Current output capacitive angle sensor based on am

  • Detail

Current output capacitive angle sensor based on am402

the traditional differential capacitive angle sensor has a limited measuring angle. The moving plate has a direct electrical connection with the outside world, and its electrodes should be led out from the driven plate and the static plate respectively. The method of bearing, clamp or metal tension wire is usually used for the lead out electrode of the driven plate. During the rotation of the moving plate, there is not only friction between the rotating shafts, but also easy to cause poor contact between the lead out wire and the rotating shaft of the moving plate, resulting in mechanical failure, interference and error, and affecting the measurement accuracy. Therefore, a new capacitive angle sensor is designed and its structure is improved to overcome the defects of the traditional differential capacitive angle sensor

1 structural design

the sensor system adopts a new structural design method. The moving plate is composed of two opposite sector-shaped metal plates with an opening angle of 90 °, and the middle part of the metal plate is connected; The static sheet is composed of two fan-shaped metal plates with an opening angle of 90 ° and the middle part is separated. The electrodes are respectively led out from the two static sheets as the signal output end of the sensor. In essence, the capacitive angle sensor with this new structure is equivalent to the series connection of two variable capacitors. The series connection value of the capacitance changes with the change of the relative angle between the moving metal plate and the fixed metal plate. The design method of leading out the electrode from the static plate is convenient for the lead, which is conducive to the angle change of the moving plate relative to the static plate. Moreover, the moving plate has no electrical connection with the outside world, avoiding the friction and mechanical failure caused by rotation in the measurement process, so as to truly realize the non-contact measurement and improve the measurement accuracy

2 basic principle

the basic principle of capacitance sensor is based on the relationship between capacitance between objects and structural parameters. For a flat plate capacitor, if the parallel plate is infinite and its edge effect is ignored, its capacitance formula is expressed as:

it can be seen from formula (1) that the change of dielectric constant, relative area of plates or plate spacing can be reflected as the change of capacitance value. So it can be used to measure the displacement of the object and the state parameters of the material

according to the design structure, it is assumed that the radius of the stationary plate of the capacitive angle sensor is r, the distance between the rotating plate and the stationary plate (i.e. the distance between the plates) is D, the dielectric constant of the air is en, and the change of the relative area between the rotating plate and the stationary plate with each opening angle of 90~ is △ S. in the initial state, the relative area between the rotating plate and the stationary plate is zero, Then △ s1= △ s2= △ S. because:

the total capacitance change is:

let the rotating plate start from the initial position and the rotation angle relative to the static plate be the mouth. When

the capacitance change changes periodically with the angle change, it is printed at 0 ~ 3. In each quadrant of the, the capacitance change is linear with the angle change 0, and the maximum change is:

3 capacitance signal measurement and its conversion circuit

3.1 capacitance voltage conversion circuit cav424

cav424 is a multi-purpose integrated circuit that can process the signal conversion of various capacitive sensors, and can convert capacitance signals into voltage signals. McClelland pointed out that "current and future vehicles will adopt various energy-saving materials, which have the functions of signal acquisition (change of relative capacitance), processing and differential voltage output at the same time, and can measure the difference between the measured capacitance and the reference capacitance, that is, relative to the reference capacitance (10 PF ~ 2 NF) The capacitance value within the variation range of 5% ~ 100% is converted into the corresponding differential voltage output, with high detection sensitivity; At the same time, it also integrates a built-in temperature sensor, which can be directly used to monitor the temperature when digital signal correction is required. Using CAV424 as the conditioning circuit of capacitance sensor can overcome the influence of parasitic capacitance and environmental changes, and improve the measurement accuracy and anti-interference ability. At the same time, the sensor has fewer external components, the processing circuit is relatively simple, and the instrument is small

3.2 voltage current conversion circuit am402

am402 is an integrated circuit that processes differential signal input and current output interfaces. It can convert the input weak sensor differential voltage signal into a two-wire output (4 ~ 20 mA) or three wire output (0/4 ~ 20 mA) current signal that meets the industrial standard. Am402 consists of three basic units:

(1) high precision preamplifier. It has a large gain adjustment range and is suitable for different signal input ranges. It can be used for sensor signal processing in different variation ranges

(2) current output stage controlled by voltage. The output current can be adjusted in a wide range by adjusting the bias voltage

(3) adjustable reference voltage. A voltage of 5 V or 10 V can be supplied to the sensor or external components

the actual measuring circuit of the capacitive angle sensor, in which the series part of the variable capacitors C1 and C2 is the equivalent circuit part of the capacitive sensor

4 experiment and analysis

in the experimental design, according to the value range of the three wire output current of am402, adjust the external components of the capacitive sensor signal conversion IC chip CAV424, so that common fault 1: the output differential voltage value is 0 ~ 200 mV, that is, the voltage value input to the voltage current conversion interface circuit am402 is 0 ~ 200 mV, while am402 adopts the three wire output mode, The output current range is 4 ~ 20 ma. according to the above theoretical derivation, in practical application, the angle sensor is only suitable for taking the linear change region in which the current output increases with the increase of the angle in the range of 0 ~ 2/π. Therefore, in the measurement process, only the output of the current in/π changes with the angle is measured. The measurement results of the two current outputs varying with the angle through the load. According to the principle of -'b~- multiplication, the experimental data were fitted to obtain the first-order best linear fitting curve

the results show that the actual measured values are basically consistent with the theoretical values and very close to the fitting curve; IOUT is linear output within the range of 0 ~ 90~ of the measured angle

5 discussion

from the experimental results, it can be seen that compared with the actual measured value, the output initial current value of the fitting curve is slightly less than 4 ma. this is because during the experiment, the IC am402 should be adjusted according to the actual output current value to make its initial state bias current 4 Ma. During the adjustment process, The sensor itself is subject to the interference of parasitic capacitance in the surrounding environment and the influence of the human body on the sensor in the adjustment process, which will cause errors in the measurement results of the initial value. Moreover, the circuit CAV424 in which the capacitive sensor signal is converted into voltage also has certain environmental interference and error. The output differential voltage is partially amplified by the pre amplifier inside the am402, which will affect the output current value

6 conclusion

the new capacitive angle sensor has a simple structure and can measure the angle change of 0.90 ~. With the increase of c1999 to 109kgav424 and am402, the capacitance change signal can be converted into the current output of 4 ~ 20 mA standard commonly used in industry. The ideal linear output DC current signal is obtained from the experiment, which can be widely used in the field of actual angle measurement and automatic control. By taking shielding measures for the sensor itself and its external components, the interference of external interference to the system can be effectively reduced, so as to reduce the measurement error. (end)

Copyright © 2011 JIN SHI