How to choose measurement technology for industrial motion control? You need to understand these questions

This article will focus on current and voltage sensing in various motor control signal chain topologies based on motor power rating, system performance requirements, and end applications. In this context, the implementation of the motor control signal chain varies due to sensor selection, galvanic isolation requirements, analog-to-digital converter (ADC) selection, system integration, and system power and ground partitioning.

Motor control applications cover a range of motor types, from simple inverters to complex servo drives, but all motors include a motor control system for a specific power level, and a processor that drives a pulse width modulator (PWM) module with varying levels of sensing and feedback. Figure 1 is a simplified diagram of the application spectrum, showing various systems with increasing complexity from left to right, starting with simple control systems such as pumps, fans, and compressors that can be implemented using simple microprocessors without the need for sophisticated feedback.

As system complexity increases (i.e., moving toward the higher end of the spectrum), complex control systems require precise feedback and high-speed communication interfaces. Examples include sensored or sensorless vector-controlled induction motors or permanent magnet motors, and high-power industrial drives (such as large pumps, fans, and compressors) designed for efficiency as shown in Figure 1. At the highest end of the spectrum are complex servo drives for applications such as robotics, machine tools, and placement machines. As system complexity increases, the detection and feedback of variables becomes increasingly critical.

Figure 1. Industrial drive application map

We may encounter various problems when designing a system to meet the needs of various industrial motion control applications. A general motor control signal chain is shown in Figure 2.

Figure 2. Generic motor control signal chain

Isolation requirements are very important and often have a significant impact on the resulting circuit topology and architecture. There are two key factors to consider: why and where to isolate.

The isolation classification requirement depends on the former. Safety isolation of high voltage (SELV) may be required for protection against electric shock, or functional isolation for level translation between non-lethal voltages, or isolation for data integrity and noise rejection. The isolation location is usually determined by the expected performance of the system. Motor control is often performed in a harsh environment full of electrical noise, and the designs used are often required to withstand hundreds of volts of common mode voltages, may switch at frequencies in excess of 20kHz, and have very high transient dv/dt rise times.

For this reason, higher performance systems and inherently noisier high power systems are often designed with the power stage isolated from the control stage. Whether a single or dual processor design is used affects the location of the isolation. In lower performance, low power systems, isolation is often done at the digital communication interface, which means the power stage and control stage are at the same potential. Low-end systems have lower bandwidth communication interfaces that need to be isolated. Isolating the communication ports of high-end systems is often difficult due to the higher bandwidth requirements and the limitations of traditional isolation technology. But things are changing with the advent of magnetically isolated CAN and RS-485 transceiver products.

Two key elements in high performance closed-loop motor control designs are the PWM modulator output and the motor phase current feedback. Figures 3a and 3b show where safety isolation is needed, depending on whether the control stage shares the same potential as the power stage or is referenced to ground. In either case, the high-side gate driver and current sense nodes require isolation, but the isolation levels are different in Figure 3a, where only functional isolation is required, while in Figure 3b, human safety isolation (i.e., galvanic isolation) of these nodes is critical.

In addition to the system power and ground partitioning described above, the signal chain implemented to sense current and voltage will also vary depending on sensor selection, galvanic isolation requirements, ADC selection, and system integration. Signal conditioning for high-fidelity measurements is not an easy task. For example, recovering small signals or transmitting digital signals in such a noisy environment is very challenging, and isolating analog signals is an even greater challenge. In many cases, the signal isolation circuitry introduces phase delays that limit the system's dynamic performance. Phase current sensing is particularly difficult because this node is connected to the same circuit node as the gate driver output in the core of the power stage (inverter module), and therefore has the same needs in terms of isolating power supplies and switching transients.

The most commonly used current sensors in motor control are shunt resistors, Hall Effect (HE) sensors, and current transformers (CT). Although shunt resistors have isolation and suffer losses at higher currents, they are the most linear, lowest cost, and suitable for both AC and DC measurements. Signal level attenuation to limit shunt power loss typically limits shunt applications to 50A or less. CT sensors and HE sensors provide inherent isolation, allowing them to be used in higher current systems. However, they are more expensive, and solutions using these sensors are not as accurate as solutions using shunt resistors due to their inherent poor initial accuracy or poor accuracy over temperature.

