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Service Robotics and drives challenges

A look into power MOSFET and package technologies for best performance of drives in service robotics applications

A perfect storm of circumstances such as market readiness, technology maturity and delivered value fosters the spread of robots both in the industrial and the consumer segments. Semiconductor technologies for robotics have been significantly developed in the recent years. New power semiconductors allow more effective, reliable and multi-dimensional service robot designs.

Robotics

Service robots are highly complex systems. They stress the boundaries of ever compact designs with high efficiency and reliability. These robots are as small in size as robust in technical parameters and requirements. Energy efficiency together with long life batteries, small form factor and excellent thermal management of the hardware are key in designing a robot that meets and exceeds customer expectations. Taking into consideration the software element, for connected service robots data protection, authentication and authorization also atop consumers’ priorities.

The success of a robotics project often depends on the availability and scalability of the semiconductor solutions required. In this article we discuss the use cases and benefits of different drives technologies for robotics with a special focus on MOSFETs, packages and high switching frequency solutions such as gallium nitride (GaN).

Common system architecture in service robotics

The most common architecture for robots in most of the cases features a central processing unit (CPU), power/battery management unit, battery chargers, wireless communication (COM) modules, human machine interface (HMI), sensors, and drives modules (brushed and brushless motors). Some robots do not include all components discussed here; however, this still represents a good system overview.

The main CPU is the central brain and carries most of the intelligence of the system. This processor is responsible for the system coordination, command different modules to execute their tasks in a scheduled and independent way. The remaining modules execute instructions, and report status to the main CPU.

Main CPU
Figure 1: Common robotics system architecture block diagram

Most of the service robots are battery driven to enable flexibility in their movement. These robots feature on-board charger enabling direct connection to the AC grid. In these cases, a charger is included in the robot to generate a high voltage DC level that the power management unit will process further down. There is an emerging trend of wireless charging capability for this application, especially for those that are required to work continuously as wireless charging allows the robot to charge while still performing.

As mentioned above, most of today’s robotics systems are battery driven so a power/battery management unit is quite common in the architecture. The battery management unit takes care of the overall condition of the battery (including health and safety aspects), and also protects it against system overvoltage or overcurrent. In the battery module, security – including authentication– is a key factor to be considered. Batteries also rely on general purpose microcontrollers to implement auxiliary functions like metrology or monitoring in the battery system. Apart from a battery management unit, a power management unit supplies power to the different components in the robot by controlling the required voltage rails – 12V, 5V or 3,3V – in a stable manner for the rest of modules. It is possible with buck converter controllers or linear regulators – fixed and adjustable.

Being equipped with wireless communication modules, robots are interconnected to other systems like other robots or control units that command complete robot fleets in real time. The communication is usually Wi-Fi or Bluetooth based. In many cases, a local controller is responsible for the communication process, working as a gateway between the robot’s main controller and the external world.

More and more robots have a certain level of interaction with humans. Human-machine interfacing can be achieved with either a simple display or even a high resolution display, but also LED lights can be used to provide information or feedback to the user. Once the robot is provided enough intelligence to be able to interact verbally with the user, both voice input and output devices are required.

Also, different types of sensors can be considered in service robotics designs. Commonly, position sensors (Hall sensors, encoders), speed, angle or current sensors are used in drives. If the robot needs a precise understanding of its environment, further types of sensors are required such as radar sensors for motion sensing (distance and direction), barometric pressure sensors or 3D image sensors for object recognition. Sensing of the surroundings boosts the robot’s autonomous capabilities, especially when it is deployed in complex environment such as crowded warehouses.

And finally, drive modules are also part of a common system architecture. When accurate positioning, high speed or quiet operation are required the designer will decide for a brushless DC (BLDC) motor together with a set of position sensors. Otherwise, if a low performing motor control (slow, low accuracy) is enough, the designer will select a brushed motor, profiting from the lower cost of such a solution. And there are robotics applications in which both brushed and brushless motors co-exist to meet the performance and cost efficiency goals at the same time.

After briefly describing the main technological structure behind service robots, the next section will expose how conduction losses affect the overall performance of a robot and what semiconductor solutions and technologies are available to mitigate these losses.

How to reduce switching and conduction losses

A way of optimizing the battery life in robots is to increase efficiency of the robot drives, this is to reduce power losses. In drives applications, both conduction and switching losses are in the focus. Infineon has been innovating continuously to improve figures of merit of MOSFETs with special focus on reduced RDS(ON) (drain to source ON resistance) and gate charge (capacitance) of the MOSFET, minimizing both types of losses from one generation to the next.

Depending on the control method, different losses are observed. When synchronous rectification is used, the lowside MOSFETs are turned on if the current freewheels through their body diodes. This dramatically reduces the conduction losses of the body diode (PLoss = IF x VF) as the RDS(ON) value of the MOSFETs gets lower and lower with new generations; however, the low-side diode is still one of the main sources of losses. To address this issue, Infineon has developed MOSFETs with integrated Schottky diodes that reduce the forward voltage, therefore minimizing the power loss in the diode. These products are referred to as OptiMOS™ FD (Fast Diode) and can be identified by the suffix – LSI, e.g., BSC010N04LSI.

