5-DOF Robotic Arm
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INTRO-
A 5- Degree of Freedom robotic arm with use of planetary gearboxes, compound gearboxes and pulley systems.
Multi-dexterous arm with a continuum gripper to hold irregular objects such as an egg, phone, plant and more
The gripper was designed in such a way that it does not drop the object if power goes away.
BREAKDOWN OF THE ROBOT
1
2
3
4&5
1. Shoulder Pitch Planetary Gearbox
The shoulder pitch movement allowed the arm to reach the farthest point when extended.
The torque requirements at this joint was the highest and this was achieved by using a 6:1 torque multiplier. A 2:1 pulley system was designed compunded with a 3:1planetary gearbox helping achieve a 6:1 gear ratio. All the drivetrain components were custom made. The gears and pulleys were 3D printed, while the hubs carrier plate and shaft was machined manully using lathe and bridgeport mill. The gears were optimized to have minimum backlash and were printed out of PLA with 5 perimeter walls strong enough to survive the project needs.
Appropiate PTFE bronze bushings were added to teh sun and the planet gears. M6 shoulder socket screws were used as planet gear pins and the carrier plate and hubs were manually machined after cutting them under waterjet.
2. Base-Rotation Planetary Gearbox
The base rotation was achieved using a 5.25:1 planetary gearbox mated to a NEMA 21 stepper motor combined with a TB6600 driver. The gearbox was custom designed and chunky gears were used to survive the heavy mass and resist the tipping movemnet that the arm would produce due to shit in centre of mass. A 4" turntable bearing was used and custom pins were made out of 5/16" steel rods. On one end M4 threads were added to screw them with the turntable bearing and the bottom steel plate and the other end had male threads to attach the aluminum base plate to. Each planet gear had 608-2RS tight fit bearings for smoother rotation and silicone grease was added to reduce friction and prevent binding of gears.
3. Elbow Pitch Compound Pulley System
The elbow pitch movemnet was essential for the robotic arm to reach different heights and to pick the objects.
This motion required high torque multiplication as the placement of motor limited our ability to use only a NEMA 17.
20:1 torque multiplication was achieved by using a compound pulley system. The first stage pulley helped achieve a 4:1 multiplier which them transferred the motion to a shaft, this shaft on the other side was connected to another 5:1 torque multiplier and therefore helped us achieve a 20:1 torque multiplier. To prevent the 8mm steel shaft from binding into the aluminum tube, low friction teflon bronze bushings were used with some silicone grease appiled to them.
In order to tension stage 1, slots on teh aluminum tube were drilled to get maximum tension while on stage 2, a fixed pin was used to achive the required tension in teh belt. This was one of the major problems we faced and a better adjustable design woudl have helped deliver most amount of torque using the GT3 belts.
4. & 5. End-Effector Differential Gearbox
To achieve the last 2 degrees of freedom, a differential gearbox system was desigend. I designed teh bevel gears in Solidworks and using 2 MG 995 continous servos and pulleys I was able to turn teh bevel gears and achieve pitch, roll or a combination of both by varying the speed and direction of rotation.
In order to tension the pulleys the waterjetted aluminum arms had slots. The two aluminum plates got their rigidity froma box structure by using steel studs and 4 set of nuts for each stud. The studs also acted as anchor points for the 3D printed motor mounts. This was directly connected to the gripper's base which helped it achieve roll and pitch. Silicone grease was used to reduce the friction and bronze bushings were used to prevent the 3D printed gears from cutting because of aluminum.
Continuum Gripper
The continuum gripper was a really successful part of the project. Its flex design helped grip almost anything without breaking it, like holding a raw egg. It could wrap around a bency, water bottle, a smartphone, a plant set of allen keys, pens etc. The ratcheting mechanism helped prevent opening of the grip of power was lost. This means that the opeing and closing of the 'fingers' was only possible if there was power to the system.
Combined with the differntial end effector and a high degree of freedom arm, we could pour water from a water bottle into a cup. The two motors basically controlled the opening and closing of the fingers and tension respectively. The tension and motion was transferred to the fingers using a winch mechanism facilitated by a fishing line. The yellow rubber band provided the required elasticity. The tension required to grasp an object, however, was not calculated due to the non-linear properties of rubber.
Videos of the Robot
