ELECTRONIC EXTREMUM UNIT OF AN AUTOMATIC DEVICE FOR MEASURING VIBRATION CHARACTERISTICS

A. V. Trikhanov, V. M. Razin

(Presented by the scientific seminar of the Faculty of Automation and Computer Engineering)

The electronic unit for extrema, start, and stop is the most important unit of an automatic device for determining the mean value of the displacement \(A\) of the body of a pneumatic hammer [1].

The quantity \(A\) is determined for a fixed number \(n_{\phi}\) of oscillations of the body by the formula [1]:

\[ A=\frac{1}{n_{\phi}}\sum_{i=1}^{n_{\phi}}(x_i-x'_i), \]

where \(x_i\) and \(x'_i\) are, respectively, the values of the maximum and minimum of the vibrogram of the hammer body for the \(i\)-th oscillation.

Computation of the quantity \(A\) by the indicated formula begins with the first maximum and ends at the \(n_{\phi}\)-th maximum; in this case, a shift along the vibrogram by any number of oscillations is possible.

Counting the quantity \(A\) can readily be realized with the aid of an electronic digital printing device of the ETsPV-1 type [2], by replacing its decimal counter with a reversible one, adding a special counter for counting the number of oscillations, and also adding the indicated unit for determining extrema, start, and stop.

The unit for determining extrema, start, and stop is intended for determining the time instants at which the vibrogram passes through extrema, for controlling the operation of the input device and the reversible counter. At the time instants of the extrema it generates a starting pulse for the ETsPV-1.

The unit must satisfy the operating conditions both with an electronic model having a fundamental vibration frequency of \(0.1 \div 0.2\) cps, and when operating with an actual hammer, where the fundamental vibration frequency varies from 15 to 50 cps.

In accordance with its functions, the unit for determining extrema, start, and stop has the functional diagram shown in Fig. 1. The operation of the unit is illustrated by the timing diagrams shown in Fig. 2.

A voltage proportional to the velocity of displacement of the hammer body, \(\dot U_k\), is fed to the input of the unit. At the instant when it passes through zero, Schmitt triggers TSh1 and TSh2 operate; moreover, they are adjusted so that trigger TSh1 operates more precisely when \(\dot U_k\) passes through…

Fig. 1. Functional diagram of the extremum, start, and stop unit

Fig. 2. Timing diagrams of the extremum, start, and stop unit

...zero from the positive region into the negative one, and the Schmitt trigger SHT2 from the negative region into the positive one. Owing to the hysteresis of the characteristic of a Schmitt trigger, the indicated functions cannot be implemented with sufficient accuracy by a single trigger, especially a tube trigger.

The signals from the Schmitt triggers are taken from the right-hand outputs, since they provide steeper fronts of the voltage steps. The leading edge from SHT1 is the signal of the minima of the vibrogram, and the trailing edge of the pulses from SHT-2 is the signal of the maxima of the vibrogram. The pulses from the Schmitt triggers pass through differentiating circuits and amplifier-shapers. The amplifier-shaper is triggered by positive pulses. These pulses correspond to the leading edges of the pulses from the Schmitt triggers. Consequently, in order to record the maxima and minima, the signal from Schmitt trigger 1 is inverted by the NOT circuit (Fig. 1).

The amplifier-shapers are provided to amplify the amplitude of the voltage pulses from the differentiating circuits to a level sufficient for triggering the control register. The shaper \(\Phi_{\max}\) delivers a negative voltage pulse at the instants of the maxima of the vibrogram, and the shaper \(\Phi_{\min}\) at the instants of the minima of the vibrogram.

These pulses alternately transfer the control register \(P_{\mathrm{ru}}\) from one state to the other. The state of the control register determines the counting mode of the reversible counter. The zero state corresponds to subtraction, and the one state to addition. The output-voltage levels of the control register are applied through cathode followers.

The control register is also used to generate the initial starting pulse for starting the interval-cutoff unit. For this purpose the steps from \(P_{\mathrm{ru}}\) are differentiated in the circuits \(DC_3\) and \(DC_4\). The negative pulses at their outputs are cut off. The remaining positive voltage pulses are combined by means of an OR circuit to start the interval-cutoff unit of the digital voltmeter.

