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Free floating DOF's

TBW

As discussed in the explanation, a pure torque does not have a point of application. It does not apply a net force, and it does not need a force support point to react against. It does require a torque reaction, but this does not apply in any particular point. This concept has surprisingly deep consequences, which can be put to very good use in simplifying both the exoskeleton itself and its use in practice.

  The torque-only actuation is decoupled from all other movements, in particular from the translations of the shoul­der joint and of the torso. Not only are these not driven, but they are in fact left completely free floating.

the alignment problem in conventional exoskeletons

In conventional floor mounted exoskeletons, the position of the human shoulder joint is pinned to a fixed position in space by the mechanism. Typically, a therapy session will start with some ten minutes of care­ful readjustment of the robot's shoulder pivot location to coincide with that of the patient. Even small misalignments can be painful during subsequent movements.

  But it gets worse : the human shoulder does not even have a single pivot point. When lifting the arm by more than about 30° of abduction, the shoulder joint wants to rise, and if constrained the shoulder feels forced and even painful, even after careful "fitting".

wearable exoskeletons

The "overconstraining" problem goes away if we can drive the shoulder rotations by a pure torque, and leave the shoulder itself free to move in the three translational DOF's.

  The first solution that comes to mind is to make a wearable exoskeleton for the human arm. There are how­ever two major problems with this solution. The first one is that the torque has to be reacted somewhere. This will almost have to be on a body har­ness. These will never be very comfortable, and most likely re-introduce a fixed position of the shoulder pivot rela­tive to the torso.

  The second problem is that the motors, and most likely some batteries, will have to be worn as well. This will make the wearable exoskeleton very heavy and cumbersome. The Armbot is therefore still a floor fixed device.

the floor mounted mechanical solution

Designing a simple mechanism which can apply pure torques to the shoulder and elbow from a fixed base without constraining any of the other motions is not trivial. In the Armbot it is achieved by a relatively simple pantograph mechanism.

  The mechanism decouples the torques from the translations by a 90° change in direction in some torque- and pushrods, via small bellcranks at the top of a swiveling column. The resulting exoskeleton does not fight, or even "see" any patient misalignments or any of the natural synergies such as the shoulder lifting during abduction. The video shows the idea. A separate page gives more details.

  TODO Add the bit about no motors and the Galloway mechanism.

the original idea : skewed axes

The torque-only idea was not the one that started the development. The original motivation was to use the skewed axes ideas from the ADL gimbal to eliminate the open circle segments, sometimes even big full ring bearings often seen near the human wrist interface, and even somemetimes even around the upper arm.

  Eliminating these large bearings led to a mechanically so simple and lightweight robot arm.

  , with no motors moving with the limbs, and with no cable drives or pulleys, only simple pushrods.

  Several mechanical innovations were needed to make this ideal possible, as detailed below. But the major breakthrough came with the torque-only idea.

pushrods only ( 2009 )

  Upper arm abduction, flexion and internal-external rotation, as well as elbow flexion are powered without loading the shoulder joint and are absolutely painless. There is no setup or alignment time. Any patient can put their arm into the upper arm- and elbow brackets without any need for further adjustment.

  Motors and force sensors are located in the base. The mechanism itself is very lightweight, consisting only of a few lightweight aluminium tubes. In addition, it is completely naturally balanced.

While at Moog, around 2009 I started the development of a novel arm exoskeleton. It is mechanically balanced. The mechanism gently floats when it is not powered. Bias forces ( or rather, torques ) are added to the mecha­nism by motors on the floor, with force sensors near the motor and admittance control to free up the motion completely.

  By the use of a special elbow linkage ( modified Galloway ), ± 45° joint angles are transmitted over a series of joints with a total added angle of over 180°, all the way from the motors to the elbow and even to the wrist, by simple lightweight pushrods.