected to the instrument ground. The shield protects the instrument from high-frequency pickups and parasitic capacitive couplings to ground; in addition, it plays the role of a guard ring, ensuring uniformity of the electric field of the sensor and, consequently, increasing the resolving power of the sensor across the width of the lamella.

The capacitance of such a sensor, without allowance for the shielding effect, is determined by the approximate formula

\[ C=\frac{27.8\,\varepsilon l}{\ln \frac{h}{r}+\sqrt{\left(\frac{h}{l}\right)^2-1}}, \tag{1} \]

where \(l\) is the length of the plate; \(r\) is the radius of the plate; \(h\) is the gap.

The capacitive sensor is connected into the measuring bridge with tight inductive coupling between the arms. The shielding circuit of the sensor and bridge is shown in Fig. 2. Owing to this arrangement it is possible to eliminate interference and the influence of parasitic connections to ground. In addition, since the bridge shield, the shield of the cable connecting the sensor to the bridge, and the guard ring are at the generator potential, a uniform field of the sensor can be obtained, which increases the linearity of the sensor characteristic and its resolving power.

The measuring bridge is supplied from a generator of stable frequency and stable voltage, assembled on transistors \(T_1\)—\(T_4\), which delivers to the load a voltage of \(4\) V at a frequency of \(1.5\) MHz.

To stabilize the output voltage of the generator, an automatic gain-control system is provided in the circuit. Part of the output voltage of \(T_4\) changes the base operating condition of \(T_3\), connected in parallel with \(T_2\) to the common emitter load, and, consequently, changes the dc operating condition of \(T_4\), which entails a change in its amplification factor. The stability of the generator voltage is \(0.5\%\).

The generator circuit used in the instrument also makes it possible to modulate the generator output voltage by applying a signal from a calibration generator to the base of \(T_3\).

The calibration generator, intended for calibrating the profile of collectors in dynamic operation, supplies rectangular pulses to the base of \(T_3\), which subsequently pass through the entire amplification path. The pulse repetition frequency is \(3\) kHz, and their height corresponds to a deviation of the gap between the sensor and the collector by \(10\ \mu\) and is set by the clipping level of diode \(D_1\).

The calibration generator consists of an LC generator assembled on \(T_5\), a diode limiter \(D_1\), and is connected to the base of \(T_3\) by toggle switch \(P_4\).

The measuring circuit of the profilometer (Fig. 2) consists of a high-frequency amplifier \(T_6\)—\(T_9\), an amplitude detector \(D_3\), a video amplifier \(T_{11}\)—\(T_{15}\), a circuit for automatic correction of the conversion characteristic of the sensor \(D_4\), \(D_5\), \(T_{10}\), and a dc amplifier for measuring the collector profile under static conditions \(T_{16}\)—\(T_{18}\).

To match the amplifier input to the bridge output, a cascode emitter follower \(T_6\), \(T_7\), having a high input resistance, is used.

The high-frequency amplifier \(T_8\), \(T_9\) is tuned to the generator frequency and provides at the detector output a voltage of \(2\) V, necessary for operation of the detector on the linear portion of its characteristic. The amplifier gain is 300. The amplifier is enclosed by negative feedback loops in order to stabilize the gain.

The correction circuit, made with diodes \(D_4\), \(D_5\) and transistor \(T_{10}\), is intended for linearizing the sensor characteristic. At a gap of \(0.3\) mm, diodes \(D_5\) and \(T_{10}\) are cut off, and the signal at the output of the high-frequency amplifier is determined only by its gain. When the gap is reduced, the voltage at the amplifier output increases linearly with the decrease in the gap, and the sensor characteristic remains linear over the interval from \(0.3\) to \(0.2\) mm. A further decrease in the gap leads to the fact that, as the output voltage of the amplifier increases, \(T_{10}\) is increasingly cut off by the voltage from \(D_5\) and shunts the input circuit.

Fig. 2. Schematic diagram of the profilometer.

of the resonant stage so that its output voltage remains linearly dependent on the gap up to \(0.12\) mm.

At gaps of less than \(0.12\) mm, when further correction by means of \(T_{10}\) becomes impossible, diode \(D_4\) is unlocked and shunts the tank circuit of the resonant amplifier, reducing its gain and thereby straightening the sensor characteristic.

The correction circuit developed makes it possible to eliminate the measurement error caused by the nonlinearity of the sensor characteristic.

The amplitude detector \(D_3\) is made according to the usual half-wave circuit with an open input and operates on the linear portion of the detection characteristic over the entire range of measured deflections.

The load of the detector is the emitter follower \(T_{11}, T_{12}\), which has a high input resistance and ensures constancy of the detector load resistance both for direct current and for video frequency current.

The load of the emitter follower is the video amplifier \(T_{13}—T_{15}\). The main requirement imposed on it is reproduction of a wide frequency spectrum with small frequency distortions. The negative-feedback circuit, encompassing the entire amplifier, ensures constancy of the gain with an accuracy of \(1\%\) in the frequency range from 10 to \(2 \cdot 10^5\) Hz. This ensures a small measurement error for both the low-frequency and high-frequency components of the collector profile.

To measure the profile of the collectors in the static mode and to set the initial gap, a direct-current amplifier \(T_{16}—T_{17}\) is included in the instrument circuit. The load of the amplifier is an M-24 microammeter. By adjusting the “compensation” control, the pointer is brought to the mark 50, which corresponds to the installation gap of \(200\ \mu\)m. When measuring the profile in the static mode, the value of the large division is 20, and of the small division \(4\ \mu\)m.

For ease of operation, a synchronization and rotational-speed measurement unit is included in the profilometer circuit.

A photodiode FD-2, installed next to the capacitive sensor, is used as the sensor. One of the lamellas is blackened with India ink. In this case the photodiode responds to the difference in the reflection coefficients from the lamellas and delivers a photocurrent pulse to the input of the amplifier \(T_{19}—T_{21}\), from whose output the amplified pulse is fed to the oscilloscope synchronization.

By selecting the lamella, the oscilloscope sweep can be synchronized relative to any lamella.

The voltage pulse taken from \(T_{21}\) is limited by diodes \(D_7, D_8\) and triggers the flip-flop \((T_{22}—T_{23})\), which delivers to the integrating circuit, consisting of diodes \(D_6, D_9\), a \(200.0\) capacitance, and \(47\) kΩ and \(1\) kΩ resistors, pulses of \(500\ \mu\)s duration and \(5\) V amplitude. When the \(100\ \mu\)A indicator is connected by toggle switch \(P_3\) into the integrating circuit, the deflection of the pointer is proportional to the rotational speed.

The value of the large division in this case is \(1000\) rpm, and of the small division \(200\) rpm.

If necessary, the range of measured speeds can be expanded by introducing additional elements into the integrating circuit, and the accuracy of reading can be increased by dividing the speeds into subranges.

The profilometer is powered from a stabilized source—18 V.

Structurally, the profilometer is made as a small-sized portable instrument with dimensions \(350 \cdot 150 \cdot 175\) mm (Fig. 3). The principal technical data of the instrument are as follows: limits of measured deflections

Fig. 3

profile \((200 \pm 150 \,\mu\text{k})\); instrument sensitivity \(50 \frac{\text{mV}}{\mu\text{k}}\); error in static profile measurements on the \(200 \,\mu\text{k}\) scale not more than 3%; error in setting the working gap 1%.

Fig. 4

A typical oscillogram of the commutator profile is shown in Fig. 4, from which it can be seen that the instrument has high sensitivity to very small profile irregularities and high resolving power, making it possible to distinguish both group protrusions and protrusions of individual lamellae.