Fixed number timing method (periodic measurement method): 1 time unit has great error at high speed.
Synchronous counting and timing method: it combines the advantages of timing counting and timing, and achieves high accuracy in the whole measurement range. More than 5/10000 tachometers adopt synchronous counting and timing method.
In order to ensure the reliable performance of the generator, the output electromotive force of the motor tachometer generator has the characteristics of high slope, linear characteristics, no signal area or small residual voltage, small output voltage asymmetry during forward and reverse rotation, and insensitivity to temperature.
The electronic speedometer consists of a speed sensor and a meter head (display). Most of the output pulse signals can be matched with the voltage and current input pointer table and digital table through frequency-current conversion, or directly sent to PLC. The frequency-current conversion methods include resistance-volume method, charge pump method and ASIC method. The first two methods are also used in magnetoelectric tachometer.
Most ASIC are a combination of resistance-volume division method and charge pump method. When there are certain requirements for display accuracy, reliability, cost and flexibility, the pulse frequency can be directly used to run the tachometer. Frequency operation methods include timing counting method (frequency measurement method), fixed number timing method (period measurement method) and synchronous counting timing method. ?
Extended data:
Classification of motor speed measurement
1, hollow cup rotor asynchronous motor tachogenerator:
When the rotor does not rotate, the magnetic field generated by the cup-shaped rotor current excitation is perpendicular to the axis of the output winding, and the output winding does not induce electromotive force; When the rotor rotates, the magnetic field generated by the cup-shaped rotor coincides with the axis of the output winding, and the electromotive force induced in the output winding is proportional to the rotating speed of the cup-shaped rotor, while the frequency is the same as the excitation voltage, regardless of the rotating speed.
2. Cage rotor asynchronous motor tachogenerator;
Similar to AC servo motor, due to poor output linearity, it is only used in occasions with low requirements.
3, synchronous motor tachogenerator:
Alternator with permanent magnet as rotor. Because the output voltage and frequency change with the rotating speed at the same time, and the rotating direction cannot be distinguished, it is inconvenient to use, so it is rarely used in automatic control systems, and is mainly used for direct measurement of rotating speed.
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