Motor drive engineering for Cobot: reliability and efficiency
Industrial robots have brought programmable automation to manufacturing plants and to warehousing, sorting and packaging areas, as companies seek to increase productivity and reduce operating overhead.
More recently, collaborative robots (cobots) have arrived and created a new world of application opportunities. Designed to work safely alongside humans, providing the unique ability to act as collaborators that contribute to the processes that must be completed to achieve results.
This friendly image of the robot as a co-worker – in contrast to the unapproachable, imprisoned superhumans used for dangerous jobs such as heavy lifting, welding and spray painting – encourages the adoption of cobots in a broader range of tasks. It raises the exciting prospect of combining human qualities such as natural dexterity and visual acuity, combined with the superior speed, repeatability and strength of the robot.
Cobots are also supposed to have a greater advantage in reliability than humans, as they do not tend to get sick, be late, or lose concentration while working. As important as performance, efficiency and safety, reliability needs to be designed to ensure that the cobot will provide the entrepreneur with the expected return on investment (ROI).
Design requirements for the motor controller
Figure 1. Power, communication, detection and driving subsystems of the cobot.
Figure 1 describes the underlying electrical architecture of a typical industrial cobot. The motors and motor controller circuitry that move each of the cobot's joints must be able to withstand the necessary loads and movements without stressing or overheating. Motors that offer a high torque-to-weight ratio and small dimensions in relation to the demands of each joint are usually preferred to allow a lightweight and compact robot design that can be safe and easily navigated around people.
However, these requirements place heavy demands on the motor controller circuitry, where the power electronics are located. The module size should be as small as possible, consistent with size and light weight objectives. Size restrictions limit thermal performance, which increases stress on power semiconductor components. Frequent starts and stops, as well as emergency stops when safety sensors detect the proximity of a human companion, generate heat within the power semiconductors that must be dissipated to preserve the device.
Cobot applications typically operate with a bus voltage lower than 60V, meaning the motor driver power stage is driven by MOSFETs in a three-phase inverter topology. High power efficiency is certainly an important consideration in today's markets, which is why MOSFETs with low resistance are desirable. On the other hand, high availability and uptime are critical requirements that directly impact productivity and return on investment, so it is important to choose devices designed with high robustness and reliability.
When driving high-power motors, MOSFETs are vulnerable to failures caused by rotor locks, parasitic effects, and voltage drops that can expose devices to excessive currents and long switching times. To resist these threats, the chosen device needs a strong Safe Operating Zone (SOA) and avalanche robustness. The improvement of these parameters usually occurs at the expense of an increase in RDS (on). Therefore, it is justified to sacrifice some efficiency in search of greater thermal dissipation, greater surge capacity, over current and greater surge capacity. Seeking all of these qualities along with a compact footprint to minimize controller module size narrows the selection of suitable devices.
In fact, the limitations that determine the selection of power semiconductors for cobots are remarkably similar to those for high-power cordless power tools, such as drills, electric screwdrivers, sanders, and grinders. These must also adhere to strict limitations on physical dimensions while delivering high currents and withstanding high temperatures, especially during repeated starts and stops. German sports equipment pioneer Jaykay GmbH encountered similar limitations when creating its range of innovative e-fin paddleboards and overcame them by using Nexperia MOSFETs optimized to deliver robustness and reliability in the compact motor controller.
Selection of Power Semiconductors
Given the complexity of the demands in current applications and the latest advances in device technology, focused on the traditional figure of merit (FOM) RDS (un) becauseg, is no longer the best starting point when selecting MOSFETs. The differentiating factor of each design is finding the best silicon-encapsulation combination.
While optimization and improvement of silicon have paved the way for new MOSFETs with greater current and voltage controllability than previous generations, packaging technology plays a critical role in meeting the needs of high-power applications. . Compared to traditional power packages, Nexperia's LFPAK technology combines significantly lower stray inductance and electrical resistance with improved thermal performance. The key innovation, which enables these improvements, is Copper Clip technology which replaces conventional bonding threads in the source connection, as shown in Figure 2. Compared to thread bonding, wire bonding Copper clip prevents current buildup, thereby eliminating hot spots and ensuring more uniform current distribution, while also acting as a heat sink for the semiconductor (die).
Figure 2: Transparent view of an LFPAK56 package
The combination of the most advanced silicon and the LFPAK56 package has allowed the creation of a pair of MOSFETs in a half-bridge configuration composed of HS (High Side) and LS (Low Side) for the control of three-phase motors with a packaging of 5mm x 6mm. The package uses flexible pins to improve overall reliability and the internal copper clip that connects the silicon drain and source terminals to the package conductors has higher electrical performance and thermal dissipation than traditional bonding wires, increasing the confiability. Although the drain tab of the MOSFET remains the dominant pathway for dissipating heat from the semiconductor, the source pin of the LFPAK clip contributes significantly. Therefore, ensuring an adequate amount of copper in the connections from the PCB to the source pin helps maximize overall thermal performance.
In high-power applications, where space limitations conflict with demands for high performance and robustness, copper clip packaging technology is a valuable part of the solution that helps maximize the value of silicon. Robustness and reliability also come down to the thermal cycles and, in some cases, the current flow that the devices must withstand in the event of failure.
Conclusion
For robot manufacturers, bringing a safe and cost-effective system to market requires smart design decisions at every stage, including a precise, flexible and responsive control strategy, a cost-effective and optimally sized motor, and high-quality semiconductors. Reliable and efficient power. Given the expected growth of this market, with increasing power density, power semiconductors are a critical component to ensure that the motor and, by extension, the entire cobot operate efficiently, reliably and safely.