In addition to the sensor type, there are many options for motor current measurement nodes. The average DC link current is sufficient for control needs, but in more advanced drives, the motor winding current is used as the primary feedback variable. Direct phase winding current measurement is ideal and can be used in high-performance systems. However, winding current can be measured indirectly using shunts on each low-side inverter leg or using a single shunt in the DC link. The advantage of these methods is that the shunt signals are all referenced to a common supply, but extracting the winding current from the DC link requires sampling to be synchronized with the PWM switching. Direct phase winding current measurement can be made using any of the above current sensing techniques, but the shunt resistor 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 shows the various current feedback options described above. Although only one of them is required for control feedback, the DC link current signal can also be used as a backup signal for protection.

Figure 4. Isolated and non-isolated motor current feedback

As mentioned previously, the system power and ground partitioning will determine the isolation classification required and therefore the applicable feedback. The target performance of the system will also affect the sensor selection or measurement technology. Looking across the performance spectrum, many configurations are possible.

Power and control stages on common potential, detection option A or B

Using pin shunts is one of the most cost-effective techniques for measuring motor current. In this case, the power stage shares the same potential as the control stage, there is no common mode to deal with, and the output of Option A or Option B can be directly connected to the signal conditioning circuitry and ADC. This type of topology is common in low-power and low-performance systems with an ADC embedded in a microprocessor.

Control stage ground, detection option C, D or E

In this case, personnel safety isolation is required. Sensing options C, D, and E are all possible. Option E provides the best quality current feedback of all three options, and as a high performance system, there may be an FPGA or other form of processing in the system to provide a digital filter for the isolated modulator signal. For the ADC selection of option C, a discrete isolated sensor (most likely a closed loop HE) is typically used to achieve higher performance than what can be achieved with current embedded ADC products. Option D in this configuration is an isolated amplifier compared to a common mode amplifier because safety isolation is required. An isolated amplifier will limit performance, so an embedded ADC solution will suffice. This option provides the lowest fidelity current feedback compared to options C or E. In addition, while the embedded ADC can be considered "free" and the isolated amplifier can be considered "cheap", the implementation usually requires additional components for offset compensation and level shifting for ADC input range matching, which increases the overall cost of the signal chain.

In motor control designs, many topologies can be used to sense motor current, and there are many factors to consider, such as cost, power level, and performance level. An important goal for most system designers is to improve the current sensing feedback to improve efficiency within their cost targets. For higher-end applications, current feedback is critical not only for efficiency, but also for other system performance measures such as dynamic response, noise, or torque ripple. Obviously, there is a continuum of performance from low to high in the various topologies available, and Figure 5 is a rough mapping diagram showing low-power and high-power options.

Figure 5. Current sensing topology performance spectrum

Shunt resistors coupled with isolated Σ-Δ modulators provide the best quality current feedback, where the current level is low enough to fully satisfy the shunt requirements. Currently, there is a clear trend among system designers to switch from HE sensors to shunt resistors, and designers are also favoring isolated modulator solutions compared to isolated amplifier solutions. Simply changing 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 the ADC previously used in HE sensor-based designs to limit the level changes in the signal chain. However, as mentioned earlier, no matter how good the ADC performance is, the performance will be limited by the performance of the isolated amplifier.

Furthermore, replacing the isolated amplifier and ADC with an isolated Σ-Δ modulator eliminates performance bottlenecks and greatly improves the design, typically taking it from 9 to 10 bits of good quality feedback to 12 bits. In addition, the digital filter required to process the Σ-Δ modulator output can be configured to implement a fast OCP loop, eliminating the analog overcurrent protection (OCP) circuit. Therefore, any BOM analysis should include not only the isolated amplifier, the original ADC, the signal conditioning between the two, but also the OCP devices that can be eliminated.

