Measurement technology in industrial motion control

Industrial motion control covers a range of applications, including inverter-based fan or pump control, factory automation with more complex AC drive control, and automation applications (such as robots with servo control). These systems need to detect and feedback multiple variables, such as motor winding current or voltage, DC link current or voltage, rotor position and speed. Among consideration factors such as value-added functions (such as status monitoring), terminal application requirements, system architecture, target system cost or system complexity will determine the choice of variables and the required measurement accuracy. According to reports, motors account for 40% of the total energy consumption, and international regulations are increasingly focusing on the system efficiency of the entire industrial motion application. Therefore, these variables are becoming more and more important, especially current and voltage.

This article will focus on current and voltage detection in various motor control signal chain topologies based on motor rated power, system performance requirements and end applications. In this case, the implementation of the motor control signal chain will vary depending on sensor selection, galvanic isolation requirements, analog-to-digital converter (ADC) selection, system integration, and system power consumption and grounding division.



Figure 1. Industrial drive application map

Industrial drive application map

From simple inverters to complex servo drives, motor control applications cover a range of motor types, but all motors include motor control systems with specific power levels, as well as different levels of detection and feedback, which can drive pulse width modulators ( PWM) module processor. Figure 1 is a simplified diagram of the application map, showing various systems that gradually increase in complexity from left to right. The first is a simple control system, such as pumps, fans, and compressors that can be achieved using simple microprocessors without precise feedback. machine. As the complexity of the system increases (that is, to the higher end of the graph), complex control systems require feedback and high-speed communication interfaces. Examples include vector-controlled induction motors or permanent magnet motors with or without sensors, and high-power industrial drives (such as large pumps, fans, and compressors) designed for the efficiency shown in Figure 1. At the end of the map are complex servo drives for applications such as robots, machine tools, and placement machines. As the complexity of the system increases, the detection and feedback of variables becomes more and more critical.

drive architecture system partition

We may encounter various problems when designing systems that meet the needs of various industrial motion control applications. The general motor control signal chain is shown in Figure 2.



Figure 2. General Motor Control Signal Chain

Isolation requirements are very important and usually have a significant impact on the resulting circuit topology and architecture. Two key factors need to be considered: the reason and location of the isolation.

The requirements for isolation classification depend on the former. High-voltage safety isolation (SELV) may be required to prevent electric shock, or functional isolation for level conversion between non-lethal voltages, or isolation for data integrity and noise elimination. The isolation location is usually determined by the expected performance of the system. Motor control is usually carried out in a harsh environment with full charge and noise. The design used usually needs to withstand hundreds of volts of common mode voltage, may switch at frequencies exceeding 20 kHz, and has extremely high transient dv/dt rise time. For this reason, high-performance systems and high-power systems with high inherent noise are usually designed to have a power stage isolated from the control stage. Whether it is a single-processor or dual-processor design will affect the isolation position. In low-performance low-power systems, isolation is usually performed on the digital communication interface, which means that the power stage and the control stage are at the same potential. Low-end systems need to isolate the communication interface bandwidth is low. Because high-end systems require higher bandwidth and traditional isolation technologies have limitations, it is often difficult to isolate the communication ports of high-end systems. But with the advent of magnetically isolated CAN and RS-485 transceiver products, the situation is changing.

In the high-performance closed-loop motor control design, the two key components are the PWM modulator output and the motor phase current feedback. Figure 3a and Figure 3b show the location where safety isolation is required, depending on whether the control stage shares the same potential with the power stage or is grounded. In any case, the high-side gate driver and the current detection node need to be isolated, but the isolation level in Figure 3a is different, these nodes only need to be functionally isolated, and in Figure 3b, the personnel of these nodes are safely isolated (that is, galvanic isolation). ) Is essential.



Figure 3a. Control level based on power level



Figure 3b. Control level based on grounding

Current and voltage detection measurement technology and topology

In addition to the system power and ground division described above, the signal chain implemented to detect current and voltage will also vary due to sensor selection, galvanic isolation requirements, ADC selection, and system integration. Signal conditioning for high-fidelity measurements is not easy. For example, recovering small signals or transmitting digital signals in such a noisy environment is very challenging, while isolating analog signals is a greater challenge. In many cases, the signal isolation circuit will cause phase delay and limit the dynamic performance of the system. Phase current detection is particularly difficult because the circuit node connected to this node is the same as the output node of the gate driver in the power stage (inverter module), so the requirements for isolating power and switching transients are also the same. The measurement signal chain (technology, signal conditioning and ADC) that needs to be implemented in the motor control system is usually determined based on the following three key factors:

1. The point or node in the system that determines the measurement demand.

2. The motor power level and the final selected sensor (whether it has an isolation function). The choice of sensor largely influences the choice of ADC, including converter architecture, functionality, and analog input range.

