JP3296121B2 - Electronic balance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called electromagnetic balance type electronic balance having an electromagnetic force generator for generating an electromagnetic force for balancing a balance mechanism against a load.
More specifically, the present invention relates to an electronic balance of a type in which a pulse current flows through a force coil of an electromagnetic force generator.

[0002]

2. Description of the Related Art In an electronic balance of an electromagnetic force balance type, a displacement sensor detects a deviation from a balance point of a balance mechanism caused by a load to be measured, and a current corresponding to the detection result is applied to a force in a magnetic field. It has a servo mechanism that balances the balance mechanism by generating an electromagnetic force corresponding to the load to be measured by flowing through the coil, and the load to be measured is determined based on the amount of current flowing through the force coil in the balance state of the balance mechanism. I'm asking.

[0003] In such an electromagnetic force balance type electronic balance, conventionally, a DC current is applied to a force coil.
With the C servo system, there is a limit in obtaining the stability and high resolution of the system. Therefore, a pulse current with a constant period is applied to the force coil, and the duty of the pulse current is adjusted according to the displacement detection result of the balance mechanism. A so-called pulse modulation type that changes the frequency is often used.

[0004] In the pulse modulation type electronic balance, conventionally,
Basically, a current pulse is raised at a constant period, and a current pulse is detected at the time of detecting a coincidence of a voltage value between an analog voltage signal obtained by performing a PID operation on a displacement detection value and a constant sawtooth voltage signal. While ending
By counting the clock generated during that time,
Measuring the pulse width of a flowing current is often used.

Thus, the resolution of the duty of the pulse current flowing through the force coil is determined by the number of clocks generated in one cycle of the pulse current. Usually, the period of the pulse current is about 1 to 2 ms, and the clock frequency is 30 MHz.
z is only about 1/30000 to 1/60000, which is insufficient compared with the resolution required for an electronic balance.

Therefore, various proposals have conventionally been made to increase the resolution of the electronic balance to be higher than the resolution of the pulse duty. In Japanese Patent Application Laid-Open No. 47-41772, the current pulse is not terminated at the time when the analog voltage obtained by performing the PID operation on the displacement detection value coincides with the sawtooth voltage signal.
After the coincidence point, the time when the clock for measuring the current pulse width is first generated is added, and the measurement result of the load is obtained by adding each measurement result of a fixed number, for example, 200 current pulse lengths. Thus, the quantization error of each current pulse is eliminated by averaging.

In Japanese Patent Publication No. 58-29856, the current pulse is terminated at the time when the analog voltage signal obtained by the PID operation of the displacement detection value coincides with the sawtooth voltage, and the counting result of the clock generated during that time is used. The next PID is stored by storing the included quantization error in a capacitor or the like.
By adding the result to the calculation result, the error is eliminated.

In Japanese Patent Publication No. 5-32684, a plurality of pulse currents having different peak values (current values) are generated, and the displacement of the balance mechanism is taken in as a digital signal, and a digital PID operation is performed to perform a predetermined operation. By obtaining the data of the bits, of the data, for example, by allocating the upper bits to determine the duty of the pulse current having the higher peak value and distributing the lower bits to determine the duty of the pulse current having the lower peak value, A plurality of pulse currents having different current values and duties are supplied to a force coil, and a high-resolution electronic balance is obtained as a whole, even if the duty resolution of each pulse current is limited.

[0009]

Among the above proposals, Japanese Patent Laying-Open No. 47-41772 discloses that a quantization error included in the measurement result of the width of each current pulse is determined by a predetermined number. The purpose is to reduce the number of current pulses to be averaged by summing or averaging the measurement results of the pulses, but the number of current pulses to be averaged is appropriately set in advance regardless of the physical condition of the system, The error disappears only when the direction (positive / negative) of the quantization error during the period is the same, and an accurate metric value cannot be obtained.

Japanese Patent Publication No. 58-29856 describes PI
There is a disadvantage that the circuit configuration for reflecting the quantization error of each time on the analog voltage signal obtained by the D operation is complicated, and the cost is greatly increased.

Furthermore, in Japanese Patent Publication No. 5-32684, the cost of hardware is increased by the need to generate pulse currents of a plurality of peak values. SUMMARY OF THE INVENTION An object of the present invention is to provide an electronic balance which is simple in hardware and can achieve high resolution without increasing the cost.

