Ai-cheng ZOU，Qun-ying WANG，Yun ZHANG

Guilin University of Aerospace Technology，Guilin 541004，China

1.Introduction

CNC cutting machine has been widely used in manufacturing industry．In order to improve the cutting quality and reduce waste generation，the height between cutting torch and the material to be processed must always keeping the setting value in the cutting process［1］．But because the temperature near the torch is very high，ordinary sensors can not be used，so it needs to design a new height sensor device．A feasible method is to place a steel planar inductive ring which can withstand high temperature near the torch．The capacitance which usually very small between the ring and the material to be processed can characterize the height of which its［2-3］．That can be expressed as:

Where，ε is the dielectric constant between torch and material to be processed，S is the size of ring．

This paper designed a detection circuit for measuring the micro capacitors．Through measuring the capacitance value，we can calculate the height．

2.Micro capacitance detection circuit

2.1.Overall block diagram of circuit

Overall block diagram of the micro capacitance detection circuit is shown in Figure 1．The sine excitation source signal(V1)，generated by the sine wave generating circuit，after processed by calculation capacitance detection circuit became the sine wave signal(V2)．The frequency of V2is equal to V1，and the amplitude of V2is proportional to the value of the height between the torch and material to be processed．After processed by the phase detection circuit，V2became DC voltage signal(Vout)，whose amplitude is equal to V2．Then can get the height．

Figure 1．Overall block diagram of circuit

2.2.Sine wave generating circuit

Micro capacitance detection circuit requires precise sine signal which maintains frequency of 10 kHz，amplitude of 3 V stability［4］．This paper designed the sine wave generating circuit based on XR-2206 as shown in Figure 2．The circuit can output sine signal from the pin 2 stability．R2is used to regulate the amplitude of the output sine wave signal，whose range of error is±1%．And R7is used to regulate the frequency of the output sine wave signal，whose range is 200 Hz～2 MHz．And R9is used to regulate the symmetry of the output sine wave signal．

Figure 2．Sine wave generating circuit

In the circuit V1(t)=Vmsinωt，ω =2π ×105．

2.3.Calculation capacitance detection circuit

Calculation capacitance detection circuit is shown in Figure 3．There are several characteristics given as follows．

1)“T”Network Resistance．R1，R2and R3formed the“T”network resistance，which can use small resistance to get greater resistance，thereby reducing the noise voltage output．The equivalent resistance of the“T”network resistance is Rf=R1+R1R3/R2+R3．R1=R3=100 k，R2=1 k，the equivalent resistance of the“T”network resistance is 10．2 M．And the noise of the output voltage is mainly determined by R1．If the“T”network resistance is replaced by a resistance R(10．2 M)，the noise of the output voltage is mainly determined by R．Obviously，R is much greater than R1，so the“T”network resistance can greatly reduce the output noise voltage．

Figure 3．Calculation capacitance detection circuit

2)“Double T”filter．C2，C3，C4，R5，R6and R7constitute the“double T”filter，which notch frequency is 10 kHz．The transfer function of the circuit is

Since Cref=Cx，the relation between the frequency of input excitation signal(V1(t))and the amplitude of the output voltage signal(V2(t))as shown in Figure 4．Since V1=10 kHz，the amplitude of output signal V2(t)reached maximum and equal to the amplitude of input signal V1(t)，the signals of another frequency will decay．

Figure 4．Relation between V1(t)and V2(t)

2.4.Phase detection circuit

The phase detection circuit，was used to calculate the DC amplitude of output signal V2in Figure 4，as shown in Figure 5．The circuit consist of two parts，one is Switch phase detection circuit which consists of U1，U3 and U4，another is filter amplifier circuit which consists of Low pass filter U2，operational amplifier U3 and sliding rheostat R8．

Figure 5．Phase detection circuit

The transfer function of the phase detection circuit is

The“K”is

The“T”is periodic of the excitation signal．

Conduct Fu Liye decomposition on K，we have

so

From the above equation，we know the DC component of V4(t)is 2*Vm/π，Processed V4(t)by low pass filtercircuit，can getthe DC component of V4(t)．

3.Simulation and test

3.1.Simulation analysis

Simulated the circuit composed of sine wave generating circuit，calculation capacitance detection circuit and phase detection circuit by the software SPICE．In sine wave generating circuit，adjust the parameters of the XR-2206，get V1(t)=3sin2π*105t．In calculation capacitance detection circuit，set Cx=Cref=10 μF．And adjust circuit parameters of phase detection circuit，get

The simulation result of Voutas shown in Figure 6．When changing Cx，we can get a series of simulation value of Vout，as shown in Table 1(Column“Simulation result”)．

3.2.Test analysis

Make printed circuit board and test．can get the testing value of Vout，as shown in Table 1(Column“Test result”)．

Figure 6．Waveform of Vout

Table 1．Simulation and test result of Vout

4.Conclusion

Compare“Simulation result”and“Test result”，get their error as shown in Table 1(Column“Error”)．The error between “Simulation result”and“Test result”within 0．020 V when Cxchanges from 2 μF to 20 μF．Through theoretical calculation and experimental analysis，we know that the circuit has the ability of anti noise and stray capacitance，and has high precision，can meet the requirements．

［1］ SUN Lining．Design of Transform Circuit for Displacement Capacitance Sensor［J］．Machinery ＆ Electronics，2013(1):34-35．

［2］ MO Bing．A Detecting Circuit for Signal of Micromachined Gryoscope with Capacitive Detection［J］．Chinese Journal of Scientific Instrument，2014(2):325-326．

［3］ ZHAO Jiazheng，BU Jiawu． Auto-adjustment control system of the arc voltage in NC plasma Cutter［J］．Modern Manufacturing Engineering，2007(1):28．

［4］ LI Ping．Simulations of DC motors in ripple-free deadbeat control［J］．Computer Engineering and Design，2009(8):1998-1999．