JPS6160366B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a digital thermometer using a thermistor as a temperature sensor.

Generally, temperature measurements such as body temperature require high accuracy (0.1°C) and fast temperature measurement speed.
Conventional mercury thermometers typically measure temperature in about one minute at the most. Therefore, digital thermometers using a thermistor as a temperature sensor, such as the one shown in FIG. 1, have come on the market.

In FIG. 1, Rth is a thermistor as a temperature sensor, and constitutes a bridge circuit 1 together with resistors R1, R2, and R3. The voltage between a and b of this bridge circuit, that is, the unbalanced voltage, is amplified by the preamplifier 2, and a voltage Eo proportional to the temperature is generated.
get. This analog voltage Eo is converted into a digital value by a double integration type analog-to-digital converter (hereinafter referred to as an A/D converter), and the temperature is displayed on the digital display 6. For example, adjust the bridge circuit 1 in advance so that the bridge output voltage is 0V at 0°C, and adjust the preamplifier gain so that the preamplifier output terminal voltage Eo is 200mV at 20°C. decide. Then, if this Eo is displayed as "20.0" with an A/D converter, the temperature can be displayed.

The double integration type A/D converter uses operational amplifier 4.
It is composed of an integrating circuit 3 and a control circuit 5. The input terminal of the operational amplifier 4 can be connected to the output voltage Eo of the preamplifier by means of a switch S', and by means of another switch S'' the reference voltage E _R
It is now possible to connect to. This switch
The on/off of S' and S'' is controlled by the control circuit 5 as follows. That is, during the input integration period, the switch S' is on (S'' is off), and the resistor Ri and capacitor C control the input voltage. Integrate the voltage Eo over a certain period of time. Next, switch S'' is turned on (S' is turned off) during the reference integration period, and the voltage E _R , which has the opposite polarity to the input voltage Eo, is integrated until the integral output voltage becomes zero. In this case, the integral output Since the slope of is constant, the reference integration time is proportional to the input voltage.The number of clock pulses within this time is counted by a counter in the control circuit 5 and latched to obtain a digital output.This double integration method has the advantage that fluctuations in the clock period and circuit elements Ri, Re, and C do not cause errors.

However, in order to improve the measurement accuracy of a digital thermometer as shown in FIG. 1, it is important that the output voltage o of the preamplifier A1 be proportional to the measured temperature.

By the way, as is well known, a thermistor is a type of semiconductor whose resistance value changes with temperature changes, and because the resistance value changes with temperature, it is said to be relatively sensitive, and thermistors are generally inexpensive and small. Moreover, it has the characteristic of relatively fast response. Therefore, thermistors are used not only for measuring body temperature but also for industrial thermometers.

However, the resistance of a thermistor changes logarithmically with temperature. Therefore, as mentioned above, the preamplifier A
In order to make the output voltage Eo of 1 proportional to the temperature, for example, a correction resistor may be inserted in parallel with the thermistor Rth to improve the linearity of the bridge circuit with respect to temperature, or the amplification degree of the preamplifier A1 may be further increased. It was necessary to take measures such as keeping it stable against temperature changes.

An object of the present invention is to provide a digital thermometer that can measure temperature with high accuracy without requiring a special circuit for correcting the linearity of the thermistor as described above.

In the present invention, in order to utilize the thermistor's characteristic that the resistance value changes logarithmically with respect to temperature without modification, first, a constant voltage Ei that is unrelated to temperature is prepared, and this voltage is integrated over a certain period of time through the thermistor Rth. That is, a resistance measurement is made and then a logarithmic transformation is performed by discharging this integrated voltage E _N through a predetermined resistor Ro. In this way, the logarithm of the thermistor's resistance value at that time, that is, the temperature itself, can be directly determined.

The present invention will be described below with reference to the illustrated embodiments.

FIG. 2 is a block diagram showing the principle of the thermometer of the present invention. Thermistor Rth is a normal thermistor with negative characteristics, one end of which is connected to a constant DC voltage Ei
(negative voltage in this example), and the other end is connected to the input terminal of the operational amplifier 10 via the switch S1. Therefore, the thermistor Rth functions as one element constituting the first integrating circuit together with the capacitor C connected between the input and output terminals of the operational amplifier 10. Between the input and output terminals of the operational amplifier 10,
A series circuit of a switch _S2 and a resistor Ro is connected in parallel to form a discharge path (second integrating circuit) for the capacitor C.