[0012]

A configuration for achieving the above object will be described with reference to a basic conceptual diagram shown in FIG. 1. An electronic balance according to the present invention comprises a balance mechanism 1 using a load.
The displacement from the balance state is detected by the displacement sensor 2, and the current flowing through the force coil 3 is controlled in accordance with the output of the displacement sensor 2, thereby providing a servo system for balancing the balance mechanism 1. Includes pulse current generating means a for generating a pulse current whose duty changes at a constant period and with a predetermined resolution according to the output of the displacement sensor 2, and the pulse current is supplied to the force coil 3. In the balance configured so as to balance the balance mechanism 1 by the pulse current generation means a, the period of the pulse current is increased and decreased in synchronization with the period of increase and decrease of the duty of the pulse current. That counts the number of current pulses with different duties that flow through it and calculates the ratio Measurement data determination means for calculating load measurement data from the occurrence rate calculation means b, the ratio calculated by the pulse occurrence rate calculation means b, and one of the pulse duties of each of the current pulses flowing in the period. c
It is characterized by having.

[0013]

When the servo system is operated by passing a current from the pulse current generating means a having a resolution lower than that required for the balance to balance the balance mechanism 1 to the force coil 3, the pulse is generated. Due to insufficient duty resolution, the balance mechanism 1 attempts to balance while increasing or decreasing the duty of the pulse current flowing through the force coil 3. That is, for example, when the resolution of the duty of the current pulse is 1/10000, the data relating to the duty of the current pulse necessary to balance the balance mechanism 1 under a certain load state is, for example, “10000.73”. At this time, the duty of the current pulse increases / decreases (reciprocates) at a predetermined cycle between “10000” and “10001”, and the cycle of the increase / decrease is determined by the characteristics of a system mainly including the balance mechanism 1. For example, if the period is 0.1 s, and the period of the pulse current is 1 m
In the case of s, “10000” is repeated 27 times and “1000” is generated while 100 current pulses are generated during 0.1 s.
"1" appears only 73 times, and "100" appears within this cycle.
The number of each pulse having a duty of “00” and “10001” is counted, and their appearance ratio is calculated, and the ratio and the duty of one of the current pulses flowing in this cycle, for example, “10000” are calculated. , "10
Thus, measurement data of a load of 000.73 "can be obtained, and measurement with a higher resolution than that of the duty of the pulse current becomes possible.

[0014]

FIG. 2 is a block diagram showing the configuration of an embodiment of the present invention. The displacement from the balance point of the balance mechanism 1 supporting the plate 1a for applying the load to be measured is measured by a displacement sensor 2
Is detected by The balance mechanism 1 includes a permanent magnet 31
A force coil 3 movably placed in a magnetic field created by a magnetic circuit 31 mainly composed of a is attached. A pulse current described later is passed through the force coil 3, and the electromagnetic force generated thereby acts on the balance mechanism 1 in a direction to compensate for displacement from a balance point due to a load to be measured.

The displacement detection signal from the displacement sensor 2 is input to a PID calculation circuit 12 via a preamplifier 11 and subjected to PID (proportional / integral / differential) calculation processing. The result of the PID calculation is supplied to the pulse width conversion circuit 13 and is also taken into the computer 20.

As will be described later, the pulse width conversion circuit 13 generates a rectangular wave voltage signal having a pulse width corresponding to the magnitude of the input PID operation and having a constant period, and drives the electronic switch 14. The electronic switch 14 operates to flow a constant DC current from the constant current source 15 to the force coil 3 when the rectangular wave voltage signal from the pulse width conversion circuit 13 is, for example, H level, and to the ground when the rectangular wave voltage signal is at L level. I do. Thus, a pulse current having a constant peak value and a constant peak value having a duty corresponding to the magnitude of the PID calculation result flows through the force coil 3.

The filter 16 provided between the electronic switch 14 and the force coil 3 converts the pulse current into a direct current in order to reduce the vibration of the balance mechanism 1 due to the pulse current. In FIG. 2, elements that are not related to the features of the present invention, such as temperature compensation, are not shown.

FIG. 3 shows a specific circuit configuration example of the pulse width conversion circuit 13. In this example, the PID operation circuit 12
Analog voltage signal from the sample and hold circuit 13a
, And the value is digitized by the AD converter 13b. The result of the PID operation digitized by the A / D converter 13b is latched by the latch circuit 13c as PWM data for determining the duty of the pulse current, is sampled by the computer 20, and is simultaneously stored as a preset value in the down counter 13d. Supplied. The down counter 13d counts down a separately supplied high frequency clock signal CL of about 10 MHz, and supplies a reset signal to the flip-flop 13e when the content becomes zero. The clock signal CL is also supplied to the counter 13f at the same time, and the counter 13f counts the clock signal CL, and sets the set signal to the flip-flop 13e at every fixed period T when the count value reaches a fixed value, and the down-counter A load signal of a preset value is supplied to 13d.