Power Loss
Figure 2: Power loss breakdown showing conduction (‘Cond-‘) and switching (‘SW-‘) losses in high-side (HS) and low-side (LS) MOSFETs as well as body diode (D) losses. Conduction of low-side body diode is dominant and can be reduced by usage of LSI parts.

Figure 2 shows a power loss breakdown measured in a threephase inverter using block commutation PWM (6 steps) with synchronous rectification. The supply voltage is 18 V and the selected MOSFET for the comparison is BSC010N04 in both LS and LSI versions.

The candle diagram clearly shows that both conduction (‘Cond-‘) and switching (‘SW-‘) losses have an important role in both high-side (‘HS’) and low-side (‘LS’) MOSFETs. There are three main findings related to this:

Switching losses are negligible in the low-side MOSFET as softswitching is granted.

Conduction loss in low-side diode is by far the most dominant source of losses.

LSI (Fast Diode) version of MOSFET with integrated Schottky diode reduces the conduction loss by approximately 25 percent. This reduction depends on system conditions such as current level.

Switching losses are strongly connected to switching frequency. Common frequencies in robotic inverters range from 10 kHz to 40 kHz. The higher the switching frequency, the higher the losses. Infineon’s best-in-class OptiMOS™ solutions offer low RDS(ON) and low-charge MOSFETs to significantly reduce both types of losses; however, losses are inevitable and also heat will always be produced in the power switches. Thus, thermal management is one of the key challenges in drives designs, especially when considering high power density devices for example in small robotic arms.

Infineon DirectFET™ packages, as shown in Figure 3, are dual-side cooling packages with direct connection between the metallic package and the silicon die inside that directly connected to the PCB on the bottom side, minimizing thermal resistances to the exterior. These packages spread efficiently the heat from the junction to the bottom of the PCB, and from the top through the metal package into the air or an optionally used heat sink for more rigorous cases. In addition to its extra low profile, this package is the perfect choice for space constrained designs. Figure 3 shows a comparison of thermal resistances between DirectFET™ and D2Pak packages. DirectFET™ has less than half of the thermal resistance (8.1°C/W) than D2Pak (16.8°C/W).

DirectFET
Figure 3: DirectFET™ packages allow optimized thermal design in high density drives. Comparison of thermal resistances between DirectFET™ and D2PAK packages

High switching frequency drives solutions

Engineers can take several advantages of using gallium nitride (GaN) devices in their applications. GaN attributes such as lower on-resistance giving lower conduction losses than silicon alternatives, less capacitance resulting in less switching losses, or improved body diode reverse recovery make it a perfect choice for high switching frequency power applications. Increasing the switching frequency can help to improve the performance of a drive by reducing for example the torque ripple. In other applications such as power supplies, this technique can also be used to reduce the size of magnetic components effectively.

As the switching frequency increases, controllers have to be adapted. PWM resolution must be considered to ensure that the complete loop can stay under the required accuracy. Infineon offers microcontrollers like XMC4100 family with high resolution PWM modules for such high resolution loop purposes, especially when switching frequency increases. Also, the processing capability of the microcontroller must be considered when switching frequency raises. Assuming a cycle-by-cycle control, less time is then available to finalize new duty cycle calculations. Infineon offers a broad portfolio of controllers with wide range of performance from XMC1000 family ARM®-Cortex™-M0 at 32 MHz to XMC4000 family ARM®-Cortex™-M4F at 144 MHz and AURIX™ when higher level of functional safety and performance is required. Increasing the control loop execution frequency leads to better dynamics of the motor, resulting in more accurate control.

Normalized execution
Figure 4: Normalized execution time for cosine and division functions with a standard ARM® Cortex™-M0 without MATH co-processor (Standard) and with XMC1300 utilizing the integrated MATH coprocessor and DIVIDE unit.

Infineon’s product offering also covers a special MATH coprocessor (including both a CORDIC unit for trigonometric calculations and a division unit) dedicated to motor control calculations. This co-processor reduces the execution time of control loops in XMC1000 family compared to standard implementation (hardware versus software calculation). A comparison of the execution time of cosine and division functions – often utilized in motor control algorithms like field oriented control (FOC) – is shown in Figure 4.

Summary

Engineers stress the design parameters of drives to be able to develop the next generation of robotic solutions and devices. They can choose from different semiconductor solutions to fine-tune their designs. The technical parameters such as switching frequency and thermal resistance of end products set the requirements for drives. To build a well-optimized system, designers have to minimize the losses – both conduction and switching losses – and optimize the thermal management.

MOSFETs with integrated Schottky diodes can reduce the forward voltage resulting in minimal power loss in the diode. Engineers can also take advantage of new package designs like DirectFET™ that offer optimized thermal management New wide bandgap solutions like GaN devices will stablish the foundation of higher switching frequency drives, helping both in the accuracy and footprint aspects.

For Infineon’s A-Z service robotics product offering of more than a hundred products including drives and robotics related solutions, please visit www.infineon.com/service-robotics or download the Service robotics and AGVs brochure here.

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