New compensation temperature inspection circuit design improves temperature detection accuracy

A new type of compensated temperature inspection circuit is introduced. The circuit overcomes the influence of the measuring wire on the measurement result in the traditional three-wire detection method through clever design, and improves the temperature detection accuracy. At the same time, the circuit completes the inspection of multiple temperature signals by group sharing, reducing the complexity and cost of the temperature inspection circuit. The experimental data verifies the accuracy and practicality of the detection circuit.

1 Introduction

Temperature detection is achieved by detecting the resistance value of the temperature sensor and converting the relationship between the resistance value and the temperature curve. In order to reduce the measurement complexity of the temperature inspection circuit, the three-wire measurement method is often used in engineering for temperature measurement. The schematic diagram of the measurement circuit is shown in Figure 1.

Rx1~RxN are the resistance values ​​of temperature sensors 1~N respectively. Taking Rx1 measurement as an example, assuming that the resistance of the three wires connected to temperature sensor 1 is RL1, when the analog switch K1 is closed, there is:

It can be seen from formula (1) that the measurement accuracy of Rx1 is affected by RL1. In engineering practice, the value of RL1 is often difficult to measure or unknown. If RL1 cannot be ignored relative to Rx1, the measurement result will have a large deviation from the actual resistance value of the temperature sensor. In addition, for N temperature measurements, N constant current sources are required, which is costly.

2 Three-wire conductor voltage drop compensation mechanism

Taking the sampling of a single temperature sensor resistance value as an example, the working mechanism of the three-wire conductor voltage drop compensation is analyzed.

The schematic diagram of the three-wire conductor voltage drop compensation circuit is shown in Figure 2.

Rx is the temperature sensor resistance value. When the chip select A1A0 of the analog switch is 00, the three switches shown in the figure are closed, and the circuit measures Rx. It can be seen from the figure that

since T1-P2 is sent to the non-inverting input terminal of the operational amplifier after passing through the analog switch, the input current is very small (<10nA) and can be ignored, that is, i4=0.

Similarly, i3=0, from which we can conclude:

V3 and V4 are followed by the back-end, subtracted and inverted circuits to obtain Vo. The calculation steps are as follows:

In engineering practice, the three wires connecting the same temperature sensor have basically the same material and length, so RL0=RL1, and thus V8=-I×Rx, and finally the resistance value of the temperature sensor is obtained:

It can be seen from the above formula that the measured Rx value is not affected by the wire resistance.

From the above analysis, it can be seen that this detection method compensates for the wire voltage drop, eliminates the influence of wire resistance on the measured resistance, and improves the measurement accuracy.
 

3 Inspection function mechanism
 

As can be seen from FIG1 , when there are N temperature sensors to be measured, the traditional temperature inspection circuit requires N constant current sources.
 

Based on realizing the wire voltage drop compensation to improve the measurement accuracy, this paper also proposes a new type of patrol circuit.
 

FIG. 3 is a schematic diagram of measuring with four temperature sensors as one group.

Rx1~Rx4 are the resistance values ​​of four temperature sensors in the same group, which share a constant current source and a set of acquisition circuits.
 

At time t1, A1A0=00, so that the first group of analog switches is closed, Rx1 is connected to the acquisition circuit, and other temperature sensors to be measured are disconnected from the acquisition circuit. The constant current source I is sent to the acquisition circuit through Rx1 and the analog switch. Although the constant current source is connected to the lower end of other resistors through GN, since no closed path is formed, the current flowing through Rx1 is still I. According to the measurement and analysis method of the wire voltage drop compensation in the previous section, we get:

In the above formula, Vo(t1) is the voltage value of Vo at time t1. By obtaining the Vo value at time t1, Rx1 is obtained, thereby completing the measurement of the 1# temperature sensor. Similarly, when A1A0=01, A1A0=10, A1A0=11, the measurement of the 2#~4# temperature sensors is completed.
 

From the above analysis, it can be seen that, under the premise that the sampling rate meets the requirements, an 8-to-1 or 16-to-1 multi-channel analog switch can be used. In this way, every 8 or 16 temperature sensors form a group, sharing a constant current source circuit and a group of acquisition circuits, which greatly saves circuit resources and reduces circuit complexity and cost.


4 Engineering Implementation of the New Compensation Temperature Patrol Circuit
 

Figure 4 is a detailed design of a compensated temperature inspection circuit with four temperature sensors as a group. In this design, the analog switch uses two groups of 4-to-1 multi-way analog switches ADG409, and the constant current source is generated by a constant voltage source ADR291 (2.5V voltage output) + operational amplifier to generate a constant current I=2.5V/R12=2.5mA. The MCU completes the measurement of 1#~4# temperature sensors by controlling the chip selects A1 and A0 of the analog switch. The measured value Vo is sent to the MCU for temperature fitting and conversion through the ADC.

5 Solutions to several problems in the design

1) Switching process processing

Since all switches of the multi-channel analog switch are disconnected at the moment of channel switching, the input ends of the two voltage follower circuits are suspended and the output voltage is unstable. Therefore, the capacitor C5 is added in the design to form an integration circuit to eliminate the influence of switch switching.

2) Elimination of the influence of switch on-resistance

The on-resistance of the general analog switch is about tens to one hundred ohms, which is equivalent to the resistance of the temperature sensor, so the influence of the switch resistance must be considered.

In Figure 2, the value of V3 is at the rear end of RON2, instead of directly connecting to the rear end of RON1. Although a switch RON2 is added, V3=V1-RON2×0=V1 at this time, instead of V3=V1-RON1×I=V1, thereby eliminating the influence of the switch on-resistance.

6 Experimental verification

The actual equipment after the above inspection circuit was used to conduct field tests and comparisons on the magnet temperature of a certain type of motor. The test data is shown in Table 1.

It can be seen from the test data that the temperature inspection circuit has a high accuracy (in most cases, the error is ≤0.033%, and the error of individual measurement points is ≤0.167%).

7 Conclusion

Through the above analysis and the final test results, the new compensation temperature patrol circuit has the advantages of high measurement accuracy, low complexity and low cost. The circuit has been successfully applied to multiple industrial occasions of temperature detection and has broad application prospects in temperature detection.