Angular position detection system based on AD2S83 and its application in radar servo system-EEWORLD

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【Abstract】 The resolver-to-digital converter (RDC) integrated circuit AD2S83 is introduced, and its application in radar servo system is briefly described.
Keywords: resolver-to-digital converter (RDC), AD2S83, radar servo system

1 Introduction

The resolver based on the principle of electromagnetic induction is a precision micro-control motor. In the radar antenna angular position servo system, it completes the detection function of the shaft angular displacement information. Since it is an analog electromechanical component, it is required to use it in a computer-controlled digital servo system. A certain interface circuit, namely the resolver-digital converter (RDC), is required to realize the conversion of analog signals to digital quantities of the control system. With the rapid development of electronic technology, the American AD company has developed it into a series of monolithic integrated circuits, thus making up for the shortcomings of the RDC built by discrete components in the past, which was large in size and low in reliability, and brought great convenience to engineering applications. The resolver and AD2S83 can form a high-precision radar antenna angular position detection system, and the analog speed signal output by AD2S83 can also be used as a speed feedback signal to form a speed loop in the radar servo system.

2 Resolver

The resolver is a component that works according to the principle of electromagnetic induction. There are windings on its stator and rotor, which are arranged concentrically and coupled to each other. The resolver uses orthogonal two-phase windings. It is mainly used in the angular position servo control system as an angular position generation and detection element. If the excitation voltage of the resolver is Uf = Ufmsinω·t, the induced electromotive force in the orthogonal A and B phase windings is

eA=Emcosθsin(ωt+α)(1)

eB=Emsinθsin(ωt+α)(2)

Among them, θ is the rotor angle of the resolver, and α is the phase angle between the secondary electromotive force and the primary excitation voltage.

3 Introduction to AD2S83

AD2S83 is a monolithic integrated circuit launched by AD Company of the United States, manufactured with BIMOSⅡ process, which combines advanced CMOS logic circuit with high-precision bipolar linear circuit. It has low power consumption (300mW), and its digital output resolution can be set by the user to 10, 12, 14 or 16 bits, and has a speed output signal that can be used by the user as a speed feedback signal of the speed loop to replace speed measuring components such as tachometer generators, thereby reducing the size of the system.
After AD2S83 is connected as shown in Figure 1, it constitutes a tracking RDC working in a type II servo loop. Its digital output can automatically track the shaft angle input at the selected maximum tracking rate without static error. Because it uses a ratio tracking method when converting the resolver signal into a binary number, the output digital angle is only related to the ratio of the SIN and COS input signals, but not to their absolute values. Therefore, AD2S83 is insensitive to the amplitude and frequency changes of the input signal, and does not need to use a stable and accurate oscillator to generate the reference signal, while still ensuring accuracy. The presence of a phase-sensitive detector in the conversion loop ensures a high suppression capability for the orthogonal components in the reference signal. In addition, it has a strong ability to suppress noise and harmonics. The most prominent advantage of AD2S83 is that it allows users to select the corresponding parameters to optimize the performance of the entire system.

4 Design of speed output and angular position detection circuit

The key to the design of speed output and position detection circuit is to correctly select the peripheral components of AD2S83. The following introduces the selection of AD2S83 peripheral components. It should be noted that the components closest to the ideal value should be selected and work within the allowable temperature range. The selection of components with an error level of 5% will not reduce the performance of the converter. See Figure 1.
4.1 Selection of high-frequency filter components R1, R2, C1, C2
The role of the high-frequency filter is to eliminate DC bias and reduce the noise entering the AD2S83 signal because they affect the output of the phase-sensitive detector. Its role is particularly important when there is noise from the switching power supply and brushless motor. The selection of component parameters is as follows:

Where fREF is the reference frequency.
When R2＝R3, C1＝C3, R1 and C2 can be omitted.
Note: Since the high-frequency filter has a 3-fold attenuation on the signal input to the phase-sensitive detector, it will affect the gain of the loop.
4.2 Selection of gain proportional resistor R4
If R1 and C2 satisfy equations (3) and (4), then

Among them, 100×10－9＝current/LSB;
EDC＝160×10－3 10-bit resolution;
＝40×10－3 12-bit resolution;
＝10×10－3 14-bit resolution;
＝2.5×10－3 16-bit resolution.
4．3 Selection of R3, C3
Appropriate R3 and C3 make the signal have no obvious phase shift at the reference frequency. The two components are:

4.4 Selection of Maximum Tracking Rate
The input resistor R6 of the VCO is used to set the maximum tracking rate of the converter. If the speed output is 8V at the maximum tracking rate, then R6 is:

Where T cannot exceed 1/16 of the maximum tracking rate or reference signal frequency, and n is the output resolution.
4.5 Selection of closed-loop bandwidth
When selecting the closed-loop bandwidth (fBW), it is necessary to ensure that the ratio of the reference frequency to the closed-loop bandwidth does not exceed the indicators shown in Table 1.

