Why is current sensing essential in mobile and collaborative robots?

Why is current sensing essential in mobile and collaborative robots?

Robots are increasingly deployed in manufacturing and warehousing facilities. Factories are expanding their use of mobile robots to help move items autonomously 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 stress. Current sensing plays an important role in mobile robots and collaborative robots to help realize these benefits.

Mobile robots typically run on lithium-ion batteries between 48V to 80V on the mains power rails and may experience high surge currents in excess of 150A on the mains rail. Secondary rails on mobile robots can use anywhere between 3.3V to 80V to power peripheral devices such as lighting, motors, vision systems, CPU, memory, and other related subsystems. Current levels on the secondary busbars are usually much lower, in the tens of amps range.

On the other hand, collaborative robots usually operate between 24V and 60V. The current level within the system – specifically the current in the electric motors – is usually about 20 amps or less per node. Accurate current measurements are even more important in collaborative robots because the higher accuracy provides tight system control to enable safe and efficient robot operation.

Current sensing plays an essential role in robotics systems for use cases such as motor phase current measurements, battery management systems, and general terminal monitoring.

Motor drives in mobile and collaborative robots

In motor control applications, current-sensing integrated circuits now have a front-end that takes advantage of a technique called enhanced pulse-width modulation (PWM) rejection. This technology reduces the output error caused by switching common-mode voltage signals, which are so common with current inline phase measurements. As described in Figure 1it improves electrical properties such as displacement, error, and temperature deviation, enabling advantages such as improved system performance and ultra-accurate measurements.

Figure 1 PWM rejection improves electrical properties such as offset, gain error, and temperature deviation. source: Texas Instruments Inc

Looking closely at the motor of cars, Figure 2 Shows five potential sites for current-sensing integrated circuits in three-phase motor systems within mobile or collaborative robots. Starting at the top left is a high sided DC link, which is phase-aware and monitors current loads in the total motor system as well as short circuit conditions. Subsequent implementation of current sensing is on the upstream side of each phase, and monitors the current passing through each phase of the motor. Monitoring each phase enables the system to better detect which phase may be operating incorrectly. For high-side measurements, current-sensing ICs will usually see the highest levels of system voltages.

Figure 2 Below is a summary of the kinetic current sensing methods commonly used in robotics systems. Source: Texas Instruments

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

The last configurations in Figure 2 are low-side phase and low-side DC coupling. Low side measurements are usually made at lower voltage levels because the integrated circuits are close to ground; These integrated circuits can monitor the low side current. The monitor on the underside gives a comprehensive readout of the current measurements in the system; It also provides lower levels of protection and control after loading. It is possible to use one or more of these configurations in an engine system.

Mount point discovery in mobile and collaborative bots

Figure 3 Shows how a mobile robot system can monitor peripheral devices such as lighting, radar, processing systems, and other related subsystems. Typically, the power system supplies DC power to the secondary busbars and conduits. Power is routed to a DC/DC converter and then to a load switch, which connects and disconnects the load from the source to save power and increase efficiency when the peripheral is not needed.

Figure 3 Below is an overview of load point current sensing methods used in robotics systems. Source: Texas Instruments

When the switch is enabled, the current sensing IC monitors the current and voltage coming through the switch and transmits the voltage, current, power, and other important information to the microcontroller via I2c. This data helps ensure system integrity and peak efficiency. You could also use a current sensing IC here, but it will require more hardware in most cases, such as an analog-to-digital converter (ADC) or a general-purpose I/O pin on the microcontroller. However, in specific cases, where you need fast over-current detection, a current-sensing integrated circuit has a 1-s comparator.

Emerging safety trends in robotics

The International Organization for Standardization (ISO) 3961-4 specifies safety requirements for driverless mobile robots and systems for warehouse robots, while ISO 15066 specifies safety requirements for collaborative industrial robot systems and their working environment. ISO standards differ because the ability of mobile robots to move around a warehouse or area with a greater degree of freedom can lead to an increased likelihood of an accident with the robot.

Due to ISO standards, the Automotive Electronics Council (AEC) -Q100 integrated circuits can help ensure that the integrated circuits are of the highest quality, and that the information generated by these integrated circuits 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 transistor (SOT)-23 or SC-70 packages can help reduce size limitations.

The use of current-sensing integrated circuits can help designers add improved capabilities by enabling strict control and health monitoring. Current sensing is constantly expanding, and as the technology continues to grow, the use of current sensing will become more important, because more electronics will need to be monitored.

Kyle Stone is a Product Marketing Engineer at Texas Instruments.

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