1. measuring principle
The electronic digital tachometer consists of a digital speed sensor and an electronic counting circuit. The sensor converts the rotating speed of the rotating shaft into an electric pulse signal, which is amplified and shaped by the circuit and sent to an electronic counter to display the corresponding measured rotating speed value. General frequency method is used for medium and high speed, and periodic method is used for low speed measurement.
The speed measurement adopts (1) frequency method: that is, the electric pulse signals output by the speed sensor are counted within the sampling time of the electronic counter. Control the counter gate with standard time. When the displayed value of the counter is n, the measured rotation speed n is
Design and application of hydraulic power head core drilling rig
Where: z is the number of electric pulse signals sent by the sensor for each revolution of the rotating body; T is sampling time, min.
In order to improve the measurement accuracy, the number of electric pulse signals output by the speed sensor per revolution can be increased or the sampling time can be extended.
(2) Measuring rotational speed by periodic method: When measuring a certain angle, the electronic counter measures the time interval of a certain rotation angle. Here, the measured period t is used to control the gate, and the filling time τ0 enters the counter count n.
In order to improve the accuracy of periodic measurement, the measured period is multiplied by the frequency division of the periodic signal. Therefore, the measured rotational speed n is
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Where: k is the period multiplier (100,1kloc-0/,102, 103 ...).
2. Speed sensor
Its function is to convert the rotating speed of the measured rotating body into an electric pulse signal and push the counter to count.
According to the working principle, commonly used speed sensors can be divided into photoelectric type, magnetoelectric type, capacitive type, Hall element type and eddy current type. They are characterized in that they are connected with the measured rotating shaft through couplings or marks, and the angular displacement of the rotating shaft is converted into electric pulse signals in a non-contact way. When measuring, two sensors with a difference of 90 degrees are often used to judge the rotation direction. According to the phase relationship, the circuit judges the rotation direction and controls the addition and subtraction of the counter.
3. Digital speed measuring circuit
See Figure 7-5 for the principle block diagram of digital speed measuring circuit.
Figure 7-5 Principle Block Diagram of Digital Velocity Measurement Circuit
The electric pulse signal sent by the speed sensor is sent to the C gate through the amplifier shaper, which is also used to control the addition and subtraction of the counter and distinguish the rotation direction. The signal of the standard clock oscillator controls the opening and closing of the gate C through the bistable A. Therefore, the counter counts a group of speed signals at every standard time. If the standard time is 1 min, the counter is revolutions per minute. At the same time, the signal of the standard clock oscillator is also sent to the input of bistable B. When the door C is closed, the bistable B also flips, sending a signal to close the door D. At this time, the counter displays the measured revolution value through the display screen. The adjustable low-frequency starting oscillator is used to control and adjust the display time of speed measurement. It resets the counter to zero, flips the bistable B, opens the door D, and repeats the next speed measurement process.
Digital tachometer is convenient and accurate, and it is easy to connect with computer for control and parameter calculation. The measuring range is from hundreds of thousands to tens of days per minute, so it is widely used in engineering testing.