Abstract: A novel capacitive angle sensor was designed, and the sensor signal was converted into a 4-20 mA standard industrial current output using a capacitive signal conversion integrated circuit CAV424 and a voltage-to-current conversion interface circuit AM402. This system effectively reduces interference and measurement errors, achieves accurate angle measurement within the 0-90° range, and provides a DC current output with good linearity.
0 Introduction
Traditional differential capacitive angle sensors have limited measuring angles. The moving element has a direct electrical connection to the external environment, and its electrodes must be led out from both the moving and stationary elements. Leading the electrodes from the moving element typically uses bearings, clips, or metal wires. During the rotation of the moving element, friction exists between the shafts, and poor contact between the leads and the shaft can easily occur, leading to mechanical failures, interference, and errors, thus affecting measurement accuracy. Therefore, a novel capacitive angle sensor was designed, with improved structure to overcome the shortcomings of traditional differential capacitive angle sensor designs.
1. Structural Design
This sensor system employs a novel structural design. The moving plate consists of two opposing sector-shaped metal plates with a 90° angle, connected in the middle. The stationary plate comprises two sector-shaped metal plates with a 90° angle and separated in the middle. Electrodes are led out from the two stationary plates, serving as the sensor's signal output terminals. Essentially, this novel capacitive angle sensor structure is equivalent to two variable capacitors connected in series. The series capacitance changes with the relative angle between the moving and stationary metal plates. The design of leading electrodes from the stationary plates facilitates wiring, is beneficial for the angle change of the moving plate relative to the stationary plate, and ensures that the moving plate has no electrical connection to the outside world, avoiding friction and mechanical failures caused by rotation during measurement. This truly achieves non-contact measurement and improves measurement accuracy.
2. Basic Principles
The basic principle of capacitive sensors is based on the relationship between capacitance and structural parameters between objects. For a parallel plate capacitor, if the parallel plates are infinitely large and edge effects are ignored, its capacitance formula is expressed as:
As shown in equation (1), changes in dielectric constant, relative area of plates, or distance between plates can all be reflected as changes in capacitance. Therefore, this principle can be used to measure the displacement of an object, the state parameters of a substance, etc.
Based on the design structure, let the radius of the stationary plate of this capacitive angle sensor be R, the distance between the rotating plate and the stationary plate (i.e., the electrode spacing) be d, the dielectric constant of air be En, and the change in relative area between the rotating plate and the stationary plate at each 90° angle during rotation be Δs. Initially, the relative area between the rotating plate and the stationary plate is zero, then ΔS1 = ΔS2 = Δs. Because:
The total change in capacitance is:
Let the angle θ be the angle through which the rotating plate rotates relative to the stationary plate from its initial position. When
The change in capacitance varies periodically with the angle. Within each quadrant of the angle range (0–3 Å), the change in capacitance is linearly related to the angle change (0°), with the maximum change being:
3. Capacitance signal measurement and conversion circuit
3.1 Capacitor-to-Voltage Converter Circuit CAV424
The CAV424 is a versatile integrated circuit for processing and converting signals from various capacitive sensors, converting capacitance signals into voltage signals. It simultaneously performs signal acquisition (relative capacitance change), processing, and differential voltage output functions. It can measure the difference between the measured capacitance and a reference capacitance, specifically the capacitance value within a range of 5% to 100% relative to the reference capacitance value (10 pF to 2 nF), and convert this value into a corresponding differential voltage output with high detection sensitivity. It also integrates a built-in temperature sensor, which can be used directly to monitor temperature when digital signal correction is required. Using the CAV424 as the conditioning circuit for a capacitance sensor overcomes the effects of parasitic capacitance and environmental changes, improving measurement accuracy and anti-interference capabilities. Furthermore, the sensor requires fewer external components, the processing circuit is relatively simple, and the instrument is small in size.
3.2 Voltage-to-current conversion circuit AM402
The AM402 is an integrated circuit that processes differential signal input and current output interfaces. It can convert weak differential voltage signals from sensors into industry-standard two-wire (4–20 mA) or three-wire (0/4–20 mA) current signals. The AM402 consists of three basic units:
(1) High-precision preamplifier. It has a large gain adjustment range, is suitable for different signal input ranges, and can be used for sensor signal processing with various ranges of variation.
(2) Voltage-controlled current output stage. The output current can be adjusted over a wide range by adjusting the bias voltage.
(3) Adjustable reference voltage. It can provide a voltage of 5 V or 10 V to the sensor or external components.
The actual measurement circuit of this capacitive angle sensor, in which the series connection of variable capacitors C1 and C2 is the equivalent circuit part of the capacitive sensor.
4. Experiment and Analysis
In the experimental design, based on the range of the AM402 three-wire output current, the external components of the CAV424 signal conversion integrated circuit chip were adjusted so that its differential output voltage was 0–200 mV. This means the voltage input to the AM402 voltage-to-current conversion interface circuit was 0–200 mV. The AM402 uses a three-wire output, with an output current range of 4–20 mA. According to the aforementioned theoretical derivation, in practical applications, angle sensors are only suitable for linearly varying current outputs within the range of 0–2/π, where the current increases with the angle. Therefore, during the measurement process, only the current output within the range of 0–2/π as the angle changes was measured. The measurement results of the load's two current outputs changing with the angle were used. Based on the principle of least squares, curve fitting was performed on the experimental data to obtain a first-order optimal linear fitting curve.
The results show that the actual measured values are basically consistent with the theoretical values and are very close to the fitted curve; within the measured rotation angle range of 0 to 90 degrees, Iout exhibits linear output.
5. Discussion
The experimental results show that the initial output current value of the fitted curve is slightly less than 4 mA compared to the actual measured value. This is because during the experiment, the AM402 integrated circuit needs to be adjusted according to the actual output current value to achieve an initial bias current of 4 mA. During this adjustment, the sensor itself is affected by parasitic capacitance in the surrounding environment and the influence of the human body on the sensor, thus causing errors in the initial measurement results. Furthermore, the CAV424 circuit, which converts the capacitive sensor signal into voltage, also suffers from environmental interference and errors. The differential voltage output is amplified by the preamplifier section inside the AM402, affecting the output current value.
6. Conclusion
The novel capacitive angle sensor features a simple structure and can measure angle changes from 0 to 90 degrees. Using the CAV424 and AM402, the capacitance change signal can be converted into a standard 4–20 mA current output commonly used in industry. Experiments yielded a relatively ideal linear DC current signal, which can be widely used in practical angle measurement and automatic control. By shielding the sensor itself and its external components, external interference can be effectively reduced, thereby minimizing measurement errors.
