Offer readers contactless electronic tachometer designed to measure the frequency of rotation of the propeller without any mechanical linkage of the sensor to the motor shaft.
The tachometer consists of two main parts — a sensor and a frequency counter (Fig. 1).
Fig. 1. A block diagram of the tachometer:
1 – sensor, 2 – meter, 3 – indicator, 4 – calibrator.
The sensor produces a pulse signal at a frequency multiple of the speed of rotation of the screw. The multiplicity in this case is determined by the number of blades. For this tachometer, you can use two types of sensors: optical and electrostatic.
Designed specifically for the described device of the electrostatic sensor converts the charge accumulated on the blades of a rotating screw when the friction of the air, in the switching voltage. For this purpose, the sensor has a sensing element (Fig. 2) — thin, metal plates or wire antenna, installed in parallel to the plane of rotation of the screw.
Fig. 2. The principle of operation of electrostatic sensor (a) and optical sensor (b). On the picture below:
1 — propeller, 2 — sensing element (antenna) of the electrostatic sensor, 3 — amplifier, 4 — source of light 5 — light receiver with a sensitive element of optical sensor, 6 — amplifier.
With the passage of charged blades past the antenna, there will be induced an alternating voltage, whose frequency will be determined by the expression (K*N)/60, where K is the number of blades of the rotor, N — rotational speed of screw (rpm).
Antenna electrostatic sensor is a source of low (order of a few millivolts) voltage with high internal resistance equals the insulation resistance. To ensure the normal operation of the frequency this voltage is fed to an amplifier with high input impedance (Fig. 3).
Fig. 3. Schematic diagram of the electrostatic sensor.
High input impedance is achieved by using the matching of the cascade, which is a combination of streaming repeater field on the transistor VT1, and the emitter follower bipolar transistor VT2. Operational amplifier DA1 provides amplification of signals to a level sufficient to operate the frequency counter.
The optical sensor consists of a light source, a sensitive element photodiode or photoresistor and amplifier.
The light source and the sensing element is arranged so that the beam passed through the plane of the screw. During the rotation of the blade to periodically intersect the beam falling on included between the base and the emitter, the sensitive element (Fig. 4), periodically changing its resistance and thus forming the base of the transistor alternating voltage.
Fig. 4. Schematic diagram of the optical sensor.
The resulting pulses are amplified two-stage amplifier to a value sufficient to operate the frequency counter.
The frequency counter converts aldatzeko pulses to direct current proportional to the pulse repetition frequency. Its main element is the standby multivibrator transistors VT5 and VT6 (Fig. 5).
Fig. 5. Schematic diagram of the frequency counter.
When entering the standby multivibrator of the sensor signals it produces pulses of constant duration determined only by the values of resistors and capacitances of the circuit.
The rotation of the propeller at the output of the standby multivibrator formed by a sequence of pulses with constant amplitude and duration, the frequency of which is proportional to the speed of rotation of the screw.
The pulse sequence contains a constant component, the magnitude of which depends on the duty cycle — the ratio of the repetition period of pulses to their duration, that is, the speed of rotation of the screw.
The DC component is extracted by integrating the pulse sequence. The integrating element is the gauge PA1, which serves both for indicating the speed of rotation of the screw. In the case of the used coil head 100 µa with an additional resistor R22. Can be applied and a rough tool. Variable resistor R21 is used in the calibration of the tachometer. Decoupling the integrator and standby multivibrator is used the emitter-follower transistor VT7.
The device is powered from batteries or a rectifier with a voltage of 9.5 V.
In the manufacture of a tachometer can be made any design, but the most appropriate one is a structure in the form of two units — sensor and a frequency indicator connected between a three-wire cable.
The electrostatic sensor must be carefully screened. The sensor antenna can be made of cut copper wire, narrow strips of brass or foil fiberglass. When conducting measurements, it should be parallel with the plane of rotation of the screw at a distance, to ensure the normal operation of the device.
To improve the accuracy of measuring the speed of rotation of the screw before you start, you need to calibrate the tachometer, which the Committee put the calibrator (internal or external). The calibrator is a multivibrator (Fig. 6) generating short pulses, the repetition frequency of which is determined by the values of resistors R24, R25 and capacitances C6, C7 and is selected based on the range of measured velocities. For sufficient measurement accuracy, calibration should be carried out in two or three points of the speed range. Thus the necessary pulse repetition frequencies for two-bladed propeller is determined by the expression f=N/30.
Fig. 6. Schematic diagram of the calibrator and the table of values for R25 of calibration points.
In the table (see Fig. 6) shows the values of the resistors R24 and R25 for different speeds of rotation of the screw. Accurate frequency setting is postroiku resistor R30, the control of frequency is performed using high-precision digital frequency meter.
To obtain several values of the frequency by the step change of the resistors R24 and R25, or using multiple generators.
B. Evstratov, engineer
Recommend to read BRO-11M – STEP INTO THE SKY Did you know that a lot of aircraft Amateur-built are based on BRO-11M - glider initial training famous Lithuanian aircraft designer B. I. Oscines. Very easy to manufacture and reliable... FIGHTER TANKS “CENTAUR” In the 1980s, the Italian army had developed tactical and technical requirements for prospective family of tracked and wheeled armored combat vehicles that were planned to start... 
When developing propeller systems snowmobiles, trikes, aircraft and model aircraft designer is required to know the exact values of some parameters. And most importantly — the frequency of rotation of the propeller. It is necessary and when the force of the engines, and the selection of the propeller. The frequency of rotation is also one of the main parameters in the process of operation of the motor: the value of this parameter is possible to objectively judge the reliability of the engine.