The AD7401A isolated Σ-Δ modulator is based on ADI's iCoupler® technology and has a differential input range of ±250mV (typically ±320mV full scale for OCP), which 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 a data stream density, with a data rate of up to 20MHz. The original information can be reconstructed through 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 on the order of 2s, which is very suitable for IGBT protection.

From a signal measurement perspective, some of the main challenges today are related to the selection of shunt resistors, as a balance needs to be struck between sensitivity and power consumption. Large resistor values ​​will ensure that the entire or as large an analog input range of the Σ-Δ modulator is used, thereby maximizing the dynamic range. However, large resistor values ​​also result in voltage drops and reduced efficiency due to the I2×R losses in the resistor. Nonlinearities caused by the heating effect of the resistor itself can also be a challenge with larger resistors.

Therefore, system designers are faced with a trade-off and further aggravation, often needing to select an appropriately sized shunt resistor to meet the needs of various models and motors at different current levels. Maintaining dynamic range is also a challenge when faced with peak currents that are several times the rated current of the motor and need to reliably capture the values ​​of both. The ability to control the system's turn-on peak current can vary greatly from design to design, from tightly controlled fluctuations of, say, 30% above the rated current, to factors as high as 10 times the rated current. Acceleration and load or torque changes can also generate peak currents. However, peak currents in a system are typically in the range of 4 times the rated current that the drive is designed for.

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

Following the trend of using shunt resistors in low-power drives, motor drive manufacturers are also looking to increase the power rating of drives that can utilize this topology for performance and cost reasons. The only way to do this is to use smaller shunt resistors, which requires the introduction of higher performance modulator cores to discern the reduced signal amplitude.

System designers, especially servo designers, continue to explore ways to improve system response by reducing analog-to-digital conversion time or by reducing group delay using digital filters associated with isolated ∆-modulator and shunt resistor topologies. As mentioned earlier, there is a trade-off between conversion performance and bandwidth or filter group delay. A simpler, faster filter provides faster response but reduces performance. System designers analyze the effect of filter wavelength or decimation ratio and then make trade-offs based on their end application needs. Increasing the clock rate of the modulator can help, but many designers have achieved operation at the maximum clock rate of 20MHz supported by the AD7401A. One disadvantage of increasing the clock rate is the potential for radiation and interference (EMI) effects. At the same clock rate, a higher performance modulator can improve the trade-off between group delay and performance, thereby achieving faster response time with less performance degradation.

Clearly, higher performance isolated Σ-Δ modulators can meet the needs and developments in industrial motor design and improve the power efficiency of motor drives by reducing the size of shunt resistors, improving sensorless control schemes, and enabling control of high-efficiency interior permanent magnet motors (IPMs).

Analog Devices' AD7403 is a new generation of the AD7401A that provides a wider dynamic range at the same 20 MHz external clock rate. This gives designers more flexibility in selecting the shunt resistor size, optimizing the matching of the drive to the motor, improving the measurement accuracy of the rated current and peak current, reducing the impact of a single shunt resistor size for a range of motor models, and being able to replace HE sensors with shunt resistors at higher current levels. In addition, dynamic response can be improved by shortening the measurement delay. Compared to the previous generation AD7400A and AD7401A, the AD7403's isolation scheme can also use a higher continuous operating voltage (VIORM), which can improve system efficiency by using a higher DC bus voltage and lower motor current.

As mentioned previously, implementing a sigma-delta modulator requires a digital filter in the system. This is typically implemented using an FPGA or digital ASIC. The advent of the ADSP-CM408F mixed-signal control processor, which includes Sinc3 filter hardware to interface directly to the AD740x family of isolated sigma-delta modulators, has the potential to accelerate the adoption of resistive shunt current sensing coupled to isolated sigma-delta modulators. As discussed in this article, resistive shunt current sensing has historically been considered expensive by designers due to the increased complexity of the digital domain system and the associated (FPGA) cost. The ADSP-CM408F is a cost-effective solution that will allow many designers who were previously constrained by cost targets to consider this technology .

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