To design a linear drive system to the robotic gripper that will have both a slow close and quick snap open, the team first setup their design constraints. The design constraints of the project are shown below.
Design Constraints
- Snapping Motion without a ramp up period
- No pneumatics
- Single motor control
- Spin CCW to close
- Spin CW to snap open
- Provide user control on the exact time to release
- Instant sliding block velocity of 1 m/s when the mechanism releases
The listed constraints above were formulated internally amongst the team. The snapping motion without a ramp up period was set by the team because if you were to try and toss a water bottle with your fingers, those fingers would actually need to release almost instantaneously at the right moment to match the swinging motion of your arm with the opening motion of you thumb and index finger let go of the water bottle. This constraint therefore eliminated a quick release linkage system, shown in Figure 1. This type of mechanism does release quickly, but not until the link being driving by the wheel with a dowl is aligned directly above the drive wheel's center of rotation.
The next constraint limiting the use of no pneumatics was set based on the fact that it is not common for many electro-mechanical robotic arms to be equipped with a supply of compressed air nor the hosing need to supply this air at the end of the robotic arm. Pneumatics are great for a snapping motion, but without a supply of compressed gas you are going nowhere.
Single motor control was another constraint that the team limited themselves to because of two things, one in weight of the overall drive mechanism and the other is control. Typically, a robotic arm does not have extensive strength for the amount of weight they can manipulate and move around. By adding a heavy end effector to the end of the arm, you will be decreasing the amount of weight the arm has left over to manipulate objects in the world. The second reason for a single motor is control. robotic arms generally only have one power supply at the end of the robotic arm for just the addition of a single motor. Plus having one motor means the team can provide easier control to the user as they only need to control the direction of the motor. The intent of the design is to have the user just need to swap the direction of the spin on the motor itself and the gripper will act accordingly.
Providing a user the ability to release the water bottle at the right moment is key, as mentioned earlier. Without the ability to time the snap open correctly, flipping a water bottle would be almost impossible. Using a typical linkage mechanism quickly release the linkage system would be very challenging, as the point as to where the motion turns out to snap would need software that knows the motors position, which would need to be based on encoders from a motor, which are not always accurate. Therefore a software based release procedure would not be robust and could lead to the user dropping the water bottle at the incorrect moment.
Instantaneous sliding block velocity of 1 m/s was derived based on a paper