New current sampling scheme based on TLC2274 and its implementation in DSP

Introduction

Current sampling is used in most motor speed regulation and other control systems for current feedback control. Currently, digital signal processing (DSP) is increasingly used in high-performance motor frequency conversion speed regulation systems. Among them, the C2000 series of DSPs represented by Texas Instruments (TI) TMS320C/LF240(X) are frequently used. Most of the existing current sampling methods use the analog-to-digital sampling scheme in literature [2], as shown in Figure 1 below:

 

Figure 1(b)	Figure 1(c)	Figure 1(d)

The principle of the scheme shown in Figure 1 is: first use a current transformer or current sensor (such as the LTS series sensor of the Swiss LEM company, etc.) to sample the two-phase current value; then pass the sampling result through the operational amplifier to convert the current value to -2.5~+ In the voltage range of 2.5v, a voltage offset of +2.5v is finally added to form a voltage of 0~5v and sent to the DSP for sampling. The advantage of this method is that the circuit is simple and easy to implement, but its disadvantage is low sampling accuracy and large error. As shown in Figure 1(b), when one of the peak values ​​of the AC phase current is converted into a DC voltage, one is 5v and the other is 0v. Due to the existence of the voltage dead zone, a large error occurs near 0v.

New current sampling scheme

The operational amplifier used in the new current sampling scheme is TLC2274[1]. TLC2274 is a high-performance quad operational amplifier manufactured by Texas Instruments using advanced LinCMOS technology and has Rail-to-Rail output capability. It has better noise, power consumption and input offset voltage performance than currently commonly used CMOS operational amplifiers. The low noise and high input impedance of the TLC2274 are very suitable for calculation and amplification of small signals such as voltage/current sensors. And its minimum operating voltage can be as low as plus or minus 2.2V.

Based on the following circuit, shown in Figure 2. Figure 2 shows a double subtraction current sampling circuit. The output voltage of the first operational amplifier U8B is:
 
choose R2 = R1, R3 = Rf1,
then we have

 
Figure 2 Double subtraction current sampling circuit

In the same way, the output of the second op amp U8A is:
 

The main idea is: Ui= v output by the LEM sensor, this voltage is successively applied to two subtraction circuits composed of TLC2274. The first circuit uses Ui to subtract the median reference voltage Uref (2.5v) of the sensor sampling result, and then Then linearly amplify to the voltage range (0~Ud) required by the A/D sampling of the DSP. For TMS320C/F20x and C/F24x series DSPs, the value of Ud is 5v; for TMS320LC/F240x Ud is 3.3v. The second path is the opposite, subtracting the sensor output voltage Ui from the median reference voltage Uref, and also linearly amplifies it to a suitable voltage range. Z1 and Z2 are two 3.3v Zener diodes, which limit the output voltage of the op amp. When the value of Ui is greater than Uref, Uo1 outputs a positive voltage, and the voltage range is 0~Ud. However, due to the existence of diode D2, current cannot be injected into the operational amplifier, so the second operational amplifier cannot output negative voltage, but clamps The bit is at 0v; when the value of Ui is less than Uref, Uo2 outputs a positive voltage. Similarly, due to the existence of diode D1, the first op amp cannot output negative voltage and is also clamped at 0v. At a certain moment in a sinusoidal cycle, only one signal outputs a voltage of 0~Ud, which is twice as wide as the sampling window of the method in Figure 1, thus improving the sampling accuracy.

The two outputs are sent to the two A/D sampling channels in the DSP, but only one output value is useful. We can program to distinguish useful signals. The software flow is shown in Figure 3:
 
Figure 3 Software flow chart

In this way, the two-phase current actually requires 4 A/D channels, which requires two more A/D sampling channels than the method in Figure 1. However, the current DSP provides enough A/D sampling channels, taking TMS320LF2407(A) as an example. , which has 16 A/D channels. The B-phase current sampling circuit is the same as the A-phase, so we can use a TLC2274 to achieve two-phase current sampling. The subroutine implemented by DSP is attached at the end of the article.

Improvement of the circuit

It is not difficult to find through experiments that TLC2274 can work on a single power supply, so some improvements can be made to the double subtraction circuit shown in Figure 2. Since the TLC2274 is a Rail-to-Rail output, we can directly use the system's 3.3v to provide the working voltage Vcc, and the Vdd can be directly connected to ground. After removing D1, D2, Z1, and Z2, do the experiment and verify that they still work normally. However, the sampling accuracy will be reduced after doing this, so it is recommended to use positive and negative dual power supplies.

Attached is the AD sampling subroutine.bss
Uo1, 1
.bss Uo2, 1 ; A-phase current intermediate value.bss
Uo3, 1
.bss Uo4, 1 ; B-phase current intermediate value.bss
Ia, 1 ; A-phase current.bss
Ib, 1; B-phase
current.bss Ic, 1; C-phase current
; ... ;Suppose the above variables are in the same DP
ADC_Proc:
CALL StartADC; Call the AD sampling program
CLRC C
LDP #Uo1
LACL Uo1
SUB Uo2
BCND AP1, C; Uo1>Uo2
LACL Uo2
NEG ; Uo2 negation
B AP2
AP1: LACL Uo1
AP2: SACL Ia ; get Ia
　　...
　　RET
StartADC:

References
1 slos106c TLC2274, TLC2274A, TLC2274Y Advanced LinCMOSE rail-to-rail quad operational amplifiers, www.ti.com
2 brpa076, Implementation of a Speed ​​Field Orientated Control of Three Phase AC Induction Motor using TMS320F240, www.ti.com
3 . Liu Heping, TMS320LF240x DSP structure, principle and application, Beijing University of Aeronautics and Astronautics Press, 2002