(2) Digital ratio method
The digital phase is implemented by logic gates and counters, and the phase difference is directly output in digital form. The phase comparison principle: two co-frequency signals θ1 and θ2 are amplified and shaped to obtain two sets of pulse signals u1 and u2, which respectively control the opening and closing of the counter through a logic gate circuit. The counting result of the counter is the time interval Δt between θ1 and θ2, which is proportional to the phase difference δ(t). If the phase of the phase signal is T, then δ(t)=2πΔt/T.
The main features of the digital ratio phase measurement method are: 1 Since the value of Δt depends not only on the phase difference δ(t) of the two signals, but also on the frequencies of the two signals. Therefore, in order to obtain higher-accuracy measurement results, it is necessary to ensure high accuracy of both phase-phase pulse signals and clock signals. In a phase-to-phase period T, any factor that causes a change in the frequency of the phase signal will affect the measurement. 2 Although the digital ratio compensates for some of the shortcomings of the analog phase, the measurement stability and reliability are improved, but it can only be applied to the same frequency ratio phase.
(3) Microcomputer subdivision ratio phase method
Since the 1980s, the computerization of test instruments has become an important development trend of measurement technology. In the machine tool transmission error measurement, the microcomputer subdivision ratio phase method has begun to be widely used.
The microcomputer subdivision ratio phase method is a microcomputer application of the digital phase comparison method. Because the computer has powerful logic, numerical calculation function and control function, it is easy to realize the high-frequency clock subdivision, phase comparison and output of the two signals, so the peripheral circuit is relatively simple to manufacture. The transmission error is δ(t)=2πNt/N. In the phase-in phase, the high-frequency pulse φ is no longer generated by the external oscillating circuit, but directly uses the internal clock CP of the computer; the counting of the pulse CP no longer uses the logic gate counter, but uses a programmable timer/counter in the computer. The microcomputer subdivision ratio phase measurement method has the following advantages: 1 The two-way phase-to-phase signal does not need the same frequency (ie, the transmission ratio of the tested transmission chain can be any value), and the transmission ratio is a constant in the calculation of the transmission chain error. The phase-to-phase phase difference can be any value, and the phase difference must be less than 360°. 3 The integration of clock subdivision and phase comparison is realized, which greatly simplifies the hardware interface line. Since the number of divisions of the programmable counter can be controlled by computer software, the sampling frequency can be easily adjusted to accommodate the measurement of the transmission chain error at different speeds. 4 system subdivision accuracy and measurement accuracy is high, easy to form an intelligent, multi-functional measurement system.
3.2 Machine tool transmission error counting method
The analog phase and the digital phase are both in the same frequency ratio phase. In order to obtain the same frequency ratio phase signal, the gear ratio must be first divided. To ensure that the 2Ï€ phase inversion does not occur in each error range, the range division is also required. Since the crossover reduces the measurement resolution, it is necessary to multiply the frequency before the frequency division, which makes the measurement system more complicated. In addition, measurements cannot be made for non-integer gear ratios due to the inability to divide the frequency.
The digital counting measurement method adopts the non-same frequency ratio phase, so it is not necessary to divide the two pulse signals, and the number relationship between the output pulses of the two sensors can be directly used to calculate the computer bed transmission error.
(1) Direct counting measurement
The principle of direct counting measurement method: set the number of output signals per revolution of the input and output shaft sensors to λ1 and λ2, respectively, and select the output axis θ2 as the reference axis, and the sampling interval T is equal to the period of the θ2 pulse signal or an integral multiple thereof. According to the definition of the transmission error, the transmission error at the jth sampling is: δ(j)=[N1(tj)-N2(tj)(iλ1/λ2)]2π/λ1.
Since θ1 and θ2 are time-discrete pulse trains, the count N1(tj) of the θ1 pulse in the sampling time interval (N2 θ2 pulses) varies with time during measurement, and is usually a non-integer. Thus, the error Δ2π/λ1 caused by the fractional part Δ is ignored. In addition, the (iλ1/λ2) of the actual transmission system is not necessarily always an integer, that is, the frequency of the pulse θ1 is not necessarily an integer multiple of θ2. If the N1 theory is regarded as an integer processing, a theoretical error is caused, thereby limiting the application range thereof.
(2) Microcomputer subdivision counting measurement method
The measurement steps of the microcomputer subdivision counting measurement method are as follows: 1 the previous θ2 pulse is used as the door opening signal, the latter θ2 pulse is used as the closing signal, and the counter counts the number of pulses θ1 of the θ1; 2 the clock sequence θ1 is performed by the clock pulse CP. Interpolation subdivision, respectively, counting the fractional period count value TΔ of the θ1 pulse signal and the integer period count value T2; 3 calculating the transmission error: δ(t)=(N0+TΔ/T2-iλ1/λ2)2π/λ1.
The microcomputer subdivision counting measurement method has the following advantages: 1 can effectively reduce the measurement error Δ; 2 can fully utilize the internal resources of the computer and software control to simplify the external hardware circuit; 3 integrate measurement sampling, data processing and result analysis to achieve Intelligent measurement.
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