An Improved Isolated Delta-Sigma Modulator Current Sampling Method

“Servo control systems are widely used in industrial robots, CNC machine tools, Electronic manufacturing, printing machinery, textile machinery and other fields, and play an important role in industrial production. In the fields of industrial robots and CNC machine tools, servo control systems with fast response speed, small torque jitter and high torque are becoming more and more critical. The servo control system consists of a position loop, a speed loop and a current loop. The current loop is an inner loop, and the performance of the current loop determines the response speed of the entire control system.

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Servo control systems are widely used in industrial robots, CNC machine tools, electronic manufacturing, printing machinery, textile machinery and other fields, and play an important role in industrial production. In the fields of industrial robots and CNC machine tools, servo control systems with fast response speed, small torque jitter and high torque are becoming more and more critical. The servo control system consists of a position loop, a speed loop and a current loop. The current loop is an inner loop, and the performance of the current loop determines the speed and response speed of the entire control system.

A factor that affects the performance of the current loop is the current sampling method. The current mainstream current sampling methods include Hall current sampling, isolated op amp current sampling and isolated delta-sigma modulator current sampling. TI’s isolation delta-sigma modulator AMC1305 has high performance, high insulation level and strong anti-interference performance, and is widely used in industrial servo, inverter and solar inverter fields.

The current sampling method of AMC1305 is shown in Figure 1. AMC1305 converts the current information into a single-line data stream, and the data stream passes through the filter module in the control chip such as DSP, and restores the current information. Compared with the first two current sampling methods, the isolated delta-sigma modulator is used for current sampling, which has the advantages of high and strong anti-interference ability. However, the current sampling method of the isolated delta-sigma modulator has low bandwidth and slow response speed. When the motor is overloaded or short-circuited, it takes a long time to reflect the current digital quantity, which may cause protection failure.

Figure 1: Schematic diagram of the current sampling of the AMC1305

In order to improve the performance of the current sampling method of AMC1305, taking into account the current loop control and overcurrent protection functions, two sets of digital filters can be set in the DSP and other controllers (see Figure 2), one with lower bandwidth and higher , used for internal current loop control; another group, with higher bandwidth and lower, used for overcurrent protection.

Figure 2: A method of current sampling filter with both current loop control and overcurrent protection functions

Using the current sampling method of AMC1305, the Sinc3 filter is usually used to extract the current information. The Over Sample Rate (OSR) of the Sinc3 filter determines the current sampling and bandwidth (see Figure 3). Using the same filter structure, the OSR and filter frequency response are shown in Fig. 3(a). The larger the OSR, the lower the bandwidth. Under the same filter structure, the effective number of bits (ENOB) under different OSRs is shown in Figure 3(b). The larger the OSR, the larger the ENOB.

Figure 3: OSR, Bandwidth, and Effective Bits Relationship for Sinc3 Filters

When AMC1305 is used for current sampling, Sinc3 filter is used, different OSR, bandwidth and ENOB of current sampling are shown in Table 1.

Table 1: OSR, Bandwidth, and Effective Bit Relationship for Sinc3 Filters

Through the above analysis, the current loop has high requirements, the OSR of the Sinc3 filter is set to 128 or 256, and the ENOB is greater than 13 bits; the overcurrent protection function has high requirements for rapidity, the OSR of the Sinc3 filter is set to 8 or 16, and the response bandwidth is greater than 330kHz.

This paper proposes an improved isolation delta-sigma modulator current sampling scheme, which has the following advantages:

● While meeting the high requirements of current loop control, the rapidity requirements of overcurrent protection are met;

● Simple implementation, no need to modify external circuit