SRI Robotics Super Flex Exosuit

SRI Robotics Demonstrating Super Flex Exosuit For
SRI Robotics Demonstrating Super Flex Exosuit For

SRI’s Alexander Kernbaum presented an overview of SRI Robotics and gave more detail on the Super Flex Exosuit at the Feb 2016 HomeBrew Robotics Club meeting hosted by Google.  The SRI Robotics group is roughy 60 people, which is a small percentage of the overall company.   SRI does not sell products but generates multiple spin-off companies.  Siri for Apple for example was a development of SRI (though the Apple website will claim it was made in Cupertino).  Here is what Alexander Kernbaum, Senior Research Engineer – Robotics and Medical Systems had to say about his work on the Super Flex exosuit:

Super-suits not comfortable – People feel cool in the exoskeleton but can’t wait to take it off.  The technology has reached the point where we can create wearable robotic devices that assist the movement of the user, but they can’t be worn for a long time.  Imagine going on a 6 hour hike.  An able bodied powered exoskeleton for the lower body would be useful for about 1 to 2 hours (depending on the configuration) while climbing uphill.  What would you do with the device for the rest of the hike?  An exoskeleton has to be comfortable to wear and not get in the way while going downhill, taking pictures or eating a snack, which are all parts of an average hiking experience.

A possible solution is a soft flexible exosuit.  The SRI Super Flex exosuit is the integration of several innovative systems: Flex Grip, Flex Drive and Flex Control.  The main goal for the exoskeleton was to not have any sharp, hard edges anywhere and to be comfortable for the user to wear.  The suit was originally part of the DARPA Warrior Web Project.  It pulls at the bootstraps and applies a force at the back of the heel (very similar to the ones created in the labs of Conor Walsh and Greg Sawicki).  One of the best selling points of the Super Flex is that in passive mode (all motors turned off) there is complete retention of mobility.  This is different from other exoskeletons that become stuck when they lose power.  A person can walk on a tight rope with the Super Flex in passive mode!

The next challenge of removing all hard edges was reimagining the human-exoskeleton connection.  The design of current body harnesses became an issue.  Powered exoskeletons apply shear force to the skin when generating locomotion.  Shear forces cause straps and harnesses to try to slide up and down on the operator’s body which is very uncomfortable.  The SRI team visited a circus to see how silks performers handle hanging by ropes, silks and ribbons.  SRI was inspired to create the Flex Grip, an elastic strap that distributes forces equally around a section of the leg.  This is similar to a Chinese finger trap, which relocates the forces evenly on the skin allowing for much more total force to be applied by the strap on the user.  In one test, Alexander was suspended upside down held by the Flex Grip on one leg to see how well the forces are distributed on his leg.

The next step was to power the exosuit.  The Flex Drive is a series of thin ropes that get shorter when they twist, similar to the Roman catapult.  At moderate use conditions, the prototypes have been good for about 1,000,000 activations.  The SRI team is not too happy with that number but they see it as a starting point.  The Flex Drive system weighs about a pound and the entire suit weighs about 8 pounds with a 3 pound battery.  Recently, SRI did a demonstration of the Super Flex Exosuit for

One final part of the Super Flex exosuit was not only the use of elastic materials but also 4D materials; materials that will change their mechanical properties on the fly.  The electrolaminate clutch developed by SRI weighs just a few grams and can turn from a soft to hard material with the application of high voltage at low current.  See our article from December for a broader overview of SRI and their work on 4D materials.  The technology was used in the Super Flex Exosuit to store energy during each step cycle.

What else is SRI Robotics working on?

SRI Robotics is also working on a humanoid motorcycle rider called MotoBot, developed in a partnerships with Yamaha.  The robot has 160 runs with no crashes.  The goal is to beat the fastest motorcycle driver on the planet in 3 years, (this is year 1) on the same track with the same conditions in a time trial.  The next version will be able to lean off the bike and have its knees touch the ground.  This is a really fun way to do research.  All of the problems being solved are fundamental robotics problems that can be applied to future exoskeleton models.

Another project by SRI is the Micro Factory: magnetically actuated micro robotics.  A magnetic field is applied on a surface propelling the micro robots at 68 body lengths per second.  The system can also run on flexible surfaces as long as they can have a controllable magnetic field, which is similar to the electrostatic clutch and gripper technology by SRI.  As many as 1000 micro bots can be controlled at the same time.  The microbots can be used to grab and drop components or materials (LEDs, drops of glue, etc).  They can also cut and weld.  One limitation of the current control system is that the microbots have to stay 1 cm apart.  Each microbot  can also exert 7 to 8 grams of force.

SRI Robotics have one more humanoid robot project, Proxi.  Proxi is designed to achieve the same performance as Atlas, a hydraulic biped by Boston Dynamics, but using only 1/20th of the energy.  Proxi competed in the DARPA efficiency challenge in 2015.  When walking, Proxi is still 7.5 times less efficient compared to a human.  The SRI team feels that their greatest energy loss now stems from the transmission and gear reduction.  Getting force from the motor to the actuated limb is significantly less efficient than desired.  If a robotic arm is not back-drivable, it is probably less than 50% efficient.  The initial approach was to make a very expensive custom cycloid transmission, but now the SRI team is working on a novel design for transmitting power.

The Proxi also uses brushless motors.  We talked about brushless motors and their good size-to-power ratio in Low Cost Open Source Controller For Brushless Motor.  These motors are currently most commonly found on model airplanes and drones.  The Turnigy 50cc was specifically singled out during the presentation.  Hobbyking currently has the Turnigy 50cc for sale and provides more information on the motor.  Other labs have tried designing their own motors.  The goal is to get high torque low gear ratio motor assemblies.  Different companies rate their motors differently, especially when it comes to heat dissipation and safety so it is very difficult to compare motor to motor.

The Flex Exosuit is in the hands of a great team that is trying to develop useful but still comfortable exoskeletons and is broadening its knowledge and understanding of robotics by working on multiple projects at the same time.

See also:

HomeBrew Robotics Club website:

Article on SRI Robotics by



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  • Im a South African . I have problems in the lumbar spine and
    i need two crutches to walk …its progressive spastic paraplegia
    or spastic myelopathy …my legs are very weak and i have very poor balance …i havnt seen the exoskeleton anywhere in South
    Africa …publicised or advertised ….can u help me…?..

    • Dear reader, the only exoskeletons that I have heard of in South Africa are ones that use the Ekso GT for in clinic rehabilitation: Cape Peninsula University Of Technology, Lander & Pursad Biokinetics, and Just Walk Bionics. This information is on the Ekso Bionics website under “centers”

  • Hi my name is john i have progressive ms which gives me great difficulty with leg movement i am not paralysed but walking is very difficult and i think one of your exoskeletons would be very good for me i also have ballance issues i would appreciate if you would keep me up to date with any news on this product please regards j.canning

    • The SRI International project has spun-off into its own startup. For now, the startup is somewhat private and guarded (and ironically, they are just a few miles away from the Exoskeleton Report), but when they are ready to announce their product you can be sure you will be able to read about it here!

    • Hi Nick, the Superflex spin-off is “incognito.” It is a hidden company that is developing their prototypes away from the gaze of the public. I hope that they reveal some additional information and success stories soon.

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