3 Terminal application. This can drive the need for high resolution, accuracy, or speed in the detection signal chain. For example, realizing control without sensors in a larger speed range requires more, more frequent, and more measurements. The end application will also affect the requirements for ADC functionality. For example, multi-axis control may require an ADC with a higher number of channels.

Current and voltage sensor

Commonly used current sensors in motor control are shunt resistors, Hall effect (HE) sensors, and current transformers (CT). Although shunt resistors have an isolation function and will have losses when the current is high, they are linear, cost-effective and suitable for AC and DC measurements among all sensors. The signal level attenuation to limit shunt power loss usually limits shunt applications to 50 A or less. CT sensors and HE sensors provide inherent isolation, so they can be used in higher current systems. However, their cost is higher, and solutions using such sensors are not as accurate as solutions using shunt resistors, because such sensors themselves have poor initial accuracy or poor temperature accuracy.

Motor current measurement location and topology

In addition to sensor types, there are many optional motor current measurement nodes. The average DC link current can meet the control requirements, but in more drives, the motor winding current is used as the main feedback variable. Direct phase winding current measurement is an ideal choice and can be used in high-performance systems. However, using a shunt on each low-level inverter pin or a single shunt in the DC link can indirectly measure the winding current. The advantage of these methods is that the shunt signals are all based on the shared power supply, but extracting the winding current from the DC link requires sampling to be synchronized with the PWM switch. Direct phase winding current measurement can be performed using any of the above current detection techniques, but the shunt resistance signal must be isolated. High common-mode amplifiers can provide functional isolation, but personnel safety isolation must be provided by isolated amplifiers or isolated modulators.



Figure 4. Isolated and non-isolated motor current feedback

Figure 4 shows the various current feedback options mentioned above. Although you only need to select one of them for control feedback, the DC link current signal can also be used as a backup signal for protection.

As mentioned earlier, the system power and grounding division will determine the required isolation classification, and thereby determine the applicable feedback. The target performance of the system also affects the sensor selection or measurement technique. Throughout the entire performance map, many configurations are also possible.

low performance example: power level and control level on the common potential, detection option A or B

The use of pin shunt is an economical technique for measuring motor current. In this example, the power stage and the control stage share the same potential, there is no common mode to be processed, and the output of Option A or Option B can be directly connected to the signal conditioning circuit and ADC. This type of topology is common in low-power and low-performance systems with ADCs embedded in microprocessors.

High performance example: control level grounding, detection options C, D or E

In this case, personnel safety isolation is required. Test options C, D, and E are all possible. Among all three options, option E provides qualitative current feedback, and as a high-performance system, FPGA or other forms of processing may exist in the system, which can provide a digital filter suitable for isolating the modulator signal. For the ADC selection of Option C, a discrete isolated sensor (probably a closed-loop HE) is usually used to achieve higher performance than the current embedded ADC products. Compared with the common mode amplifier, the option D in this configuration is an isolated amplifier because of the need for safe isolation. Isolated amplifiers will limit performance, so embedded ADC solutions can meet the needs. Compared with option C or E, this option provides fidelity current feedback. In addition, although the embedded ADC can be regarded as "" and the isolated amplifier as "cheap", the implementation usually requires additional components for offset compensation and level conversion to match the ADC input range, thereby improving The overall cost of the signal chain.

In the motor control design, many topologies can be used to detect the motor current, and a variety of factors need to be considered, such as cost, power level, and performance level. The important goal of most system designers is to improve current sensing feedback to increase efficiency within their cost targets. For higher-end applications, current feedback is not only important for efficiency, but also for other system performance measurements (such as dynamic response, noise, or torque ripple). Obviously, in various available topologies, there is a continuum of performance from low to high. Figure 5 is a rough map showing low-power and high-power options.



Figure 5. Current detection topology performance map

The goal, demand and development trend of motor control system designers: from HE sensor to shunt resistor

The shunt resistor coupled with the isolated-modulator can provide qualitative current feedback, in which the current level is low enough to fully meet the shunt demand. At present, system designers are obviously inclined to switch from HE sensors to shunt resistors, and compared with isolated amplifier solutions, designers also tend to adopt isolated modulator solutions. Simply replacing the sensor itself can reduce the bill of materials (BOM) and PCB assembly costs and improve the accuracy of the sensor. Shunt resistors are not sensitive to magnetic fields or mechanical vibrations. System designers who replace HE sensors with shunt resistors often choose isolated amplifiers and continue to use ADCs previously used in HE sensor-based designs to limit level changes in the signal chain. However, as mentioned earlier, regardless of ADC performance, the performance will be limited by the performance of the isolated amplifier.

