GB2133880A

GB2133880A - Measuring a force dependent frequency

Info

Publication number: GB2133880A
Application number: GB08333911A
Authority: GB
Inventors: Masami Yamanaka; Yasutoshi Masuda
Original assignee: Yamato Scale Co Ltd
Current assignee: Yamato Scale Co Ltd
Priority date: 1983-01-17
Filing date: 1983-12-20
Publication date: 1984-08-01
Also published as: DE3401269A1; AU2236683A; AU543914B2; GB2133880B; JPS59131131A; FR2541455A1; FR2541455B1; GB8333911D0; DE3401269C2

Abstract

A device for measuring a force including a force-responsive oscillator (20) which produces a first electric signal with a frequency relating to the applied force a second oscillator (24) for producing a second electric signal with a known frequency which is higher than the first signal frequency, and means (28,30) for counting the number of cycles of the second signal produced during a time period corresponding precisely to a predetermined number of cycles of the first signal and for calculating the first frequency and hence the applied force from the resultant count. The device displays good accuracy even though the force responsive oscillator has a low frequency. <IMAGE>

Description

SPECIFICATION Device for measuring a force This invention relates to a force measuring device.

There are known many kinds of force measuring device or weighing device of so-called vibration type in which a vibratory body changes its frequency of vibration in response to a force applied to it. Some are disclosed, for example, in the article of Yuzuru Nishiguchi entitled "Vibration Electronic Balances", KEIRYO KAN RI (Weighing Control) vol. 32, No.4 (1983) pp. 10-16. In such devices, the frequency of vibration of the vibratory body is measured typically by deriving the vibration as an electric oscillating signal, passing this signal through a transmission gate which opens for a predetermined time period and counting the number of cycles of the output signal of the gate.While the transmission gate is controlled with a separately generated timing signal, the time errors of the leading and trailing edges of the timing signal may result in an error in the count of the number of cycles, which tends to affect the accuracy of this measuring device. The accuracy of measurement of the number of cycles increases naturally with increase of the signal frequency and there is almost no problem in those devices utilizing piezoelectric materials. However, this problem is severe in those devices of relatively low frequency, utilizing vibratory wires.

There is proposed a force measuring device utilizing a vibratory string in our copending UK patent application No. 8325509 filed September 23, 1 983, and in this device the frequency of vibration of the wire from the maximum load is several thousand hertz. For example, when the frequency is 5,000 Hz, even if the counting period is 0.1 second, the count value is 500 and the accuracy of measurement is only 1/500. Although the accuracy of measurement may be improved by increasing the counting period, this is undesirable since it results in reduction of speed of response of the device. While the desired accuracy of measurement is in the order of 1/10,000, for example, the counting period must be two seconds for the frequency of 5,000 Hz in order to obtain this accuracy.Therefore, it cannot be used in those digital balances in which three to six measurements must be executed generally within one second.

According to the present invention there is provided a force measuring device comprising a force responsive oscillator for generating a first electric signal having a frequency dependent upon the applied force, a second oscillator for generating a second electric signal having a predetermined frequency greater than the frequency of said first electric signal, counting means for counting the number of cycles of said second electric signal for a time period corresponding to a predetermined number of cycles of said first electrical signal, and means for calculating the frequency of said first electric signal from the count value of said counting means, said predetermined number of cycles and the frequency of said second electric signal.Such a device may utilize a vibratory body which vibrates at a relatively low frequency and is capable of counting the number of cycles of the vibration at a higher accuracy than could be attained by the prior art devices.

The invention is described in more detail below with reference being made to the accompanying drawings, in which: Figure 1 is a block diagram representing a device for measuring a force according to the prior art; Figure 2 is a schematic circuit diagram representing an embodiment of the oscillator in Figure 1; Figure 3 is a block diagram representing an embodiment of the device for measuring a force according to this invention; and Figure 4 is a diagram representing waveforms used for explaining the operation of the device of Figure 3.

Figure 1 shows the prior art force measuring device described in the above mentioned copending application and 2 denotes an oscillator which produces an electric signal A having a frequency relating to the force F when it is applied to the oscillator. The output of this oscillator 2 is an electric signal of sinusoidal waveform, for example, as shown in Figure 4A, which is applied to a gate 4. The gate 4 is opened for a predetermined time T by a control signal from a timer 6 preset with this time T to pass the signal A. A counter 8 counts the number of cycles of the part of signal A which has passed the gate 4 and supplies the count value N to an arithmetic unit 10. The arithmetic unit 10 is arranged to calculate the frequency f=N/T and then calculate the applied force F therefrom, and an indicator 12 indicates the resultant force F.

