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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Aircraft tachometer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology The tachometer described in the article is designed to measure the rotational speed of the aircraft model propeller, but it can also be used to control the operation of other bladed mechanisms - rotors, impellers, obturators. The principle of operation of this device is based on measuring the frequency of interruption by the propeller blades of the modulated infrared radiation flux created by the tachometer and incident on its photosensitive element. Compared with tachometers of similar purpose [1-3], similar to the one described by the principle of operation, the proposed device is more noise-resistant. They can measure the rotational speed of propellers containing two, three and four blades. The tachometer is equipped with a pointer indicator of the rotational speed, which provides not only quantitative, but also qualitative information about the dynamics of changes in the controlled parameter [4]. The instrument has two measurement limits for propeller speed: up to 3000 rpm and up to 30000 rpm. Measurement error - no more than ±2,5%. There is a quartz calibrator, which increases the measurement accuracy and allows you to quickly monitor the performance of the device. The tachometer is made on an affordable element base and is easy to set up. The functional diagram of the device is shown in fig. 1. The crystal oscillator generates a periodic sequence of rectangular pulses following at a frequency of 100 kHz. From the output of the generator, these pulses are fed to frequency dividers by 20000 and 2000, forming pulses following at a frequency of 50 and 500 Hz, respectively. These pulses are intended to calibrate the tachometer before taking a measurement. A frequency of 50 Hz corresponds to a propeller speed of 3000 rpm (maximum at the first measurement limit), and a frequency of 500 Hz - 30000 rpm (maximum at the second measurement limit). Switch SA1 select the measurement limit, and switch SA2 - the operating mode of the device (calibration or measurements).
In the device calibration mode, pulses with a frequency of 50 or 500 Hz are fed through switches SA1.1 and SA2.1 to one of the inputs of the AND logic element, the second input of which receives pulses with a frequency of 100 kHz from the output of a quartz oscillator. At the output of the logic element, a sequence of bursts of pulses with a frequency of 50 kHz follows with a frequency of 500 or 100 Hz. This sequence is fed to the signal input of the IR transmitter, the operation of which is allowed by pressing and holding the SB1 button. The trigger in the button circuit eliminates the bounce of its contacts. Having reached the IR receiver, which is some distance away from the transmitter and located on the same optical axis with it, the IR radiation is again converted into an electrical pulse signal. It is amplified and filtered out by a bandpass amplifier. The amplified signal is detected by amplitude and converted into a sequence of pulses following with the frequency of repetition of bursts of IR radiation. After amplification and shaping by a Schmitt trigger, these pulses become rectangular with steep drops. In the calibration mode, the pulses from the output of the Schmitt trigger start a single vibrator that normalizes their duration, which, depending on the selected measurement limit, is changed by the switch SA1.2. The constant component of the output voltage of a single vibrator, directly proportional to the frequency, is measured by a voltmeter from a PA1 microammeter and additional resistors Rext1 and Rext2selected by switch SA1.3. These resistors are trimmers, with their help, in the calibration mode, the pointer of the PA1 microammeter is set to the last division of the scale at each measurement limit. When switching switch SA2 to the "Measure." instead of the calibration pulses, the input of the valve (logical element AND) receives a constant level of a logical unit, as a result of which the sequence of emitted IR pulses with a frequency of 100 kHz becomes continuous. IR radiation on the way from the transmitter to the receiver is periodically interrupted by the blades of a rotating model aircraft propeller inserted into the gap between the transmitter and receiver. Therefore, the pulse frequency at the output of the Schmitt trigger is equal to the product of the propeller speed and the number of its blades. There may be two, three or four. To take this factor into account, the signal path between the Schmitt trigger and the single vibrator is switched on using switches SA3 and SA2.2 with a pulse repetition frequency divider by two, three or four. Schematic diagram of the tachometer is shown in fig. 2. The pulse generator with a frequency of 100 kHz consists of logic elements DD1.1, DD1.2, resistor R4 and a quartz resonator ZQ1. Logical element DD1.3 - buffer. Frequency dividers are built on binary counters DD2, DD7 and logic elements DD1.4, DD4.1-DD4.3, DD6.1. Pulses with a frequency of 50 Hz are removed from the output 15 of the counter DD7, and pulses with a frequency of 500 Hz - from the output 13 of the counter DD2.
