Meeting the demand for high power density in robotics

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This article is part of the series on power management: diving into power density

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What you will learn:

  • The importance of high torque to meet power density requirements in robotics.
  • The growing role of spindle motors.
  • NASA CADRE robotics program.

The three most critical aspects for motors used in mobile robot applications are high efficiency, high dynamics, and high power density. And when it comes to high power density, that means high torque mechanisms that take up minimal space.

One of the best high power density options for mobile robots is the DC motor with continuous rated torque. These types of motors, especially for robotic arms, will need high starting torque for smooth acceleration of motion.

An example of this is robotic drives1 which powered NASA’s Mars rover known as Opportunity, which was active on Mars from 2004 until mid-2018. High power density was a requirement for the rover.

Obtaining a high torque requires the addition of reduction gears to the motors. Gearheads help control large load inertia from relatively low motor inertia. If gearheads are not used, controlling the acceleration or speed of the load would require the motor torque, and therefore the current, to be many times greater than the reduction ratio used in the gearhead. Going in this direction would be costly. Thus, modular and flexible motor-gearbox combinations are the preferred solution for robotics.

Torque index

The torque of the robotic system is the amount of force applied in a circle, in this case the output horn of the DYNAMIXEL2 or servo.

To determine if the torque from a DYNAMIXEL is sufficient for motion, the torque will need to be equal to or greater than the forces the servo will need to overcome in the robotic system.3

Before choosing their DYNAMIXEL size, designers should calculate the estimated torque requirements of their movements. A basic reference drawing is shown in the figure.

In the case of high torque for linear motion, with high force, the drive motor will need many more magnetic poles than are available in lower torque motors. This high force can be achieved through appropriate and creative mechanical design with high reduction ratio, without sacrificing the advantages of the high-speed motor.

However, additional magnetic poles require more complex assembly and design. Conventional brushless DC motors with a split iron core can be designed with multiple poles and therefore be able to deliver lower speeds at higher torque.

Spindle motors

Spindle motors4 are small, reliable, high-precision electric motors in hard drives that increase power density. This type of motor rotates the shaft/axle on which the platters sit; the platters are where the data is stored. The need for these high speed spindle motors continues to increase due to high power density demands. Robotic systems use these types of motors because large amounts of data can be stored in the robot’s body.

Another reason for the growing demand for these motors relates to the cycle time during which a machine tool must perform a specific operation. The faster a tool can be moved and rotated in a robotic arm, the faster it can complete its task.

Recent improvements in spindle motor performance are due to stronger permanent magnetic materials, high-precision machining techniques, and improved electronic control circuitry.

Ever-expanding robotic system architectures

Collaborative robots, a type of robotic automation, are designed to perform work safely alongside human workers in a shared, collaborative workspace. The role of the collaborative robot is to perform repetitive and menial tasks, with a human worker performing the more complex and thoughtful tasks. NASA’s International Space Station (ISS) is a good testing ground for collaborative robots. This type of application demands the ultimate in high power density design.

NASA’s Cooperative Autonomous Distributed Robotic Explorers (CADRE) project is developing a comprehensive network of shoebox-sized mobile robots that will enable future autonomous robotic exploration on the Moon, Mars, and the universe beyond. High power density is a must here, as weight is a huge factor in space launches from Earth.

NASA has the Autonomous Fold-Flat Explorer Robot, or “A-PUFFER”, which is a FRAME robot. Each robot will contain an on-board computer, a wireless radio for communication, and a stereo camera with multiple lenses and image sensors. These robots will sense the environment in front of them while capturing 3D images. A-PUFFER robots can be scout robots that will explore in groups while collecting data in hard-to-maneuver places, such as craters and caves on the Moon.

Summary

Most robotic applications will require high power density as well as high motor power density coupled with high torque capability. Robots should be lightweight with powerful motors capable of delivering such high torque. High-power-density hard disk spindle motors that spin disk memory and programs can act as the robot’s brain and memory storage.

NASA has entered space at high power density (no pun intended) with its automated explorer robots. These robots can travel to distant planets and explore places where human explorers cannot venture. Even higher power density in motors and power supplies will evolve as we move into the future of technology and expanded robotic capabilities.

Read more articles in the Power Management Series: Dive into Power Density

References

1. Maxon Robotics

2. DYNAMIXEL

3. Nominal torques Robotis

4. RobotWorx


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