Application of rotary transformer in rotor position measurement

Introduction

In the AC transmission system of military vehicles, the ordinary photoelectric encoder for detecting the rotor position is easily damaged due to the need to adapt to the harsh working environment such as shock, vibration, temperature and humidity changes. The resolver can solve this problem well due to its durability and high reliability. However, the resolver is an analog electromechanical component and cannot meet the requirements of digitalization. Therefore, an interface circuit is required to realize the mutual conversion between its analog signal and the digital signal of the control system. This type of interface circuit is a special type of analog/digital converter, which is what we often call a resolver/digital converter (RDC).

RDC circuit design

Main components and principles

This system uses the TS262N21E11 brushless rotary transformer produced by Japan's Tamagawa Company. The circuit mainly uses two R/D converters AD2S90 and programmable oscillators AD2S99 from ADI, and the two must be used in conjunction.

AD2S90 is the core of RDC circuit design. The internal functional module is mainly a closed-loop feedback system formed by a phase-sensitive detector, a multiplier, a voltage-controlled oscillator (VCO), and an up-down counter. When the output signals Esinwtsinq and Esinwtcosq of the two stator windings of the rotary transformer are respectively sent to the sin and cos input pins of AD2S90, they are sent to the multiplier through its internal operational amplifier. At this time, if the initial digital angle j output by the internal up-down counter is also sent to the multiplier, the output signals obtained after multiplication are Esinwtcosqsinj and Esinwtsinqcosj. The two are then subtracted to obtain the output signal Esinwtsin(qj). The error part of the signal is sent to the closed-loop feedback system formed by the phase-sensitive detector, the multiplier, the voltage-controlled oscillator, and the up-down counter, so that sin(qj)=0, that is, qj=0. At this time, the digital angle j output by the up-down counter is the displacement angle of the rotor.

AD2S99 is an excitation signal source specially provided for resolver/digital converter, with 4 signal frequencies, 2kHz, 5kHz, 10kHz and 20kHz. It not only provides synchronous reference voltage signal for AD2S90, but also provides high-quality sinusoidal excitation signal for resolver, and has advantages of signal phase compensation and signal loss detection.

Overall design of RDC circuit

The two signals sinj and cosj output from the secondary winding of the rotary transformer are used as analog signal inputs for position angle detection of AD2S90, and are connected to the corresponding input terminals SIN and SINL0 of AD2S90 through shielded twisted pair cables. At the same time, the signals of COS and COSL0 are connected to AD2S99 as feedback signals, and the output signal SYNREF (±3Vp-p square wave output signal and the phase is consistent with the SIN and COS input signals) is generated after internal processing of AD2S99, and connected to the REF pin of AD2S90 to compensate for the phase deviation from the primary side to the secondary side of the rotary transformer, otherwise the conversion accuracy of RDC will be reduced. [page]

AD2S99 provides the excitation signal of the required operating frequency for the excitation winding of the rotary transformer through the output terminals EXC and /EXC (EXC and /EXC are two ±2Vrms, 8mA sinusoidal signals, and the /EXC phase is 180° ahead of EXC). The operating frequency of the excitation signal can be set through the AD2S99 pins SEL1, SEL2 and FBIAS. In addition, the AD2S99 pin LOS is used to indicate the detection of the missing SIN and COS signals. LOS is low level during normal operation; if both SIN and COS signals are lower than the voltage threshold (Vrms) specified by LOS, LOS is pulled up to the high level VDD.

There are two output modes for the position signal provided to the controller. One is SPI output (12 bits) with a maximum transmission rate of 2Mbps; the other is the analog incremental encoder output, A, B and NM signals, which is equivalent to a 1024-line incremental encoder.

The overall circuit design plan. The following points should also be noted in the interface circuit design:

1 All analog ground signals in the circuit should be connected to the AGND pin of AD2S90 in a star manner.

2 If the resolver signal is transmitted via a shielded twisted pair cable, the shielding layer must also be connected to the AGND pin.

3 When the output signal SYNREF of AD2S99 is connected to the input pin REF of AD2S90, in order to reduce the DC offset of the SYNREF signal of AD2S99, it is necessary to add a 0.1mF series capacitor and a 100KW ground resistor.

The test waveform of the RDC circuit in the 4-pole induction motor speed measurement system. At this time, the motor rotor frequency is 63.4Hz, and the primary input excitation signal frequency of the rotary transformer is set to 10KHz. The input waveform of the RDC circuit is the sine signal and cosine signal output by the two windings of the secondary side of the rotary transformer; the output waveform of the RDC circuit is the position measurement signal provided to the DSP in the form of orthogonal encoded pulses. It can be seen that the quality of the input waveform is good and meets the design requirements of the RDC; the output rotor position signal waveform meets the QEP circuit reception requirements of the DSP, and the pulse orthogonality is good and there are no glitches, which ensures the sampling accuracy of the DSP for the position signal.

Conclusion

The biggest advantage of this digital rotary transformer is its simple and reliable hardware circuit and high accuracy and resolution. This scheme has well realized the accurate measurement of the asynchronous motor rotor position signal in the AC transmission system of military armored vehicles.