The oscillator 2 has such a structure as shown in Figure 2, for example. More specifically, when a metal wire 14 is stretched between a pair of magnetic pole pieces, oscillating current is induced in the wire 14 by natural vibration of the wire in the (arrow) direction vertical to the magnetic field. This current is amplified by an amplifier 1 6 and then fed back to the wire 14 and the wire 14 is subjected to a driving effect of the fed-back current which is synchronous to the inherent vibration of the wire 14, thereby increasing its amplitude of vibration and retaining the vibration.As well known in the art, the frequency fof this vibration is proportional to square root of the tension F of the wire 14 and the mechanical vibration is derived as an electric oscillation of the same frequency as shown in Figure 4A from an output terminal 1 8. As the structure and operation of the oscillator 2 are described in detail in the abovementioned copending application and have no direct connection to this invention, no further description will be made.

The abovementioned shortcoming of the prior art device of Figure 1 is attributable to the gate 4 timed by the timer 6. More specifically, the counter 8 counts specific points on the waveform of the incoming signal, for example, the zero crossings P1, P2.... of negative-to-positive transitions (Figure 4A). If the time points at which the gate 4 is opened and closed move due to timing error or the timer 6, at least one of the specific points may be overlooked and result in the count error. The greater the count, that is, the higher the frequency, the smaller the percent of the error.

The vibration force measuring device according to this invention is based upon this principle and, as shown in Figure 3, includes a load F responsive oscillator 20 corresponding to the oscillator 2 of Figure 1. The oscillator 20 produces an output signal A (Figure 4) of frequency frelating to the load F and supplies it to a counter 22. The counter 22, which is cleared and then enabled by an undermentioned command signal B from an arithemtic unit 30, is arranged to count specific points of the incoming waveform, for example, the zero crossings P 1, P2,... at negative-topositive transitions (Figure 4A), to a preset count N and to produce a timing signal C having its leading and trailing edges at the time points of counts one and N, respectively.In the embodiment of Figure 4, N=4 and the time points P1 and P4 correspond respectively to the leading and trailing edges of the timing signal C.

Accordingly, the duration of the timing signal C is equal to the (N-i) cycle time of the output signal of the oscillator 20.

The device of this invention further includes a clock generator 24 for generating a clock pulse train D (Figure 4) of fixed frequency f1 which is much higher than (for example, 103 to 105 times) the frequency fto be measured. The clock pulse train D is applied to a transmission gate 26 which is controlled by the output timing signal C of the counter 22. The gate 26 is opened for the duration of the timing signal C and passes the clock pulses D only for this time period as shown in Figure 4E. The clock pulses E having passed through the gate 26 are supplied to a counter 28, which is cleared by the command signal B from the arithmetic unit 30, and counted their total number N1 The output count N1 of the counter 28 is applied to the arithmetic unit 30.Here, the frequency fis calculated in accordance with an equation f=f,(N--1)/N and, then, the load F is calculated in accordance with the known relation and indicated by an indicator 32.

In contrast to the counter 8 of the prior art device, the counter 22 of this invention does not count the number of cycles within a predetermined time period, but counts a predetermined number of cycles and defines the corresponding time period. Accordingly, there should be no error even if the frequency of the measured signal is substantially low. On the contrary, the counter 28 counts the number of cycles within a predetermined time period as same as the counter 8. However, it counts the signal which was separateiy and optionally produced and has a very high frequency.

Therefore, its counting error is very small. For this reason, the force measuring device according to this invention exhibits a remarkably small error of measurement due to the error of frequency measurement and, therefore, exhibits very high measuring accuracy and resolution, as compared with the prior art device.

It should be noted that the invention is not limited to the details of the above embodiments and various modifications and changes can be made within the sope of the invention as defined in the appended claims. For example, the preset number of cycles of the counter 22 and the frequency of the clock pulses D can be optionally selected in accordance with the structure and use of the device. Moreover, the load responsive oscillator 20 need not be limited to the vibratory string type as shown in Figure 2.

Claims

1. A force measuring device comprising a force responsive oscillator for generating a first electric signal having a frequency dependent upon the applied force, a second oscillator for generating a second electric signal having a predetermined frequency greater than the frequency of said first electric signal, counting means for counting the number of cycles of said second electric signal for a time period corresponding to a predetermined number of cycles of said first electric signal, and means for calculating the frequency of said first electric signal from the count value of said counting means, said predetermined number of cycles and the frequency of said second electric signal.

2. A force measuring device according to claim 1, wherein said counting means comprising a first counter for counting the predetermined number of cycles of said first electric signal and generating a control signal defining the starting and terminating points of the time period corresponding to said predetermined cycles, gate means coupled to the output of said second oscillator and opened only for said time period in response to said control signal, and a second counter coupled to the output of said gate means for counting the number of cycles of said second electric signal delivered from said gate means.

3. A force measuring device according to claim 1 or 2, wherein said force responsive oscillator comprises a metal wire subjected to a tension dependent upon the applied force, and a mechanoelectric converter for converting vibration of said metal wire into an electric oscillation.

4. A force measuring device substantially as herein described with reference to Figures 2 and 3 of the accompanying drawings.