Elements DD8.1, DD8.2 perform a logical function AND. The trigger that generates the signal enabling the operation of the transmitter consists of logical elements DD8.3, DD8.4. Logical elements DD6.2-DD6.4, connected in parallel, and the transistor VT4 form a pulse amplifier that feeds the IR emitting diode VD4. The IR receiver consists of a photodiode VD1 and a source follower on a transistor VT1. The bandpass amplifier is built on the op-amp DA1 and the transistor VT2. The R7R8C5 circuit sets a constant bias at the non-inverting input of the op amp, and the R10 resistor sets its control current. The negative feedback circuit of the amplifier is formed by resistor R12 and decoupling capacitor C4. Capacitor C6 serves for the frequency correction of the op-amp. Transistor VT2 is an emitter follower that increases the load capacity of the op-amp DA1. In the tachometer made by the author, the voltage gain of the bandpass amplifier at a frequency of 100 kHz is 400. The cutoff frequencies of the passband at the level of -3 dB are 75 and 135 kHz. From sample to sample of the device, the values of these parameters may differ from those given by 15 ... 20%, which does not have a significant effect on the operation of the device. However, the maximum gain frequency should be within 100 ± 5 kHz. If necessary, it is corrected by a selection of resistors R10, R12 and capacitors C4, C6. Usually it is enough to choose a resistor R10. The amplitude detector is assembled on diodes VD2 and VD3, and the amplifier of the detected pulses is assembled on the op-amp DA3. The R16R24C10 circuit provides the necessary constant bias at the non-inverting input of the op-amp. Resistor R31 sets its control current. Capacitor C12 is separating. The negative feedback circuit of the amplifier is formed by resistors R27, R33 and capacitors C16, C18. The voltage gain in the middle of the passband is 5. Capacitors C12, C16 form the frequency response of the amplifier in the low-frequency region (cutoff frequency 1 ... 2 Hz), and capacitor C18 - in the high-frequency region (cutoff frequency 8 kHz). The input impedance of the amplifier is set by resistor R22. The Schmitt trigger consists of logical elements DD3.1, DD3.2 and resistors R3, R5 that set its switching thresholds. Dual binary counter DD5 and logic elements DD3.3, DD3.4 form frequency dividers by two, three and four. The single vibrator is made on the integrated timer DA2, the timing elements of which are the capacitor C13 and the resistors R25 and R26 switched when the measurement limit changes. Capacitor C15 - filtering. The electronic key on the transistor VT3 and the differentiating circuit R21C8 form short single-vibrator start pulses at the moments of increasing pulse drops at the input of the electronic key. Resistors R29, R30, R34, R35 form additional resistance for the PA1 microammeter. Capacitor C17 reduces the jitter of the microammeter needle at the lower limit of measurement. The contacts of the SB1.2 button shunt the PA1 microammeter when the button is not pressed, and it is not required to read the instrument readings. This eliminates sharp fluctuations of its needle dangerous for the microammeter at the moments of turning the tachometer on and off, switching the measurement limits and operating modes. The device is powered from a source of stabilized voltage +9 V with a maximum output current of at least 0,5 A. Capacitors C2, C3, C9, C14 - filtering in the power circuit. The tachometer parts are hinged mounted on a breadboard. The emitting diode VD4 and the photodiode VD1 are located outside the board at a distance of 150...200 mm from one another, forming a gap, which, when measuring the speed, is crossed by the blades of a rotating propeller. The device uses oxide capacitors K50-35, other similar ones can be used instead. Ceramic capacitors - K10-17, KM-6 or imported ones are suitable instead. The timing capacitor C13 is K73-17, it can be replaced with a K73-9, K73-24 or other film capacitor. Fixed resistors - C2-33. Trimmer resistors - SP2-2a or other similar ones. The device uses PGK biscuit switches and a KM2-1 double button, instead of which other similar ones can be used. Microammeter - M906 or another with a full deflection current of the arrow 100 μA. Diodes KD522B can be replaced by diodes of the same series or, for example, series KD503, KD521. Instead of the IR-emitting diode AL129A, diodes of the same purpose of the AL107, AL118 series or imported ones are suitable. Photodiode FD-256 can be replaced by photodiodes FD-21KP, FD-25K, FD-26K. Replacing the KP307G field-effect transistor - transistors of the same series with a different index or KP303 series, KT315B transistors - other low-power npn silicon structures. Instead of the KT973A transistor, it is permissible to use KT973B. When replacing op-amps KR1407UD3 and KR140UD1208 with 1407UD3 and 140UD12, respectively, one should take into account their differences in case type and pin assignment. Microcircuits of the K561 series can be replaced by microcircuits of the 564 series or imported analogues, and the KR1006VI1 microcircuit can be replaced by the imported 555 series. The adjustment of the functional units of the tachometer has no special features and is carried out according to known methods. The alignment of the optical axes of the emitting diode VD4 and the photodiode VD1 is controlled by the maximum amplitude of the signal with a frequency of 100 kHz at the output of the bandpass amplifier (the emitter of the transistor VT2) while the button SB1 is pressed. The pointer of the PA1 microammeter is set to the last division of the scale when calibrating the device within the measurement limits of 3000 and 30000 rpm, respectively, with trimming resistors R35 and R34. When measuring the rotational speed of a propeller, the blades of which are made of a material that weakly absorbs infrared radiation, the normal operation of the tachometer is achieved by reducing its sensitivity to infrared radiation. To do this, the trimmer resistor R6 reduces the amplitude of the signal at the input of the bandpass amplifier. Literature
Author: O. Ilyin
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