Abstract: This paper presents a circuit solution based on the XTR106 chip to improve the accuracy of lifting weight signal acquisition in crane lifting weight limit systems. By analyzing the working principle of the XTR106, selecting appropriate external electrical components with suitable parameter values, and building a corresponding control circuit, this solution effectively solves the nonlinearity problem of ordinary resistive bridge pressure sensors and successfully achieves 4–20mA current signal acquisition and current-voltage transmission functions. Experimental data show that this circuit solution can control the average relative error of signal gain within 0.3%, achieving accurate weight signal acquisition and laying a solid foundation for the design of lifting weight limiters.
0 Introduction
A lifting capacity limiter is a device specifically designed to protect the safety of lifting machinery. It features visual and audible alarms, displays the weight of the lifted object, and cuts off the crane's lifting circuit. It prevents equipment and personal injury accidents caused by overloading. Therefore, it is of paramount importance for modern safe production in industries such as metallurgy, machinery, mining, railways, docks, and warehouses.
The accuracy of signal acquisition directly determines the quality of the lifting capacity limiter. Only by accurately detecting the actual weight of the object lifted by the crane can an accurate judgment be made to control the safe operation of the machine. Therefore, designing a high-precision weight signal acquisition and processing circuit is crucial to the entire lifting capacity limiter system. There are many sensors for measuring weight, with pressure sensors being commonly used. The working principle of ordinary pressure sensors is to convert the change in resistance of a resistance bridge into a change in voltage, but the problem is the nonlinearity of the bridge arm resistance and the voltage output. The XTR106 is a low-cost, low-drift, high-precision, 4~20mA two-wire integrated monolithic current transmitter specifically designed for bridge sensors, featuring two 2.5V/5V bridge excitation voltages. It has amplification, second-order linearization, and current signal output functions. When used in conjunction with a resistance bridge pressure sensor, it can effectively solve the problems of bridge nonlinearity and excitation power supply, and excellently complete the signal acquisition task.
1. Working principle of bridge pressure sensor
The working circuit of the resistive bridge pressure sensor is shown in Figure 1. U is the 2.5V or 5V excitation voltage provided by the XTR106; the differential voltage U0 can be seen from Figure 1:
When R0+R1 is much larger than RΔ , that is, when the relative change in sensor resistance RΔ /(R0+R1) is very small, equation (2) can be linearized:
As can be seen from formula (3), the output voltage U0 is approximately linearly related to the resistance change RΔ /(R0+R1) of the resistance bridge pressure sensor, which has a relatively small impact on the measurement accuracy [1]. However, as the resistance change RΔ /(R0+R1) of the pressure sensor continues to increase, the nonlinearity also increases, eventually leading to very poor measurement accuracy, which cannot meet the requirements. At the same time, as can be seen from formula (3), the output voltage U0 is linearly related to the power supply voltage U of the resistance bridge, so the stability of the power supply voltage also directly affects the accuracy of the measurement results [2].
In a typical resistance bridge pressure sensor, the nonlinearity of each resistor and the overall resistance bridge contributes to nonlinear errors in the final measurement. Therefore, to obtain accurate measurement results, it is necessary to address these nonlinear issues, achieve linearized measurement, and ultimately obtain precise results.
2. Working principle of XTR106
The XTR106 is a 4-20mA two-wire integrated monolithic transmitter with two excitation voltage sources and a drive bridge [3]. Its pin diagram is shown in Figure 2.
The XTR106 provides a precision 2.5V/5V reference excitation voltage for the system. If a 2.5V excitation voltage is required, connect pin 13 to the resistor bridge; if a 5V excitation voltage is required, connect pin 14 to the resistor bridge (as shown in Figure 3). Its reference voltage source has an accuracy of 0.05% and can drive a maximum load of 2.5mA (Note: If the load exceeds 2.5mA, it will affect the 4mA zero-point output current). An external range control resistor must be connected between pins 3 and 4. A mV-level differential voltage from the resistor bridge output is connected between pins 2 and 5, with pin 5 connected to the high voltage and pin 2 to the low voltage. Pin 7 is the final 4-20mA current output terminal.
Furthermore, the most significant feature of the XTR106 is its ability to compensate for the inherent nonlinearity of an unbalanced bridge using a quadratic term. This can significantly improve the nonlinearity of the bridge sensor, with a maximum improvement ratio of 20:1. Pins 1 and 11 are directly connected to the linearization resistor RLIN , causing the bridge excitation voltage to vary with the differential output voltage of the resistor bridge. When the resistor bridge exhibits positive nonlinearity, pins 12 and 6 are connected. In this case, the excitation voltage VREF increases with the differential output voltage VIN of the resistor bridge sensor to compensate for the positive nonlinearity. When the resistor bridge exhibits negative nonlinearity, pin 12 is connected to pin 1. In this case, the excitation voltage VREF decreases with the differential output voltage VIN of the resistor bridge sensor to compensate for the negative nonlinearity. When linearization compensation is not required, simply connect pins 12 and 11 to pin 1. In this case, the excitation voltage VREF will remain constant at 2.5V or 5V.
