Home > News > Application of Current Sensor Sampling in Servo Control System

Application of Current Sensor Sampling in Servo Control System

2020-11-13
Introduction Servo technology is a tracking and positioning control technology and an important part of electromechanical integration technology. It is widely used in automation equipment such as CNC machine tools and industrial robots. With the continuous expansion of modern industrial production, the demand for electric servo systems in various industries is increasing day by day, and higher demands are placed on their performance. Therefore, researching and manufacturing high-performance, high-reliability servo drive systems is a goal that industrial advanced countries are striving to achieve. It has very important practical significance.
At present, digital servo drives are basically monopolized by Japan, Europe and the United States. Every year, China needs to import a large amount of such equipment from abroad for CNC machine tools and other industries. The price of imported drives is high and maintenance services are inconvenient. The all-digital servo driver with independent intellectual property rights in China began to be manufactured in large scale in the 1990s. Huazhong CNC hsv series digital AC servo motor drive unit has good performance. The speed ratio of all-digital AC servo system independently developed by our company is 1:5000. High-end products often use foreign AC servo systems, mainly domestic servo drive controllers in high-speed and high-precision control characteristics, compared with Japan's fanuc, Mitsubishi, Panasonic, Fujitsu and Germany's Siemens and other foreign advanced products, There is a significant gap.
Servo Control System Hardware Design The digital servo system is mainly composed of five parts: a permanent magnet synchronous motor, a power supply module, a drive and inverter circuit module, a speed and position detection circuit module, and a control circuit module. The control circuit module includes the core control chip, human-machine interface and communication module; the drive and inverter circuit module includes the inverter main circuit, voltage/current sampling circuit, overvoltage/undervoltage protection, and current limit protection. Brake circuit, digital servo control system hardware block diagram shown in Figure 1.
The tms320f2812dsp is the control core and receives information from the CNC, the encoder interface, the current detection module, and the fault signal processing module to complete the control and troubleshooting of the permanent magnet synchronous motor. Optical isolation module as the interface between the electronic circuit and the power main circuit, the svpwm signal sent by the dsp is sent to the ipm module to complete the dc/ac inversion and drive the motor to rotate. The encoder interface sends the magnetic pole position, motor steering and encoder alarm information of the permanent magnet synchronous motor recorded by the Absolute Encoder to the dsp, and sends the position information of the permanent magnet synchronous motor to the cnc. The motor phase current is measured, filtered, amplitude-converted, zero-shifted, and limited by the current detection module, and converted into 0~3v voltage signal and sent to the dsp a/d pin. Power over-voltage, under-voltage, short-circuit, power-failure, and IPM faults of the power main circuit are detected and processed by the fault detection module and sent to the i/o port of the dsp. The keyboard and display module are the man-machine interface of the controller, used to complete the input of control parameters, and display the operating status and operating parameters. The memory module is used to store control parameters and system fault information.
The servo's core control chip adopts tm320f2812, the latest motor-specific control chip from Ti. It has the following outstanding performance compared to other similar dsps:
Using high-performance static CMOS technology, the main frequency can reach 150mips, shorten the instruction cycle to 6.67ns (150mhz), and use 32-bit operation, thereby greatly improving the processing capability;
Low power consumption, supply voltage drop 1.8v (core) and 3.3v (i/o);
On-chip up to 128k words flash program memory, 18k saram and 4k rom;
With 12-bit a / d converter, the minimum conversion time is 80ns.
The inverter circuit uses Mitsubishi's ipm module. The smart power module uses a 5th generation igbt process, with an optimized gate drive and protection circuit, and an incredibly ultra-compact volume with three-phase waveforms with powerful output power. It has the following outstanding performance:
Complete power output circuit, directly connected to the load;
Built-in gate drive circuit;
Short circuit protection;
Drive voltage undervoltage protection;
Adopt fifth-generation low-power igbt die;
Ultra-small size, weighing only 65g.
Digital servo system control strategy Digital servo system is generally completed by three closed loops. The principle is shown in Figure 2. The first layer is the position loop, the second layer is the speed loop, and the third layer is the current loop; where the position and speed are all outer loops, and the current loop is the inner loop within the system. The composition is composed of core hardware and key solution software. The all-digital servo system is the core transmission part of the CNC machine tool, and it is also the most technically difficult part. Its main features are high speed, high precision, and rich and varied functions. The current loop is the core control loop of the servo system, and the key to ensure the speed accuracy and the torque stability is the design of the current loop in the digital servo. Therefore, whether a system performance is excellent is closely related to the design of the current loop.
The design of the lem sensor and current sampling circuit scheme directly results in the accuracy of the entire current loop because of the accuracy and speed of sampling, and thus has a very significant impact on the performance of the system. In the field of power parameter measurement, the Hall current sensor, which is the leading manufacturer of Lem, has become the first choice for this system design due to its stable and reliable product performance. The model number is lts25-np. This sensor uses a single-supply power supply. Compared to a dual-supply power sensor (see Figure 3), the Lamb sensor has a simpler peripheral hardware design and does not require an additional voltage boost circuit (a dual-supply sensor must Increase the voltage raising circuit to convert the negative voltage into a positive voltage before entering the dsp) to reduce the interference of the power supply to the system. Another advantage of this sensor is the small temperature drift, high precision; and built-in sampling resistor, the output is a voltage-type output, to avoid the increase in the external sampling resistor and ops to enter the dsp to reduce the accuracy.
The specific characteristics and performance parameters of the lts25-np sensor are as follows:
The original side rated current effective value ipn: 25a;
The primary current measurement range ip: 0 ~ ± 80a;
Supply voltage: +5v;
Output voltage vout: 2.5 ± 0.625v;
The conversion rate kn=np:ns is: 1:2000;
Total accuracy: ±0.2%;
Linearity: less than 0.1%;
Reaction time: less than 500ns.
The sensor has three pins: positive (+5), measuring (out), and ground (0), as shown in Figure 3. Its working principle is as follows: This sensor is a closed loop Hall current sensor, uses the Hall device as the core sensitive element, the modular product for isolating and detecting the electric current, its working principle is the magnetic balance type of Hall (or call Hall magnetic Compensation type, Hall zero flux type). When a current flows through a long straight wire, a magnetic field is generated around the wire. The size of the magnetic field is proportional to the size of the current flowing through the wire. This magnetic field can be collected by a soft magnetic material and then detected by a Hall device. Since the change of the magnetic field has a good linear relationship with the output voltage signal of the Hall device, the measured output signal can directly reflect the current in the wire. In order to prevent interference, a 1μf decoupling filter capacitor is separately connected to the power supply terminal and the ground terminal of the Hall sensor.
The current detection circuit converts the three-phase stator current of the permanent magnet synchronous motor into the dsp through the sensor and converts it into a digital form and performs a series of transformations. Since this system is a three-phase balanced system: ia+ib+ic=0; therefore only Need to detect the two-phase current, you can get three-phase current. From the mathematical model of the permanent magnet synchronous motor, we can see that the stator current detection accuracy and real-time is the key to the accuracy of the entire vector control system, so the system uses lts25-np sensor to detect the current.
In this system, the current of phase a and phase b is detected by two lem modules. In actual debugging, since the current signal passing through the sensor has high-order ripple and other interference signals, a filter must be designed to suppress high-order chopping waves and other interference signals. Considering the actual situation, this paper designs a current detection circuit with a second order low-pass filter with voltage follow-up. The specific schematic is shown in Figure 4.