To start the device in the operating mode for determining the mean value of the housing displacement, the “start” button is pressed.

When the “start” button is switched on, by means of the starting register \(P_{\mathrm{p}}\), the starting relay \(P_{\mathrm{p}}\) connects, with its contact \(1P_{\mathrm{p}}\), the voltage \(U_k\) to the input of the extremum unit.

The beginning of the count must be determined by some maximum. This condition is fulfilled in the circuit in the following way.

Before the device is started, the control register is transferred to the zero state.

The following situations are possible at startup:

a) relay \(P_{\mathrm{p}}\) has switched on after a maximum of the vibrogram,

b) relay \(P_{\mathrm{p}}\) has switched on after a minimum of the vibrogram,

c) relay \(P_{\mathrm{p}}\) has switched on either at the instant of a minimum or at the instant of a maximum.

In the initial state, when relay \(P_{\mathrm{p}}\) is de-energized, the state of the Schmitt triggers is characterized by their left-hand tubes being closed and their right-hand tubes open.

When relay \(P_{\mathrm{p}}\) operates in the interval from a maximum to the next minimum, a negative value of \(U_k\) is applied to the input of the Schmitt triggers. Consequently, in this case connection of \(U_k\) to the inputs of the Schmitt triggers does not change the state of these triggers.

At the instant of the nearest minimum, both triggers will switch to the other state.

The shaper $\Phi_{\min}$ produces a negative pulse. It is fed to the zero input of the control register. Therefore the register remains in its initial state. The initial start pulse is not produced and, consequently, the time–pulse converter is not started.

Negative pulses corresponding to the times of the maxima of the vibrogram are applied to the one input $P_{\mathrm{c}}$ of the control register. But since the register did not operate when the signal of the minimum under consideration arrived, the first negative pulse corresponding to a maximum transfers it to the other stable state.

The reversible counter is switched to addition mode. The initial start pulse is produced. In the counter, let us repeat, the value of the first maximum is obtained.

When relay $P_{\mathrm{p}}$ operates in the interval from the minimum to the nearest maximum, a positive value $\dot U_{\mathrm{k}}$ is applied to the input of the Schmitt triggers.

In this case, connecting $\dot U_{\mathrm{k}}$ to the inputs of the Schmitt triggers changes their state.

At the same time a negative pulse is produced by the shaper $\Phi_{\min}$. The control register does not change its (zero) state. The initial start pulse is not produced. At the instant of the nearest maximum, the device is started in exactly the same way as in the preceding case. The cases considered are shown in Fig. 2 by the corresponding lines.

When relay $P_{\mathrm{p}}$ operates exactly at the instant of a minimum, the Schmitt triggers operate in the same way as when the voltage $\dot U_x$ passes from the negative region into the positive one.

After the first measured maximum, the unit under consideration produces an initial start pulse for all subsequent extrema until the starting relay is de-energized, i.e., until the starting register returns to its initial (zero) state.

This means that the device will process the values of the extrema until a “stop” signal is received.

In the mode for determining the mean value of the body displacement, a signal (the “stop” signal) is applied to the one input of the starting relay after the prescribed number of oscillations $n_{\phi}$ has been reached; this signal transfers the starting register to its initial state. The starting relay is de-energized. The voltage $\dot U_{\mathrm{k}}$ is disconnected from the inputs of the Schmitt triggers, which somewhat earlier have returned to their initial state. Operation of the setup is completed.

From the reversible counter a number is read corresponding to the mean value of the body displacement.

REFERENCES

1. A. V. Trikhanov, V. F. Gorbunov, I. G. Smyshlyaeva, N. I. Sablin, V. M. Razin. A device for automatic processing of measurements of the vibration amplitude of flexible hull plates. Abstracts of reports of the IV Scientific and Technical Conference “Cybernetic Ways of Improving Methods of Measurement and Control,” Leningrad, July, 1964.

2. Electronic digital printing voltmeter of type ETsPV-1. Description and instructions for operation and adjustment. PR.3.268.001 OP and IR, 1961, Kazan.

3. Low-frequency phase meter of type NF-3. Technical description and instructions for operation and adjustment. PR 2721002 TO and IR, 1961, Kazan.