TPG Results
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Load Capacity
The maximum weight the TPG could carry was determined for two separate cases: the gripper perpendicular to the ground and the gripper horizontal to the ground. For each case, the gripper was mounted to a metal frame and given a cylindrical object to hold. The weight in this object was increased in increments of approximately 20 g until the object slipped from the gripper’s grasp. Small sandbags were used as weights. Preliminary experiments determined the appropriate object for each test case. The TPG perpendicular to the ground could carry a larger load and thus was given a heavier object to start the experiment than was the gripper parallel to the ground.
Load Capacity
In the experiment with the TPG perpendicular to the ground, the gripper was given a coffee mug weighing 410 g to hold and weight was increased in 20 g increments until the gripper failed to maintain a stable hold on the mug or dropped the mug. In this vertical position, the gripper was able to carry an average of 530 g (1.2 lbs) with a standard deviation of 23 g.
For the experiment with the TPG parallel to the ground, the gripper was given a cup weighing 40 g to hold and weight was increased in 20 g increments. In this horizontal position, the gripper was able to carry an average of 240 g (0.5 lbs) with a standard deviation of 11 g, approximately half the mass of the gripper in the perpendicular condition.
The TPG was capable of holding approximately 1 lb in the vertical orientation and 0.5 lbs in the horizontal orientation. Like the TAG, lateral deformation of the gripper, which can be seen in Figure 3.4(e), was the main reason the manipulator could not hold as heavy of a load in the horizontal orientation. Increasing the thickness of the finger walls would help solve this problem but increasing stiffness would also increase the torque necessary to open the gripper, resulting in the need for a stronger motor.
Like the TAG, the TPG had a point of failure; however, unlike the TAG, this failure was caused by repeatedly opening and closing the gripper or leaving the gripper in an open position for extended periods of time. Failure occurred at the hard/soft interface of the tendon anchor point and propagated along the wall until the finger above the anchor point was separated from the finger below. While the gripper was still capable of holding objects once this happened, it was not as effective and could not be con- trolled well. Failure due to creep occurred in under 1 hr while failure due to cycling occurred at between 100 and 125 openings. Failure happened sooner in matte grippers than in glossy grippers. Decreasing the stress concentration at this point would likely solve this issue and could be one by adding soft material around the anchor point.
Forces Exerted
In this experiment, two FlexiForce sensors were attached to each side of the PVC pipe to read measurements from all four crossbeams. For Case A, even crossbeams on the right side of the gripper were read, and for Case B odd crossbeams on the right side were read.
Five different TPGs were used to take the normal force data. Two of the grippers were 3D printed with a matte finish, which leaves a waxy residue and makes the grippers less sticky, and three grippers were printed with a glossy finish. Measurements from these grippers were taken over seven days and total almost 300 trials. Between trials, the gripper was adjusted on the force sensors to account for variability in reading.
Forces Exerted
Figure 3.11(a) shows that the normal force exerted by the crossbeams was the highest for the third crossbeam at 0.3±0.08 lbs followed by the second at 0.2±0.13 lbs. Forces at the first and fourth crossbeams were negligible. As variation in the readings was rather high, differences between the grippers were investigated, but the only statistically significant different (T-Test, p << 0.01) was between matte and glossy grippers at crossbeam 3. Glossy grippers exerted almost twice as much force at crossbeam 3 than matte grippers did.
The TPG center crossbeams exerted a larger for at the middle than at the tips, which was the opposite behavior to the TAG. Some of this difference can be attributed to the different orientation of the fingers, which was necessary to create a gripper with a closed resting state. The TPG was also smaller than the TAG and its fingers often did not close fully around the PVC pipe. Two possible sources of variability could be the relatively large sensing area compared to the crossbeam width and the inability to place the gripper in the same spot on the sensor from trial to trial, leading the gripper to measure the distributed force between crossbeams rather than the normal force exerted by the crossbeam. This could be rectified by using smaller sensors or by using a puck so that the gripper touches the same point on the sensor from trial to trial and no distributed force is recorded. Additionally, some of the grippers began to fail during testing, which could have resulted in changes in gripper behavior.
Despite this variability, the only significant difference was between matte and glossy finishes at crossbeam 3. Grippers printed with a matte finish are encased in a waxy support material, which leaves a residue on the part that decreases the coefficient of friction. Grippers printed with a matte finish were also more prone to failure because the matte finish left grooves on the walls from the 3D printer passes. Glossy grippers had a much cleaner finish and a higher coefficient of friction. It’s probable that this difference for matte.
Picking Up Objects
For each object the TPG picked up, a human positioned the gripper. The gripper was opened and an attempt was made to pick up the object. If the gripper held the object for 10 s without slippage or dropping the object, the grip was considered successful. Objects that were picked up on the first attempt were considered easy to pick up, 2-3 attempts were considered of medium di culty, and 4-5 attempts were considered di cult. Table 3.2 summarizes these findings, and a picture of the 40+ objects that the gripper picked up is shown in Figure 3.13.
Picking up Common Objects
Convex objects, like the PVC pipe and ball, were easiest for the TPG to hold because the gripper could maintain contact with the surface of the object. Other objects that were easy for the TPG to pick up included the tissue, the needle, the pen, the thumbtack, the batteries, and the pouch.
Thin objects that were also light were harder for the TPG to pick up, but not impossible. The finger nails allowed for the fingers to lift the object (e.g. the ID card) off the table while the gripper was being closed, similar to a person picking up a credit card or sheet of paper. The gripper did have to be closed slowly in order to successfully accomplish this task.
The hardest objects for the TPG to hold were thin object, especially thin objects with an unequal weight distribution and fragile objects. Thin objects, like the ID card and silverware, were difficult for the gripper to pick up for two reasons. First, lifting the object from the table was difficult because the gripper wasn’t able to achieve a large contact area with the object. Second, for thin objects that were heavier, the TPG either could not exert enough force at its tip to successfully pick up the object (e.g. the wrench) or could only grip the object in one orientation (e.g. the knife held parallel to the ground). The spoon, which was not heavy but a thin object with an unequal weight distribution, repeatedly slipped from the gripper regardless of the orientation in which it was picked up. Fragile objects, like the paper cup, would be difficult for the TPG to hold without some sort of feedback.