Robots are becoming increasingly common in manufacturing and warehousing facilities. Factories are expanding the use of mobile robots to help automate the movement of items from point A to point B without human intervention, while also increasing the use of collaborative robots to improve efficiency and reduce worker fatigue. Current sensing plays a crucial role in both mobile and collaborative robots, contributing to these advantages.
Mobile robots are typically powered by lithium-ion batteries ranging from 48V to 80V on the main power rail, where surge currents exceeding 150A can occur. The secondary power rail of a mobile robot can utilize any voltage between 3.3V and 80V to power peripherals such as lighting, motors, vision systems, CPUs, memory, and other related subsystems. Current levels on the secondary power rail are typically much lower, ranging from tens of amperes.
On the other hand, collaborative robots typically operate between 24 V and 60 V. The current level within the system (especially in the motors) is typically around 20 A per node or lower. Accurate current measurement is even more important for collaborative robots because high precision allows for tight system control, ensuring the robot operates safely and efficiently.
Current sensing plays an indispensable role in robot systems and can be used in applications such as motor drive phase current measurement, battery management systems, and general peripheral device monitoring.
Motor drives in mobile and collaborative robots
In motor control applications, current sensing ICs now feature a front-end that utilizes enhanced pulse width modulation (PWM) suppression technology. This technology minimizes output errors caused by switching common-mode voltage signals, which are very common in online phase current measurements. As shown in Figure 1, it improves electrical characteristics such as offset, gain error, and temperature drift, thereby achieving benefits such as enhanced system performance and ultra-precise measurement.
PWM suppression can improve electrical characteristics such as offset, gain error, and temperature drift.
Looking more closely at the motor driver, Figure 2 shows five potential locations for the current sensing IC in a three-phase motor system within a mobile or collaborative robot. Starting from the top left is the high-side DC link, which is phase-independent and monitors the current load and short-circuit conditions throughout the motor system. Subsequent current sensing implementations are located on the high side of each phase, monitoring the current flowing into each phase of the motor. Monitoring each phase allows the system to better detect which phase might be malfunctioning. For high-side measurements, the current sensing IC typically sees the highest system voltage level.
This is an overview of commonly used motor current sensing methods in robot systems.
The core is online current monitoring, which supports a closed-loop feedback system. The controller section can now control the system based on the in-phase current level, thus providing more stringent control capabilities. A challenge in online motor current sensing lies in switching the common-mode signal; however, PWM suppression technology helps mitigate errors that may arise from the PWM signal and can also sense high common-mode voltages up to 110 V, similar to high-side measurements. These features make it easier to implement these ICs in the system and improve overall efficiency by achieving more stringent system control.
The final configuration is a low-side phase and a low-side DC link. Low-side measurements are typically performed at lower voltage levels because the ICs are close to ground; these ICs can monitor low-side current. Low-side monitoring provides a comprehensive view of current measurements in the system; it also provides a lower level of protection and control after the load. One or more of these configurations can be used in a motor system.
Load point detection in mobile and collaborative robots
How does a mobile robot system monitor peripheral devices, such as lighting, radar, processing systems, and other related subsystems? Typically, the power system provides DC power to the secondary tracks and channels. The power is directed to a DC/DC converter and then to load switches, which connect and disconnect the power supply when the peripheral devices are not needed, saving energy and improving efficiency.
This is an overview of the point-of-load current sensing methods used in robotic systems.
When the switch is enabled, the current sensing IC monitors the current and voltage across the switch and transmits voltage, current, power, and other critical information back to the microcontroller via I2C. This data helps ensure system health and peak efficiency. You can also use a current sensing IC here, but in most cases, it requires more hardware, such as an analog-to-digital converter (ADC) or general-purpose input/output pins on the microcontroller. However, in specific situations requiring fast overcurrent detection, the current sensing IC has a 1µs comparator.
Emerging Safety Trends in Robots
The International Organization for Standardization (ISO) 3961-4 specifies the safety requirements for unmanned mobile robots and their warehouse robot systems, while ISO 15066 specifies the safety requirements for collaborative industrial robot systems and their working environments. The ISO standards differ because mobile robots can move freely in warehouses or other areas, which increases the likelihood of robot accidents.
Based on ISO standards, the Automotive Electronics Council (AEC)-Q100 ICs can help ensure the highest IC quality and the reliability of the information generated by these ICs.
Utilizing current sensing in mobile or collaborative robot platforms can improve safety and efficiency, reduce worker fatigue, and facilitate monitoring of system health. Implementing current sensing ICs presents challenges such as size, but small outline transistor (SOT)-23 or SC-70 packages can help minimize size constraints.
Using current sensing ICs can help designers add enhanced functionality by enabling stringent controls and health monitoring. The scope of current sensing continues to expand, and as technology advances, its use will become increasingly important as more and more electronic devices require monitoring.