When the reference frequency is 400Hz, the typical value of the bandwidth is 100Hz. When the reference frequency is 5kHz, the typical value of the bandwidth is 500Hz to 1000Hz. C4, C5, R5 are selected as follows:

4.6 VCO phase compensation
C6, C7, R7 take the following values: C6 = 390pF, C7 = 150pF, R7 = 3.3kΩ.
4.7 Bias adjustment
The drift of the integrator input and the bias current will cause additional position drift at the converter output. If the drift can be ignored, R8 and R9 can be omitted, otherwise R8 = 4.7MΩ, R9 = 1MΩ (potentiometer). In order to reduce zero drift, first select the peripheral components of AD2S83, disconnect the connection between AD2S83 and the resolver, then connect the COS and REFERENCEINPUT pins, the SIN and SIGNALGROUND pins, add power and reference signals, and adjust the potentiometer R9 to make the output all "0".

4.8 Selection of Output Resolution
The output resolution of the AD2S83 can be set by the user to 10, 12, 14, or 16 bits through the logic states of the two pins SC1 and SC2. See Table 2 for details.

5 Application of AD2S83 in radar servo system

Radar servo system must meet certain accuracy requirements and ensure high reliability. In some radars, it is also required to be small and light. Based on the advantages of brushless rotary transformer and AD2S83 integrated circuit, it is a good choice to select high-reliability brushless rotary transformer and AD2S83 RDC to form the angular position detection system of these radars, and use the analog speed output signal of AD2S83 as the speed feedback signal to form the speed loop of radar servo system. The system block diagram is shown in Figure 2.
According to the performance indicators of the system, after selecting the peripheral components of AD2S83 according to the above calculation method, the speed feedback and position detection circuit can be designed. When implementing this circuit, it should be noted that 100μF (ceramic) and 10μF (gall) decoupling capacitors should be connected in parallel between +Vs, -Vs and ANALOG GROUND, and between +VL and DIGITAL GROUND, respectively. They should be placed as close as possible to the AD2S83 converter, and each converter should have its own separate decoupling capacitor. The two signal grounds of the resolver should be connected to the SIGNAL GROUND pin of the converter to reduce the coupling between the sine and cosine signals. In addition, the sine and cosine signals of the resolver and the reference signal are preferably connected using twisted shielded wires.
The following describes the operation of the computer on the AD2S83 RDC. Before that, the control signal of the AD2S83 converter is briefly explained:
/INHIBIT input: The /INHIBIT signal only prohibits the reversible counter from transmitting data to the output latch, and does not interrupt the operation of the tracking loop. Releasing this signal will automatically generate a BUSY and refresh the output latch.
/ENABLE input: The /ENABLE signal determines the state of the output data. When it is high, the output data pin remains in a high-impedance state. When it is low, the data in the output latch is allowed to be transmitted to the output pin. The operation of /ENABLE will not affect the operation of the converter.
BYTESELECT input: Regardless of the state of this signal, when /ENABLE is low, the low-order byte will appear on the data output line DB9-DB16. When BYTESELECT is high, the high 8-bit byte will appear on the data output lines DB1-DB8; when BYTESELECT is low, the low 8-bit byte will appear on the data output lines DB1-DB8, and they also appear on DB9-DB16.
The process of computer reading data from AD2S83: First, apply the /INHIBIT signal to AD2S83 to prevent the latch from being refreshed. When /INHIBIT is set to a low level and delayed for 600ns, the data is valid. After setting the /ENABLE signal to a low level, the data can be read. After reading the data, the /INHIBIT signal should be released immediately and set to a high level so that the output latch can be refreshed.

6 Conclusion

This paper introduces the application of AD2S83 RDC integrated circuit, and designs the speed feedback and angular position detection system in the radar servo system composed of the integrated circuit and the rotary transformer. Practice has shown that the system has the advantages of small size, high reliability, strong anti-interference ability, and flexible and convenient operation.

References

1 Qu Jiaqi, Wang Jizhi. Sensors in Servo Control Systems. Beijing: Machinery Industry Press, 1998, 5

Reference address：Angular position detection system based on AD2S83 and its application in radar servo system

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