[ME491] Human Assistive Robotics HW4
Student_ID: 20190609
Name: Hohurn Jung
[Problem 1]
(a) Below figure shows the free diagram of passive assistive device “ShoulderX”.
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Gray colored circles & polygons: Human body
Red colored object: Passive assistive device
Black square: Heavy object
Black arrow: Elastic force
Blue arrow: Applied force on passive assistive device by human body
Green arrow: Applied force on human body by passive assistive device
When we lift up the heavy things above our head, this action requires quite high value of τℎ (torque
on the human shoulder. See the above left figure). To assist this torque of the shoulder, we should
apply force like F1 on the human arm. Then the assistive device gets the reaction force of F1. This F1
force is canceled out by the elastic force(kx) of the device, and by the elastic force, the device presses
the human shoulder in downward direction(F2). Also, F2 is the reaction force of F2 which is applied to
assistive device. Lastly, in order to attach the device firmly to the human body, F3 should be applied to
the device by the human body, and F3 is applied to the human body as a reaction force.
If we consider all forces in the overall human body, the forces that applied to the human body F1, F2
and F3 are all canceled out. Therefore, the magnitude of ground reaction force is the same although
we used a passive assistive device ( If we neglect the weight of the device. ). This device serves to
distribute the weight of an object to arms and shoulders.
(b) Below figure shows the free diagram of passive assistive device “FORTIS”.
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Gray colored circles & polygons: Human body
Red colored object: Passive assistive device
Black square: Heavy object
Black arrow: Elastic Torque by the assistive device
Blue arrow: Applied force on passive assistive device
Green arrow: Applied force on the object by passive assistive device
To reduce the magnitude of τℎ (torque on the human shoulder), F2 should be applied in upward
direction to the heavy object. F2 is generated by elastic torque(𝑘θ) of the device, and F1 is generated
as a reaction force of elastic torque and F2. Also, this device is touched on the ground, so this device
receives ground reaction force from the floor.
Because the weight of the heavy object is distributed into ground force and human, the magnitude of
ground reaction force to the human body is decreased when we compare the situation with device (a).
(c) Below figure shows the free diagram of passive assistive device “V22 ERGOSKELETON”.
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Gray colored circles & polygons: Human body
Red colored object: Passive assistive device
Black square: Heavy object
Black arrow: Elastic force
Blue arrow: Applied force on passive assistive device by human body
Green arrow: Applied force on human body by passive assistive device
To reduce the magnitude of τℎ (torque on the human elbow joint), elastic force(kx) by an elastic string
of device should be applied in upward direction to the heavy object. Because the elastic string of the
device goes over the human’s shoulder, this elastic string applies the force(F2) to the human body. In
order to attach the device firmly to the human body, F1 should be applied to the device by the human
body, and F1 is applied to the human body as a reaction force.
This device’s free diagram is similar to the case of device (a). The forces that applied to the human
body F1 and F2 are all canceled out, so the magnitude of ground reaction force is the same although
we used a passive assistive device ( If we neglect the weight of the device. ).
[Problem 2]
(i) Below figure shows a designed active exoskeleton robot that has similar function with “ShoulderX”,
“FORTIS” and “V22 ERGOSKELETON”, respectively. Red transparent block is the joint part where the
assistive force(τ𝑎) is applied by the position control method.
(a) “ShoulderX”
(b) “FORTIS”
(c) “V22 ERGOSKELETON”
Considering the joints that need the most strength to lift things, I designed each active exoskeleton
robot to apply assistive force to the shoulder, robot joints and elbow.
(a) Apply assistive torque to the shoulder joint by using position control.
(b) Apply assistive torque to each robot joint by using position control to apply force to the object in
upward direction.
(c) Apply assistive torque to the elbow joint by using position control.
(ii) Advantages and disadvantages of the designed active exoskeleton robot.
- Above three active assistive devices have similar pros and cons when compared with each passive
device, so I combined the pros and cons of active devices like the below table.
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Advantages
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Disadvantages
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We can apply appropriate magnitude of assistive torque to different kinds
of weights by using position control. If we use a passive assistive device,
we can always apply the same force regardless of the weight of the
object.
We can maintain the magnitude of assistive torque or force regardless of
the position of the object. (The assistive torque or force of the passive
assistive device is changed by the position of the object because the
passive assistive device generates assistive force by the deformation of
elastic materials.)
Generally, active assistive devices have bigger weights than passive
assistive devices. This can affect tiredness of shoulder & leg because of
pressing force by device’s weight and ground reaction force.
Free movement of the object with arms is restricted because there exists
a certain position trajectory to apply the assistive torque or force. In order
to increase the freedom of movement, we should make more states in
the program, and this will require more memory space.
[Problem 3]
(a) “ShoulderX” and (c) “V22 ERGOSKELETON”
- (a) and (c) have similar discomfort characteristics, so I combined these two devices and explained
them together.
(i) For the motion 1, 2, 4 (Level walking, Ascending & Descending stairs, sitting down & up), the
wearer will not feel such discomfort because the device is not connected with the human's leg. The
device is attached to the upper part of the hip joint. One inconvenience the wearer feels is the gravity
force by the weight of the device. To solve this inconvenience, we need to make the device lighter.
(ii) For motion 3 (Lifting & Lowering heavy objects), it is hard to pick up things on the floor. To pick up
things on the floor easily and fast, a human should stretch his or her hand in a downward direction.
However, a large amount of force is required to stretch an arm because of the elastic force of passive
devices. To solve this problem, we should implement a certain buckle that can control the existence of
elastic force. Therefore, when we want to pick up an object on the floor, unlock the buckle to remove
the elastic force, and to lift it up with assistive force, lock the buckle.
(b) “FORTIS”
(i) For the motion 1, 2, 4 which is the motion related to the leg movements, the user can feel
discomfort because of exoskeletons directly attached to the user’s leg. When walking and ascending
& descending stairs, the user should lift up his or her leg to move forward, so performance of these
actions falls when the weight of exoskeletons is heavy. Therefore, we need to implement this device
with lighter materials like carbon nanotubes, and we should also reduce the friction at each joint of the
exoskeleton for smooth walking performance. Additionally, the exoskeleton is contacted with the floor
to distribute the heavy object’s weight. Because of this, noise can occur in the walking or ascending $
descending process and this can be one big problem. Thus we should install soft materials under the
device to reduce the noise.
(ii) Similar to the case of (a) (ii), for motion 3, it can be hard to connect the heavy object and the
device when the heavy object is on the floor because of the elastic force of the device. To solve this
problem, we should implement a buckle that can control the existence of elastic force like the method
of (a) - (ii).
(iii) For every motion (motion 1,2,3 and 4), the asymmetrical shape can be one problem. For walking
motion and lifting up motion, balance of the body is very important to perform each task. However,
FORTIS’s center of the mass is located on the forward side of human body. In order to solve this
asymmetrical center of mass problem, we should add mass on the backward side of the device.
However, this method can increase the total weight of the device, so we should choose the lighter
material for all parts of the device.