Why current sensing is a must in collaborative, mobile robots

Article By : Kyle Stone

Current sensing plays an integral part in robotics systems for use cases such as motor-drive phase-current measurements, battery-management systems, and general peripheral monitoring.

Robots are increasingly common in manufacturing and warehousing facilities. Factories are expanding their use of mobile robots to help autonomously move items from point A to point B without human interaction, while also expanding their use of collaborative robots to enable more efficient work and reduce worker fatigue. Current sensing plays a critical role in mobile robots and collaborative robots to help realize these benefits.

Mobile robots typically operate on lithium-ion batteries between 48 V to 80 V on the main power rails and may experience high in-rush currents exceeding 150 A on the main rail. The secondary rails on mobile robots can utilize anywhere between 3.3 V to 80 V to power peripherals such as lighting, motors, vision systems, CPU, memory, and other relevant subsystems. Current levels on the secondary rails are typically much lower, in the range of tens of amperes.

On the other hand, collaborative robots typically operate between 24 V and 60 V. The current level within the system—specifically the current in electric motors—is usually around 20 A or less per node. Precision current measurements are much more important in collaborative robots since high accuracy provides tight system control to enable safe and efficient operation of the robot.

Current sensing plays an integral part in robotics systems for use cases such as motor-drive phase-current measurements, battery-management systems, and general peripheral monitoring.

Motor drives in mobile and collaborative robots

In motor-control applications, current-sense ICs now have a front-end that leverages a technology called enhanced pulse-width modulation (PWM) rejection. This technology minimizes the output error caused by switching common-mode voltage signals, which are very common with inline phase current measurements. As shown in Figure 1, it improves electrical characteristics such as offset, gain error and temperature drift, enabling benefits such as enhanced system performance and ultra-precise measurements.

Figure 1 PWM rejection improves electrical characteristics such as offset, gain error, and temperature drift. Source: Texas Instruments

Looking more closely at a motor drive, Figure 2 shows five potential locations for current-sensing ICs in three-phase motor systems within mobile or collaborative robots. Starting at the top left is a high-side DC link, which is phase-agnostic and monitors current loads in overall motor system as well as short-circuit conditions. The subsequent current-sensing implementation is on the high side of each phase, monitoring the current going into each phase of the motor. Monitoring each phase enables the system to better detect which phase may be operating incorrectly. For high-side measurements, the current-sensing ICs typically see the highest system voltage levels.

Figure 2 Here is a synopsis of motor current-sensing methods commonly employed in robotics systems. Source: Texas Instruments

Moving to the center of Figure 2 is inline current monitoring, which enables a closed-loop feedback system. The controller part can now control the system based on the in-phase current levels, which provides tighter control capabilities. The difficulty with inline motor current sensing is the switching common-mode signal; however, PWM rejection technology can help mitigate the error that a PWM signal could generate, in addition to sensing high common-mode voltages up to 110 V, similar to the high-side measurement. These features make it easier to implement these ICs in a system and increase overall efficiency by enabling tighter system control.

The last configurations in Figure 2 are the low-side phase and low-side DC link. The low-side measurements are typically carried out at lower voltage levels because the ICs are close to ground; these ICs can monitor the low-side current. Monitoring at the low side gives a holistic reading of current measurements in the system; it also offers lower levels of protection and control after the load. It’s possible to use one or more of these configurations in a motor system.

Point-of-load detection in mobile and collaborative robots

Figure 3 shows how a mobile robot system could monitor peripherals such as lighting, radar, processing systems, and other relevant subsystems. Typically, the power system provides DC power to the secondary rails and channels. Power is channeled into a DC/DC converter and then to a load switch, which connects and disconnects the load from the source to save energy and increase efficiency when the peripheral isn’t needed.

Figure 3 Here is a broad view of point-of-load current-sensing methods used in robotics systems. Source: Texas Instruments

When the switch is enabled, a current-sensing IC monitors the current and voltage coming through the switch and transmits voltage, current, power, and other important information back to the microcontroller through I2C. This data helps ensure the health of the system and peak efficiency. You could also use a current-sensing IC here, but it would require more hardware in most cases, such as an analog-to-digital converter (ADC) or a general-purpose input/output pin on the microcontroller. However, in specific instances, where you need fast overcurrent detection, a current-sensing IC has a 1-µs comparator.

Emerging safety trends in robots

The International Organization for Standardization (ISO) 3961-4 specifies the safety requirements for driverless mobile robots and their systems for warehouse robotics, while ISO 15066 specifies the safety requirements for collaborative industrial robot systems and their work environment. The ISO standards differ since a mobile robots’ ability to move around a warehouse or area with significantly more degrees of freedom could result in an increased probability of an incident with the robot.

Given the ISO standards, Automotive Electronics Council (AEC)-Q100 ICs can help ensure the highest ICs quality, and that the information those ICs are generating is reliable.

Leveraging current sensing in mobile or collaborative robot platforms can improve safety and efficiency, reduce worker fatigue, and help monitor system health. There are challenges in implementing current-sensing ICs 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 capabilities by enabling tight control and health monitoring. Current sensing is ever-expanding, and as technology continues to grow, the use of current sensing will become more and more relevant, since more electronics will need monitoring.


This article was originally published on EDN.

Kyle Stone is a product marketing engineer at Texas Instruments.


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