First, as shown in FIG.
Close and perform the first integral. When a negative voltage is applied to Ei, the output voltage E1 of the operational amplifier 10 is Ei/Rth as shown in FIG. A positive voltage waveform follows a function of Ct. When the counter of the control circuit 50 counts the number of pulses N, the output signal from the counter opens the switch S1 and closes the switch S2. That is, the amount of electricity charged in the capacitor C during the first integration is discharged through a predetermined resistor Ro. Since the voltage of the capacitor C changes exponentially with time t, this voltage is compared with a predetermined reference voltage E _R by a comparator in the control circuit 50 to find a point of agreement. The temperature is displayed on the digital display 60 when the ratio of the time from the start of integration until the second index changes (number of pulses N) to the time until the voltage comparison matches (number of pulses Nx) is expressed as a number of pulses. can be done.

Next, an embodiment of the present invention will be explained in detail using FIGS. 4 to 6. FIG. 4 shows an example of the configuration of the control circuit, and FIG. 6 shows a detailed specific circuit. In this case, the same components are given the same reference numerals throughout FIGS. 2, 4, and 6.

A switch S3 is connected in parallel to the capacitor C of the integrating circuit, so that the capacitor C is short-circuited except when integration is required.

20 is the terminal voltage E1 of the capacitor C when the accumulated charge of the capacitor C flows out through the resistor Ro.
This is a comparator that checks whether E1 has fallen to a predetermined value E _R and produces an output when E1 reaches E _R . 21 is a waveform shaping circuit that converts the comparison match output of the comparator 20 into a pulse with a relatively short pulse width, and 22 is a flip-flop that is reset by the waveform-shaped comparison match output pulse. This flip-flop 22
While the capacitor C is in the lit state, the output of the flip-flop turns on the switch S3 through the switch drive circuit 23, and the capacitor C
is shorted. As a result, comparator 20 does not operate.

As shown in FIG. 6, the switch drive circuit 23 connects the set Q output of the flip-flop 22 and
AND gate 25 whose two inputs are the inverted signal of the output of AND gate 32 (output of inverter 24)
Contains. This has the function of prohibiting the on/off operation of switch S2 by the set output Q of flip-flop 22 while the output of gate 32, which will be described later, is occurring, and enabling it when the output of gate 32 disappears. This is to make it happen.

30 is a clock generator with a clock frequency o;
A counter 31 counts clock pulses from the generator 30. The counter 31 is constructed by adding two stages of 1/2 counting circuits 31A and 31B to a three-stage 1/10 counting circuit. Therefore, the previous stage 1/2 counting circuit 31A is turned on when 1000 clock pulses are counted. However, 31B turns on when the count reaches 2000. Reference numeral 32 denotes an AND gate which produces an output while both 1/2 counting circuits 31A and 31B are on, and the output of this gate 32 turns on the switch S1 through the drive circuit 23.

40 reads and latches the value of the counter 31 by the comparison match pulse from the waveform shaping circuit 21;
This is a latch circuit that displays the value on, for example, a three-digit digital display 60 showing up to one decimal place.

Next, the operation of the above device will be explained with reference to the time chart shown in FIG.

The counter 31 starts counting clock pulses from the clock generator 30 by first turning on the power switch SW (FIG. 6). When the count reaches 1000, the 1/2 counting circuit 31A is turned on (point a in FIG. 3). Next, at 2000 counts, the next stage 1/2 counting circuit 3
1B is turned on (point b in Figure 3). When the count progresses further and reaches 3000 counts, both circuits 31A,
31B are both turned on (c in Figure 3).
point), this results in an output at gate 32.
The output of gate 32 is sent to the clock input terminal of flip-flop 22, causing the flip-flop to be set while switching drive circuit 23
Turn on switch S1 through the switch. When flip-flop 22 enters the set state, its output disappears and switch S3 is turned off. In this case, the Q output terminal of the flip-flop is at H level, but the AND gate 25 of the circuit 23 does not open, so the switch S2 remains off. From the moment switch S1 is turned on, the DC voltage
Ei causes capacitor C to pass through thermistor Rth.
The capacitor terminal voltage starts to be charged in the form of Ei/Rth Ct.

