Robotic Arm Technology Development for Research and Mars Exploration

RUDRA ROBOTICS is working on Mars rovers and robotic arm from past 7 years. The team rudra Srm Robotics Rover looks for the best spot for collecting the soil sample and do the research and experimentation of the Martian soil for the moisture quantity, temperature and pH. The rover is being able to perform so many tasks which will help the astronauts like accumulation of tools and objects via rover and deploying them to the coordinates provided. The robotic arm is assembled on the mobile rover and YPR (yaw, pitch and roll) is being assembled on the arm along with the mechanical gripper attached to it. During R&D phase, the team focused on designing an arm which was efficient and simple and it could also be used in industry and was successful in doing so.

Mechanism

The mechanism of this robotic arm is a four-bar mechanism which lets the arm to orient the gripper parallel to the surface. The arm is a connection of two parallelograms in a way that on a minimal actuation of the electronic actuators we get maximum actuation of the end effecter. The arm has a separate compact mechanism that provides the YAW, PITCH, and ROLL motions, giving six degrees of freedom to the arm. The arm is capable of turning angle greater than 360ᵒ around the fixed vertical axis. The gripper is a power screw mechanism that converts the actuator's rotary motion to a linear motion of a lead screw and further to parallel motion of the two claws of the same. The entire arm uses 7 electrically driven actuators, 5 rotary and 2 linear. The heavy weighted instrument on the arm are placed near fixture so as to avoid stress on arm due to self weight and also maintains the centre of gravity of the arm on near the rotational axis of the arm.

Arm

The robotic arm is mounted on a turn table consisting an external gear of module 2 and link fixtures. Turn table rotates in a radial bearing to counter both axial and radial load of the arm. The two links of the arm are connected with each with help of shafts and a mounting plate, two parallelograms are formed by both the links. The effective length of the 1st link is 500 mm and that of the 2nd link is 350 mm. The four-bar mechanism of the arm allow the gripper to perform tasks easily. One actuator is mounted on the turn table while the other one is mounted on the first link. 1st actuator provides actuation to the first link and 2nd actuator provides actuation to the second link. Both the actuators are having a stroke length of 100 mm.

Yaw-Pitch-Roll

The YPR is compact and made using materials like aluminium and MC nylon(MC901) actuated using three servo motors. The servo motors best suits for this arm in torque management, precision and lightness as compared to other comparable precision motors. One servo is used with a gear ratio 1:6 which is responsible for the yaw motion of the gripper and the other two motors having a gear ratio of 1:3 are responsible for pitch and roll motion. The complete YPR system is bolted on a 1:6 servo motor gear which gives 180ᵒ rotation to the YPR system which acts as the yaw of the end effecter.

This servo can also in give roll to the gripper when pointing downwards to lift heavy objects and rotating them. The assembly includes the two bevel gears screwed to the servo motor gears placed parallelly facing each other and connected to each other using a third bevel gear larger in diameter especially for torque increment and reduced rpm. Two parallel aluminum shafts sits in a bearing hub at one end and the other end fixed rigidly to the servo motor shaft. Third shaft transverse to the other two shafts passes through the third bevel gear and sits in the bearing hub.

The bearing hub is a specifically designed aluminum part which encapsulates three bearings, holds the three shafts and keeping the bevel gears meshed. The gripper is bolted to the third front facing bevel gear. The two servo motors are calibrated to give pitch and roll motion to the gripper. When both the actuators are given actuation in same direction pitch is obtained while actuating in different directions gives roll motion. The gripper is bolted to the third front facing bevel gear.

Gripper

Grippers are designed with special characteristics for specific applications. To help the astronauts on mars and performing various activities such as collecting rocks for analysis, the gripper was designed on Solid Works software. A 12V D.C. motor was coupled with the lead screw using an aluminium coupler. A mild steel part is linked with the lead screw. The rotational motion of the D.C. motor actuating the lead screw gives translator motion to the mild steel part.

As the D.C. motor rotates clockwise the part coupled with lead screw moves upwards making the fingers open. Anticlockwise motion of the lead screw moves the coupler downwards closing the fingers of the gripper. The gripper is designed to grab objects up to 70 mm in diameter. The gripper is capable of picking loads up to 5kg between the 25mm thick fingers with uniquely designed contour faces to allow maximum contact with the object.

One rotation of the D.C. motor gives a translation motion of 2mm to the coupler. This 2mm movement gives an actuation of 4.5mm to the gripper fingers. A four-bar mechanism is used for the actuation of fingers through the coupler to parallel move the fingers.

Stress Analysis

Analysis of the robotic arm was done on CATIA V5. Static load of 120N was applied on the two links of the arms for deformations. The results obtained showed maximum deflection of 0.00739mm.

The YPR was given a load of 75N. Results showed a maximum deflection of 0.026mm in the positions of the servo plates.

Fabrication

The robotic arm is being fabricated by using aluminium box channels of cross-section 20x20 mm. Fine grade of aluminium 6061 is used for fabrication. The length of link 1 is 500 mm and that of link 2 is 350 mm. Each link is a four-bar mechanism in itself providing more flexibility to the arm.

The two links are connected with each other using 2 parallel aluminium plates of thickness 4 mm, the plates have been cut in the form of a triangle to meet the design requirements, the plates are having three holes to accommodate the box channels of the arm using the mild steel shafts of 8 mm.

Two electric linear actuators are being used to provide the motion to the links, stroke of each actuator is approximately 100mm.

The YPR was fabricated using aluminium block which was milled to get the desired product. The bevel gears were fabricated using MC Nylon (MC901). Shafts for the YPR are 9mm hollow shafts held between the bearing and the servo shaft preventing the displacement of the gears.

Major parts of the gripper were fabricated by milling aluminium blocks. Links for four-bar were made using mild steel. Fingers were 3D printed and plates were cut out of SS sheet. Shafts of 4mm of mild steel were used of different sizes.