With such a configuration, a rectangular wave signal whose cycle matches the cycle T of the output signal from the counter 13f and whose duty is proportional to the size of the PWM data is obtained from the flip-flop 13e. The pulse width conversion circuit 13 is characterized in that the duty of the rectangular wave signal to be output is a digital amount determined by counting the clock signal CL based on the PWM data taken into the computer 20. The pulse current which is pulsed by the operating electronic switch 14 and flows through the force coil 3 is
Is a digital amount corresponding to the PWM data taken in.

The resolution of the duty of the rectangular wave signal, that is, the resolution of the duty of the pulse current flowing through the force coil 3 is determined by the number of clock signals CL generated in the cycle T. For example, if the frequency of the clock signal CL is 10 MHz and the period T is 1 ms, the duty resolution of the pulse current is 1/10000.

The computer 20 obtains measurement data of the load acting on the plate 1a of the balance mechanism 1 with a resolution higher than 1/10000 using the PWM data by a program shown below, and displays it on the display 21. Determine the weighing value to be displayed.

FIG. 4 is a flowchart showing the contents of the program written in the computer 20, and the operation of the embodiment of the present invention will be described below with reference to FIG. The computer 20 outputs P through a latch circuit 13c under a period T.
The WM data is taken in (ST1). The first P
When the WM data is PWML, it is determined whether or not the next fetched data is equal to this (ST2). Data within equal to the original data PWML is a B ₀ counter 1 counts up (ST3). If they are not equal, PWM is one count larger than PWML, that is, PWM regulated by the duty resolution of the pulse current.
It is determined whether the data is the next largest data PWMH after L (ST11). Then, data in the case of PWMH is counted up by 1 A ₀ counter (ST1
2). These counters A ₀ and B ₀ are cleared to 0 at the same time that the contents A and B are read when the other counter B ₀ or A ₀ becomes 1, respectively (ST4, ST5 and ST13). , ST14).

The acquired PWM data is converted to the original PWML.
If it is not any of the PWMHs larger than one count larger than that, it is determined whether or not the data is larger than the PWMH (ST21). If it is larger, a value smaller than the PWM data by one count becomes a new reference. PWM
L, and if smaller, the PWM data is stored as it is as a new PWML (ST22, ST22).
24). When updating the reference PWML, both counters A ₀ and B ₀ are cleared (ST23, ST25).

Each time the counter A ₀ becomes 1, the load is calculated using the value of the reference PWML and the content A of the counter A ₀ and the content B of the counter B ₀ read immediately before. Is calculated by the following equation: D = PWML + A / (A + B) (1), and based on this data D, the display 21
Is determined.

Now, it is assumed that the PWM data corresponding to the current value to be passed through the force coil 3 in order to generate an electromagnetic force necessary to balance the balance mechanism 1 under a certain load state is 10000.73. Then, the servo system operates to supply such a current to the force coil 3. However, as described above, the duty of the pulse current flowing through the force coil 3, that is, the PWM data has only a resolution of 1/10000. , And PWM data take one of the values 10000 and 10001. Assuming that the first PWM data is 10,000, and if the data is PWML, as shown in FIG. 5, after the PWM value continues several times, the PWM data eventually becomes one count larger than the PWML value. To the value of. Also, after the value of PWMH continues several times,
It changes to the value of PWML again. Such an increase / decrease cycle τ of the PWM data is determined by the characteristics of a system mainly including the balance mechanism 1. Assuming that the cycle τ of the increase or decrease of the data value is, for example, 100 times (0.1 s) the cycle T of the pulse current, the period τ is 10000 in the above example.
(PWML) 27 times, 10001 (PWMH) 73
Appears only once. In this case, the value of the actual PWM data required to balance the balance mechanism 1 can be known from the following equation: 10,000 + 73 / (27 + 73) = 10000.73 (2)

In the flowchart of FIG. 4, the timing when the content of the B ₀ counter becomes 1 in ST4 is the time of τb in FIG. 5, and the timing when the A ₀ counter becomes 1 in ST13 is also the timing of τa in FIG. It is time. Therefore, the period of until the time of for example between two .tau.a, i.e. A ₀ counter reaches the next 1 from the time when a 1 is consistent with the period τ of the increase or decrease of the data values. The counters A ₀ and B ₀ are cleared after their contents are read each time the other counter becomes 1, so that the read contents A and B of the A ₀ counter and the B ₀ counter are respectively It represents the number of PWMHs and PWMLs that appear within the data value increase / decrease cycle τ.