Next, when the 4000th count (point d in FIG. 3) is reached, the outputs of the 1/2 counting circuits 31A and 31B are turned off, the output of the AND gate 32 disappears, and the switch S1 is turned off. When the output of the AND gate 32 disappears, the output of the inverter 24 of the switch drive circuit 23 becomes H level.
Produces an output at AND gate 25. This is because flip-flop 22 is still in the set state at this point. The output of gate 25 opens switch S2. Therefore, at the 4000th count (N=1000), the first integration automatically ends, and then the second integration starts, in which the charge accumulated in the capacitor C up to that point is discharged through the resistor Ro.

Now, the clock frequency of the clock generator 30 is o, and the number of pulses defining the first integral period is N.
Then, the terminal voltage E _N of the capacitor C at the end of the first integration has the following value.

E _N =Ei/Rth. C・N/o...(1) As time t passes, this voltage becomes exponential function e-
1/Ro. It decreases according to Ct.

The discharge voltage E1 is the reference voltage E of the comparator 20.
When the output reaches _R , the output of the comparator 20 changes from on to off. At this time, the pulse is transferred to the waveform shaping circuit 2.
1. The compare match pulse from circuit 21 is sent to the reset input of flip-flop 22 to reset the flip-flop and short the integrator circuit. The comparison match pulse is also sent to the latch circuit 40 to activate the same, causing the latch circuit 40 to latch the count value from the counter 31. If this value is entered into the display 50, the temperature will be digitally displayed.

Now, if this count value is Nx and the time from the start of counting Nx to the end of the second integral is tx as shown in Figure 3, then tx = RoClog E _N /E _R ...(2), which is Substituting equation (1) into tx=RoClogEiN/Rth Co E _R (3). Therefore, since Nx=otx, Nx=toRoClogEiN/Rth Co E _R (4). Since o, Ro, C, N, Ei, and E _R are constant, Nx is proportional to the logarithm of the change in 1/Rth, as seen from equation (4). In other words, log1/Rth
Since is proportional to temperature, the count value of Nx represents the temperature.

As described above, the thermometer of the present invention directly measures temperature by measuring the resistance value of the thermistor without correcting its logarithmic characteristic, and therefore has extremely high measurement accuracy. Moreover, neither a bridge circuit nor a preamplifier as shown in FIG. 1 is required, nor is there any need for normalization or correction of the voltage Eo, which was conventionally necessary. Moreover, the uneven characteristics of individual thermistors
This can be easily corrected by adjusting the reference voltage E _R of the comparator 20.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of a conventional digital thermometer, Figure 2
The figure is a schematic diagram of the digital thermometer of the present invention, FIG. 3 is a diagram for explaining the principle of operation of double integral, and FIG.
Figure 5 is a block diagram of the digital thermometer of the present invention.
The figure is a time chart showing the operation of its main parts.
FIG. 6 is a diagram showing a more specific circuit example of the digital thermometer shown in FIG. 4. 10... operational amplifier, 20... comparator, 21
...Waveform shaping circuit, 22...Flip-flop, 23
...Switch drive circuit, 30...Clock generator, 31...Counter, 32...AND gate, 40
...Latch circuit, 60...Display device.

Claims (1)

[Claims] 1 a counter for counting clock pulses; a first switch that is turned on while the value of the counter is within a predetermined first numerical range; a thermistor that integrates the DC voltage; a second switch that is turned on immediately when a predetermined first numerical width of the counter has elapsed; and a thermistor that is connected to the capacitor by the second switch and that a resistor with a predetermined value that discharges the charge accumulated in the capacitor; a comparator that compares the voltage of the capacitor with a predetermined reference voltage during this discharge and generates a comparison match output when they match; A digital thermometer characterized by having a latch circuit that latches the contents and displays them on a display.