The voltage-current transfer function of the entire circuit is:
Where I <sub>0 </sub> is the output current in mA; V <sub>IN</sub> is the differential voltage in mV; and RG is the range adjustment resistor in Ω.
3. Application of XTR106 in Signal Acquisition and Linear Gain
3.1 Circuit Principle Analysis
Because the signal from a resistive bridge pressure sensor is extremely weak (in the mV range), only amplifiers with low power consumption, low drift, low offset, and good gain linearity can amplify the weak signal without distortion. The XTR106 possesses precisely these characteristics.
The circuit used in this system is shown in Figure 3. Since the resistance bridge of the pressure sensor is positive nonlinear, pin 12 is connected to pin 6 to correct the nonlinearity. Capacitors C1 , C2 and C3 are decoupling capacitors to improve anti-interference. Fixed resistor R4 and variable resistor RZ work together to realize the circuit zero setting, that is, the circuit outputs 4mA current when there is no external load. Fixed resistor R3 and variable resistor RS work together to realize the circuit full setting, that is, the circuit outputs 20mA current when a full load is applied. Transistor Q1 is used to reduce the accuracy error caused by temperature changes. The power supply uses 24V DC voltage, and diode 1N4148 plays a protective role to prevent the power supply from being connected in reverse and damaging the components [4].
A precision resistor RL with a load resistance of 250Ω is used to convert a current change of 4–20mA into a voltage change of 1–5V. Combining formula (4) to convert the current value into a voltage value, we can obtain formula (5):
3.2 Voltage Gain Accuracy Analysis
Connect the circuit according to the circuit schematic shown in Figure 3, conduct simulation tests, and analyze the voltage gain accuracy. Assume that the input differential voltage varies within the range of 0 to 10mV, at which point RG = 25Ω.
When the differential input voltage is controlled, the voltage is sampled every approximately 1mV, and the output terminal of the circuit is measured with a voltmeter. The differential input voltage V <sub>IN</sub> and the corresponding output voltage V<sub> O </sub> are recorded. The experimental results are shown in Table 1.
Wherein, V <sub>actual </sub> represents the voltage value actually measured by the voltmeter; V <sub>ideal</sub> represents the ideal voltage value calculated by formula (5); and Δ<sub> error</sub> represents the percentage value of the relative error calculated by formula (6).
By comparing the actual V with the ideal V and calculating the Δ error, it can be clearly seen that the maximum relative error of the sampling measurement is 0.409%, while the minimum is only 0.050%, and the average relative error can be improved to 0.289%, which is far higher than the gain accuracy of ordinary amplifier circuits. For example, the maximum gain error of the AD620 chip, which is often used in integrated circuit amplifiers, is 0.7%.
3.3 Nonlinear Analysis of Voltage Gain
Linearity analysis was performed on the experimental data, and a linearized curve was established as shown in Figure (4). It can be seen from the figure that the actual curve represented by pink color basically coincides with the theoretical straight line represented by blue color, and the maximum nonlinearity is only 0.4%.
4. Conclusion
By building the circuit and performing simulations, experiments, and data analysis, the following conclusions can be drawn:
(1) Compared with traditional discrete component circuits, the circuit based on XTR106 designed in this paper has the characteristics of small size, high integration, simple circuit design, low power consumption, low drift, low offset, nonlinear adjustable, and strong anti-interference.
(2) The circuit designed in this paper has high voltage gain accuracy. The experimental data in Table 1 show that the average relative error of the measurement results can be controlled within 0.3%. It is fully capable of serving as a signal acquisition device for crane lifting capacity limiter.
(3) The circuit designed in this paper is not only applicable to this system, but also worth referencing for other similar applications. It has good portability and scalability.
References
[1] Hua Chenquan, Ren Xuhu. Application of XTR106 in unbalanced bridge temperature measurement of platinum resistance thermometer [J]. Industrial Instrumentation and Automation Devices, 2001, (05).
[2] Huang Jichang, et al. Working principle and practical examples of sensors [M]. People's Publishing House, 1998.
[3] XTR106 Data Sheet. Burr-Brown Corporation, 1998.
[4] Xia Yiquan, Mou Zhiping. Design of transmission circuit for strain gauge pressure sensor [J]. Science and Technology Consulting, 2007, (32).
