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Embedded Motion for Lightweight Robotics

26 Feb 2014
Rob Whitlock

The utilization of robotics in manufacturing is currently a $5B industry and is projected to grow to a $20B industry (Source: A Roadmap for U.S. Robotics, From Internet To Robotics - 2013 Edition). 

A major contributor to the projected growth will come from small to mid-size users in a variety of industries where historically the demand was from the very large corporations in the automotive and aerospace sectors. 

Counter to traditional industrial robots that are big, noisy, and costly, companies are developing innovative lightweight robots designed for small to mid-sized users.  These cost effective robots can be easily moved between different work stations within a production facility and incorporates software that facilitates quick adaptation to new processes for quick usage.  In addition, safety protocols are being created to allow robots to have close interaction with humans.  It's easy to see how this can be a transformative technology that can pay big dividends in the future for mid-to-low tier production facilities.  Replacing humans on monotonous or dangerous functions will improve overall safety and productivity.  In addition, as the world moves to smaller and more complex products, future manufacturing requirements will require higher precision and adaptability that can exceed that of humans.

Embedded motion or direct drive systems, defined as an integrated frameless DC brushless torque motor coupled directly to the required load offers several key advantages as compared to classical servo motors coupled to mechanical transmissions such as gearboxes, timing belts, or ball screws.  The advantages pertaining to robotics (and many other industries) include weight savings, smaller envelope, a more rigid construction, higher reliability and lower life cycle costs. 

A torque motor is characterized with a high outside-diameter- to-length ratio and relatively high slot-pole counts. Motor torque is directionally proportional to its length but proportional to the square of its diameter.

(Torque ∝ D2L)

Therefore if you double the motor length, you double the torque.   If you double the motor diameter, torque increases by a factor of 4.  So, it pays to select a torque motor with the highest possible outside diameter.

Torque Motor Shape Factor         Servo Motor Shape Factor

The 'torque motor' will provide a higher torque at lower speed and a high slot-pole count design reduces cogging that can compromise smoothness and repeatability. 

Weight and overall system inertia is greatly reduced as couplings and mechanical transmissions are no longer needed.  Robotic form factor is also minimized since the motor is mounted internal to the robotic joint structure directly to the load or in some cases through harmonic type gearing.  The relatively large hollow rotor shaft through-bore can also offer additional space savings by running electrical cables through the motor.

As any mechanical device is prone to wear and limited life expectancy, replacing with a direct drive embedded motion option will naturally improve the overall reliability and life cycle cost of the robot as products that typically need replaced or serviced over time are removed from the system.

KBM Frameless Servo Motor FamilyIt is true that implementing direct drive solutions for robotic applications does require a thorough understanding of proper machine design and mounting guidelines.   However, machine complexity and tolerance requirements are no greater than what is found in conventional motion control systems. (Check out this blog post on mounting frameless motors)  The advantages in higher accuracy, improved performance, compactness, higher reliability and lower overall life cycle costs all lead to a compelling argument for consideration.  

About the Author

Rob Whitlock

Rob Whitlock - Author
Rob has been with Kollmorgen for 10 years working in various Applications Engineering, Sales, and Product Line Management roles. His hobbies include reading, wood turning, coaching/watching his kids play sports and spending time in the great outdoors. Rob can be reached at rob.whitlock@kollmorgen.com

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