Design of current sampling circuit in servo drive

In a servo drive control system, in order to achieve magnetic field oriented control, it is necessary to sample the current of at least two-phase motor windings. These two current samples will be used as current feedback signals to enable the servo drive to achieve current closed loop. It can be said that current signal sampling is an important module of the servo control system hardware and also a major difficulty.

Conventional current sampling circuit design

Today, most servo drives use a current sampling circuit built with a sampling resistor and a linear optocoupler, as shown in Figure 1.

Among them, RSENSE is a power sampling resistor, MC34081 is an operational amplifier, 78L05 is a three-terminal voltage regulator, HCPL-7840 is a linear optocoupler, its 2 and 3 pins are signal input terminals, 6 and 7 pins are signal output terminals, when the input and output supply voltages are both 5V, when the differential voltage of the 2 and 3 pins changes, the output signals of the 6 and 7 pins will increase and decrease linearly with the input signal.

As shown in Figure 1, when the servo motor is working normally, the current signal collected through the winding is converted into the voltage value across the sampling resistor, and the voltage value is isolated and amplified through a linear optocoupler, and then sent to the DSP for data analysis after A/D conversion through an operational amplifier, thereby realizing current loop closed-loop control. In the actual experimental process, due to the interference of external conditions such as the servo motor, the current sampling signal received by the DSP will have a relatively large degree of interference, so corresponding filtering measures must be added to the circuit.

Design of a new current sampling circuit

The sampling circuits built with sampling resistors and linear optocouplers are all analog circuits, which are easily interfered by the outside world. In the process of circuit debugging, filtering out noise is particularly cumbersome. In order to make the current sampling signal more accurate and the current loop closed better, we designed a solution to realize current sampling using high-voltage linear current sensor ir2175, and conducted comparative experiments.

Chip Overview

IR2175 is a high-voltage linear current sensor designed by IR for AC or DC brushless motor drive applications. It has a built-in current detection and protection circuit, and can perform current sampling through a sampling resistor connected in series with the winding loop. The chip can automatically convert the input analog signal into a digital PWM signal and directly send it to the processor for data processing [2]. Circuit Design

As shown in the circuit diagram of Figure 2, R2 and R3 are sampling resistors, Q1~Q6 are IGBTs, D2~D4 and D6~D7 are fast recovery diodes. The VCC of the IR2175 chip is the power supply pin, connected to +15V. Po is the open-drain PWM output pin. In this experiment, the Po terminal is directly connected to the DSP, so a pull-up resistor is required in the interface circuit to pull it up to 3.3V. Com is the ground terminal and the overcurrent signal output terminal, V+ is the sampling voltage positive input terminal, VB and VS are high-end floating power supply voltage terminals, and VBS is a power supply floating above the voltage peak of VS. Therefore, in this circuit, we use the D1 diode tube and the C1 capacitor to form a bootstrap power supply [3]. Its working principle is: when VS is pulled down to ground through the low-end IGBT, the bootstrap capacitor C1 is charged with the +15V VCC power supply through the bootstrap diode D1, thereby providing the power supply VBS. When vs is pulled to the highest voltage through the high-side switch, vbs is floating and the bootstrap diode is reverse biased, thus blocking the charging loop [2]. The diode should be a fast recovery diode with a recovery time of less than 100ns. The resistor r1 between the vs pin and the half-bridge output should be in the range of 10 to 20ω.

Figure 1 Current sampling circuit based on sampling resistor and linear optocoupler [1]

Figure 2 Sampling circuit based on ir2175

Experimental Results

In this experiment, we used CCS software to display the current sampling signal received by DSP as an intuitive waveform curve in DQ coordinates for comparative analysis.

In the design of the new current sampling circuit, when the servo motor is working normally, the input of IR2175 is a sinusoidal voltage signal, and the output frequency of the PO port is 130kHz, and the duty cycle varies with the current size. The PWM signal (as shown in Figure 3, 4, and 5) has a duty cycle range of 9% to 91%. When the voltage drop on the sampling resistor is 0, the duty cycle of the output signal is 50% (as shown in Figure 3); when the input voltage range is -260mv to +260mv, the corresponding output voltage range is 9% to 91%. When the voltage drop on the sampling resistor is greater than 260mv, the duty cycle of the output signal remains at the maximum value of 91% (as shown in Figure 4); when the input is less than -260mv, the output duty cycle remains at the minimum value of 9% (as shown in Figure 5). When the voltage drop on the sampling resistor exceeds -260mv to +260mv, the IR2175 outputs a low-level effective overcurrent signal with a typical value of 2μs.

Through the observation and analysis of Figures 3, 4, and 5, it can be seen that the PWM waveform output by IR2175 is stable and has fewer interference signals, and the sampling data transmitted to DSP is relatively accurate.

Figure 3 When the input is 0, the output duty cycle is 50%

Figure 4 Input maximum 260mv, output duty cycle 91%

Figure 5 Input minimum -260mv, output duty cycle 9%. The current signal waveforms collected by the two current sampling schemes with the same software program and debugging parameters are compared, as shown in Figures 6 and 7.

Figure 6 Conventional current sampling waveform

Figure 7 New current sampling waveform

The waveform curve of the two sampling signals obtained by the conventional current sampling circuit design is shown in Figure 6. It can be seen that it is a sine waveform. Because there are still some burrs in the waveform, the waveform is not smooth. Therefore, we add software filtering on this basis and successfully realize the current closed-loop control. After repeated experiments, it is verified that the motor runs smoothly and the current closed-loop can be realized.

The waveform curves of the two sampling signals obtained by the new current sampling circuit design are shown in FIG7 . The waveforms are very smooth and can be directly used as a current closed loop without any processing.

Both current sampling circuits can realize current loop closure, but the experimental waveforms in Figures 6 and 7 show that when the current sampling circuit is composed of sampling resistor and linear optocoupler, it is susceptible to external interference, and more filtering circuits need to be added and a lot of debugging is required, and the resulting waveform is not smooth. When the sampling circuit is composed of ir2175, the interface circuit can be greatly simplified, and because its output signal is a digital signal, the influence of external interference on it can be reduced to a large extent. Compared with the previous design circuit, it is more convenient, stable, and has a better closed-loop effect.

Conclusion

Through this experiment, we can find that the use of current sensor chips can easily solve the current collection of servo drives, and the collected signals are relatively accurate. However, when designing the PCB, we still need to pay attention to the isolation of high-voltage and low-voltage signals, and we should add appropriate protection circuits and filtering circuits.