And further replacing isolated amplifiers and ADCs with isolated-modulators can eliminate performance bottlenecks and greatly improve the design, usually from 9 to 10 bits of high-quality feedback to 12 bits. In addition, digital filters required for processing-modulator output can be configured to implement fast OCP loops, thereby eliminating analog overcurrent protection (OCP) circuits. Therefore, any BOM analysis should include not only the isolated amplifier, the original ADC, and the signal conditioning between the two, but also the OCP equipment that can be eliminated. The AD701A isolated-modulator is based on ADI's iCoupler technology and has a differential input range of ±250 mV (usually ±320 mV full scale for OCP). It is particularly suitable for resistive shunt measurement and is an ideal product choice to expand this trend. . The analog modulator continuously samples the analog input, and the input information is contained in the digital output stream in the form of data stream density, with a data rate of up to 20 MHz. The original information can be reconstructed by an appropriate digital filter (usually a Sinc3 filter suitable for precision current measurement). Since a trade-off can be made between conversion performance and bandwidth or filter group delay, a simpler and faster filter can provide a fast OCP response in the order of 2μs, which is very suitable for IGBT protection.

The need to reduce the size of shunt resistors

From the perspective of signal measurement, some of the current main problems are related to the selection of shunt resistors because of the need to achieve a balance between sensitivity and power consumption. A large resistance value will ensure the use of the modulator's entire or as large analog input range as possible to obtain the dynamic range. However, due to the loss of I2×R in the resistance, a large resistance value will also cause a voltage drop and a decrease in efficiency. The non-linearity caused by the heating effect of the resistor itself is also a challenge for using larger resistors. Therefore, system designers are faced with left-right trade-offs and further deteriorating consequences. They often need to select a shunt resistor of an appropriate size to meet the needs of various models and motors at different current levels. If you are faced with a peak current that is several times the rated current of the motor, and you need to reliably capture the values of both, maintaining the dynamic range is also a problem. The ability to control the peak current of the system at startup will vary greatly depending on the design, from a strict control such as 30% floating above the rated current to a factor of up to 10 times the rated current. Acceleration and changes in load or torque also produce peak currents. However, the peak current in the system is usually within 4 times the rated current of the drive design.

Faced with these problems, system designers are looking for a high-performance modulator with a wider dynamic range or a higher signal-to-noise ratio and signal-to-noise ratio (SINAD). The isolated-modulator products have 16-bit resolution and can ensure up to 12 effective number of bits (ENOB) performance.

SINAD = (6.02 N + 1.76) dB, where N = ENOB

In line with the trend of using shunt resistors in low-power drives, motor drive manufacturers are also trying to increase the rated power of drives that can utilize this topology for performance and cost considerations. A feasible method is to use a shunt resistor with a smaller resistance value, and this requires the introduction of a higher performance modulator core to identify the reduced signal amplitude.

System designers (especially servo designers) are still constantly exploring, trying to improve the system response by shortening the analog-to-digital conversion time, or reducing the group delay by using digital filters related to the isolation-modulator and shunt resistor topology. As mentioned earlier, a trade-off can be made between conversion performance and bandwidth or filter group delay. A simpler, faster filter can provide a faster response, but will reduce performance. The system designer analyzes the effect of filter wavelength or decimation ratio, and then according to its
The end application needs to make trade-offs. Increasing the clock rate of the modulator can help, but many designers have implemented operation at the 20 MHz clock rate supported by the AD7401A. One disadvantage of increasing the clock rate is the radiation potential and interference (EMI) effects. Under the same clock rate, a higher-performance modulator can improve the trade-off relationship between group delay and performance, thereby achieving faster response time with less performance impact.

Industry performance isolated-modulator

Obviously, by reducing the size of the shunt resistance, improving the sensorless control scheme, and realizing the control of the high-efficiency internal permanent magnet motor (IPM), the higher-performance isolated-modulator can meet the various needs and development requirements in the design of industrial motors , And can improve the efficiency of the motor driver. ADI's AD7403 product is a new generation of AD7401A, which can provide a wider dynamic range at the same 20 MHz external clock rate. This allows designers to more flexibly choose the size of the shunt resistor, optimize the matching of the driver and the motor, improve the measurement accuracy of rated current and peak current, reduce the impact of the size of a single shunt resistor applicable to a series of motor models, and be able to more Use shunt resistors to replace HE sensors at high current levels. In addition, the dynamic response can be improved by shortening the measurement delay. Compared with the previous generation AD7400A and AD7401A, the isolation scheme of AD7403 can also use higher continuous operating voltage (VIORM), which can improve system efficiency by using higher DC bus voltage and lower motor current.

Broader system solutions including ADSP-CM40x mixed signal control processor

As mentioned earlier, the implementation-modulator requires a digital filter in the system. Usually can use FPGA or digital ASIC to realize. The emergence of the ADSP-CM408F mixed signal control processor (including Sinc3 filter hardware, which can be directly connected to the isolated-modulator of the AD740x series) may accelerate the popularization of the current detection technology of the resistive shunt coupled with the isolated-modulator. As described in this article, designers used to believe that resistive shunt current detection technology is more expensive due to the increased complexity of the digital domain system and the associated (FPGA) cost. ADSP-CM408F is a cost-effective solution that allows many designers who were previously limited by cost targets to consider using this technology.

Author: Nicola O'Byrne

ADI company, application engineer