Under the switch mode control, the phase current signal contains higher harmonics that need to be filtered out. In the design, first use the pspice software to carry on the fictitious design to the filter [2 ], after confirming and verifying through the simulation, confirm and adopt the Butterworth filter structure of the second order, the system uses the electric current sensor to measure the electric current, have filtered, amplitude transform, zero deviation Move and clip, convert the voltage signal of 0~3v into the a/d pin of dsp.
The amplitude-frequency characteristics of the second-order Butterworth filter (shown in dashed box) in Figure 4 are shown in Figure 5. The frequency response curve in the passband is as flat as possible. The cut-off frequency is 300hz and the attenuation slope is -40db. /dec.

Experimental Results In the experimental system, the pwm frequency is 15khz, the dead time is 3μs, the current loop sampling period is 67μs, the speed loop sampling period is 0.67ms, the output loop of the speed loop is 1.5 times the rated current, and the current loop output Limiting is 1.2 times the rated voltage. Experimental control of an 8-pole permanent magnet synchronous motor motor, the parameters are: rated power: 1.88kw, rated speed: 2500r/min, rated current: 7.5a, rated torque: 7.5nm, rated voltage: 220v. The motor is at 10r/min, 200r/min, 1000r/min, 2000r/min and the speed regulator parameter is set to: kpv=0.5, kiv=0.02; the current regulator parameter is set to: kpi=0.2, kii=0.02 The start-stop speed curves are shown in Figures 6-9, respectively.

From the experimental waveforms shown in Figs. 6-9, it can be seen that when the motor is running at no-load, the system operates in the closed state of the speed current, and the steady state can be reached quickly. The overshoot and steady state errors are small. The experimental results show that the system Reasonable design, with good dynamic and static properties.
Conclusion In this system application, lem sensor can measure the motor current correctly and convert it into the corresponding output. All the performance indicators can meet the requirements of this system. It is a very good product. In summary, using the Hall current sensor (lem module) to sample the current, the linearity is good, the power consumption is small, the temperature stability is good, and the accuracy is generally higher, so it is an ideal current sensor. During the development of this system, we greatly thanked the engineers of the company for their thoughtful service and technical support. Hope that after the lem electronics company can provide more products and better technical support, together with China's high-end servo control industry to achieve greater development efforts.
Home > News > Application of Current Sensor Sampling in Servo Control System
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