[0027] Accordingly, in ST31 and ST32, and the A ₀ counter is calculated each time becomes 1 (1) will become that calculates the above equation (2), than the resolution of the pulse duty Can obtain the load measurement data D with a resolution two orders of magnitude higher.

Incidentally, the calculation of the expression (1) is based on A
_In addition to the timing when the ₀ counter becomes 1, the timing when the B ₀ counter becomes 1 may be used. In this case, the period shown in FIG. 6 is one cycle of the measurement, and in this case, the same measurement data can be obtained.

In the equation (1), the measured value D is obtained by adding the appearance ratio of each data to the value based on the PWML. Instead, the calculation D = PWMH- B / (A + B) (3) Needless to say, exactly the same measurement data can be obtained.

Further, in the above embodiment, an example is shown in which the analog displacement detection signal from the displacement sensor 2 is processed by the analog PID operation circuit 12 and then digitized, but after the output of the displacement sensor 2 is digitized. Needless to say, the PID operation may be performed by digital operation.

In each of the above examples, if the period τ of the increase or decrease of the PWM data becomes longer due to the characteristics of the balance mechanism 1, an appropriate dither is injected into the output of the PID operation circuit 12. do it. That is, PID
By injecting dither into the output, switching between PWML and PWMH is performed almost in synchronization with the cycle of dither. By appropriately selecting the dither cycle and setting the cycle τ to a desired cycle, the output cycle of the load measurement data can be shortened, the response can be improved, and the data averaging process can be performed. It is advantageous in the point.

[0032]

As described above, according to the present invention,
In electronic balances that employ a pulse modulation type servo mechanism that applies a pulse current to the force coil, the operation of the servo mechanism is synchronized with the pulse current duty increase / decrease cycle caused by insufficient resolution of the pulse current duty. Then, while counting the number of current pulses having different duties flowing in the cycle and calculating the ratio, the ratio and any one of the pulse duty of each pulse current flowing in the same cycle are calculated. Because it is configured to calculate the load measurement data from, the measurement value with higher resolution than the pulse duty resolution can be obtained with a simple configuration without adding any special hardware. It is possible to obtain an inexpensive and high-resolution electronic balance. Moreover, since the calculation of the measurement data is performed in synchronization with the cycle of the increase and decrease of the duty of the pulse current caused by the lack of the resolution of the duty, compared with the case of simply calculating the average value of the duty of the pulse current flowing at an appropriate time. Thus, significantly effective (accurate) measurement data can be obtained.

[Brief description of the drawings]

FIG. 1 is a basic conceptual diagram showing a configuration of the present invention.

FIG. 2 is a block diagram showing a configuration of an embodiment of the present invention.

FIG. 3 is a block diagram showing a specific configuration example of the pulse width conversion circuit 13;

FIG. 4 is a flowchart showing the contents of a program written in a computer 20 according to the embodiment of the present invention.

FIG. 5 is a diagram illustrating the operation of the embodiment of the present invention.

FIG. 6 is an explanatory diagram of an example of another operation method according to the embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Balance mechanism 2 Displacement sensor 3 Force coil 31 Magnetic circuit 12 A / D converter 13 Pulse width conversion circuit 13a Sample hold circuit 13b A / D conversion circuit 13c Latch circuit 13d Down counter 13e Flip-flop 13f Counter 14 Electronic switch 15 Constant current Source 20 Computer 21 Indicator

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G01G 7 / 02,23 / 37 G01L 1/08

Claims (1)

(57) [Claims] 1. A servo system that balances the balance mechanism by detecting a displacement from a balance state of the balance mechanism due to a load with a displacement sensor and controlling a current flowing through a force coil according to an output of the displacement sensor. In addition to the above, the servo system includes pulse current generating means for generating a pulse current whose duty is changed at a constant cycle and with a predetermined resolution according to the output of the displacement sensor, and the pulse current is In a balance configured to balance a balance mechanism by flowing the force through a force coil, the balance is synchronized with a cycle of increasing or decreasing the duty of the pulse current generated due to insufficient resolution of the duty of the pulse current generated by the pulse current generating means. And the number of current pulses with different duties that flow in that cycle A pulse generation rate calculating means for counting and calculating the ratio, a load measurement is performed from the ratio calculated by the pulse generation rate calculation means, and a pulse duty of each current pulse flowing in the cycle. An electronic balance comprising a measurement data determination unit for calculating data.