Industrial motion control encompasses a range of applications, including inverter-based fan or pump control, factory automation with more complex AC drive control, and advanced automation applications such as robots with advanced servo control. These systems require the monitoring of multiple variables, such as motor winding current or voltage, DC link current or voltage, rotor position, and speed. The choice of variables and the required measurement accuracy depend on end-application requirements, system architecture, target system cost, or system complexity. Other considerations include value-added features such as condition monitoring. Motors reportedly account for 40% of global energy consumption, and international regulations increasingly emphasize system efficiency across all industrial motion applications (see Figure 1).
Figure 1: Industrial Drive Application Map
Current and voltage sensing techniques in various motor control signal chain topologies differ depending on the motor's rated power, system performance requirements, and end application. For this reason, different sensor choices, current isolation requirements, analog-to-digital converter selections, system integration levels, and system power/ground partitioning lead to different implementations of the motor control signal chain. While isolation requirements typically have a significant impact on the final circuit topology and architecture, this paper focuses on improving current sensing (as a contributing factor) to achieve a more efficient motor control system.
Current and voltage measurement
Figure 2 shows a general motor control signal chain. Signal conditioning for high-fidelity measurements is not easy. Phase current detection is particularly difficult because the circuit node connected to this node is the same as the node output by the gate driver in the core of the inverter module, and therefore the requirements for isolation voltage and switching transients are also the same.
Figure 2: General Motor Control Signal Chain
The most commonly used current sensors in motor control are shunt resistors, Hall effect (HE) sensors, and current transformers (CTs). While shunt resistors lack isolation and introduce losses, they are the most linear, lowest-cost, and suitable for both AC and DC measurements. To limit signal level attenuation due to shunt resistor power losses, shunt applications are typically limited to 50A or lower. Current transformers and Hall effect sensors offer inherent isolation, making them suitable for higher current systems, but they are more expensive and less accurate than shunt resistor solutions due to inherently lower initial accuracy or temperature-dependent accuracy. Unlike sensor types, there are many options for motor current measurement nodes, as shown in Figure 3, with direct in-phase winding current measurement being ideal for high-performance systems.
Figure 3: Current feedback of isolated and non-isolated motors
There are many topologies available for detecting motor current, and various factors need to be considered, such as cost, power consumption, and performance levels. However, for most system designers, an important goal is to improve efficiency within a cost-controllable framework.
From Hall Effect Sensors to Shunt Resistors
A shunt resistor coupled to an isolated Σ-Δ modulator provides the best current feedback, with a sufficiently low current level. Currently, system designers are clearly shifting from Hall effect sensors to shunt resistors, and are increasingly favoring isolated modulator solutions over isolated amplifier solutions. System designers replacing Hall effect sensors with shunt resistors often opt for isolated amplifiers and continue using the analog-to-digital converter (ADC) previously used in Hall effect sensor-based designs. In this case, design performance is limited by the isolated amplifier, regardless of ADC performance.
Replacing isolated amplifiers and ADCs with isolated Σ-Δ modulators eliminates performance bottlenecks and significantly improves the design, typically boosting its high-quality feedback from 9 to 10 bits to 12 bits. Furthermore, the digital filters required to process the Σ-Δ modulator output can be configured to implement a fast overcurrent protection (OCP) loop, eliminating the need for analog overcurrent protection circuitry.
Existing Σ-Δ modulators offer a differential input range of ±250mV (±320mV full scale for OCP), making them particularly suitable for resistive shunt measurements. Analog modulators continuously sample the analog input, while the input information is contained within the digital output stream, with data rates up to 20MHz. The original information can be reconstructed using appropriate digital filters. Since conversion performance can be traded for bandwidth or filter group delay, coarser, faster filters can provide a fast OCP response of approximately 2μs, making them ideal for IGBT protection.
Reduce the size of the shunt resistor
From a signal measurement perspective, some major challenges relate to the selection of the shunt resistor, as a balance needs to be struck between sensitivity and power consumption. The nonlinearity caused by the resistor's own heating effect also presents a challenge when using larger resistors. Therefore, designers must make trade-offs, and more challengingly, they often need to select an appropriately sized shunt resistor to meet the requirements of various models and motors at different current levels. Maintaining dynamic range is also a challenge when dealing with peak currents several times the motor's rated current and reliably capturing both values.
Faced with these challenges, system designers desperately need superior Σ-Δ modulators with wider dynamic range or higher signal-to-noise ratio and signal-to-speech ratio (SINAD). The latest isolated Σ-Δ modulator products offer 16-bit resolution and ensure performance up to 12 effective bits (ENOB).
Figure 4: High-performance second-order Σ-Δ modulator AD7403
High-performance isolated Σ-Δ modulator
Higher-performance isolated Σ-Δ modulators can meet a variety of needs in industrial motor control designs and improve motor driver efficiency by reducing shunt resistor size. Analog Devices' AD7403 modulator is a good example of an industrial application (see Figure 4). It is a new generation of the AD7401A, offering a wider dynamic range at the same 20MHz external clock rate. This gives designers more flexibility in choosing shunt resistor sizes and allows them to replace Hall effect sensors with shunt resistors at higher current levels. The chip has a typical 14.2-bit ENOB. Furthermore, dynamic response can be improved by reducing measurement delay. The isolation scheme of this device supports a higher continuous operating voltage (VIORM) than its predecessor, thereby improving system efficiency by using a higher DC bus voltage and lower current.
System solutions using the ADSP-CM40x mixed-signal control processor
As mentioned earlier, implementing a Σ-Δ modulator requires the use of digital filters, typically implemented using FPGAs or ASICs. The ADSP-CM408F mixed-signal control processor from Analog Devices (ADI) will change this design approach because it incorporates Sinc filter hardware that can be directly connected to the modulator. This is expected to accelerate the adoption of current sensing techniques utilizing resistive shunts and Σ-Δ modulators.
