Manipulator

Four-Degree-of-Freedom Robotic Arm

Introduction:
The robotic arm is a private project done for self study of both mechanical and electrical hardware. It consists of a 4-DOF manipulator and a gripper assembly. The manipulator is actuated by four PMDC motors (wind shield wiper motors). The gripper is actuated by a dc linear actuator (central locking system actuator). The 3 joints of the robot (i.e. the elbow, the wrist and the shoulder) are actuated by the motors. The controller box consists of switches and relay arrangement, to run the motors in clock or anticlockwise directions.

On our left is the control box. On our right is the power supply box

Actuators:
The 4-degree-of-freedom manipulator is achieved by 4 PMDC motors:
MotorNo.--Joint--Manufacturer--Connection
1--Shoulder (Base or Swivel)--Denzo (car)--Direct
2--Shoulder (Pitch)--Denzo (car)--Direct
3--Elbow--Denzo (car)--Co-axial cable
4--Wrist--Bajaj (3 wheeler taxi - Rikshaw)--Co-axial cable

Motor 1 and Motor 2 are directly mounted and connected to the shoulder joint. Where as Motor 3 and Motor 4 are connected to the respective joints via cable drive to reduce the weight at the joints. There is no position or velocity feed back from any of the joints.


Gripper assembly:
The gripper assembly consists of:

• 12V DC, Linear actuator.
• Gripper Frame.
• Jaws.
• Links.

The gripper has two jaws connected to the gripper frame via sets of parallel links. The parallel link mechanism ensures that the face of the jaws always remains parallel to the each other. The jaws are actuated by using the linear actuator. The range of the gripper can be adjusted by adding attachments to the face of the jaws. The gripper frame is attached to the wrist joint via fasteners. There is no feed back from the gripper.

Base Frame and Links:
The base frame consists of a mild steel of 300 mm diameter and weighing about 20 Kg.
The manipulator is mounted on a platform approximately 160 mm from the base frame. The shoulder joint is formed on this platform. All the other motors are mounted here. To protect the base motor (or swivel motor) from axial load, a thrust bearing is fitted just under this platform. The elbow joint is connected to the shoulder joint via a mild steel pipe of 1 inch diameter. The wrist joint is again connected to the elbow joint by a fiber rod. The gripper frame is attached to the wrist joint.

Power supply:
It has a (230V / 12-0-12V, 5A) transformer, with a rectifier circuit using, 2 IN5402 diodes and one 4700µF/ 63 V condenser.
See appendix for details.

Controller box:
It consists of 10 switches, 2 for each motor. Four pairs of switches for the motors at the joints and one more pair for the gripper linear actuator. The switches are push-to-on type i.e. when the switch is pressed the power is supplied.

• 2 pole relays – 5 no.
• 1 pole relays – 5 no.
• Push-to-on switches – 10 no.
• IN4007 diodes – 5 no. (Plus 2X5 free wheeling IN4007 diodes)
  See appendix for details.

Lessons learnt:

1. The inertia of the base platform is too high as a result the reflected torque when the base motor stops is also high, which could damage its gears. Hence the other motors could be placed on the base frame instead of the rotating platform.
2. The shoulder joints exhibit excessive jerk. This is partly because of the high velocity and partly because of the rigid connection between motor and shoulder joints. Not much jerks are observed on the Elbow and Wrist joints. This is because these joints are driven by cable drive. The cable, by virtue of its elastic nature absorbs the shock on stopping. Hence cable drive can be used on all the joints and thus try to reduce the jerks to a minimum.
3. The power supplies needs to be of higher current value e.g. 12V 7A. The existing power supply has a current limit of 5A as a result; running more than one wiper motor is not possible. And a separate power supply for the gripper.
4. The motion of the arm in general is quite jerky; one of the reasons for this is that the motors are controlled by a human operator. Solution is to use a microcontroller or an interface with the PC.
5. The gripper actuator needs to be constantly powered to hold an object. But this will lead to the motor burning out after some time. Some arrangement by which the motor is prevented from moving when not powered needs to be implemented.e.g. worm gear.
6. Limit switches can be used instead of mechanical stops. This coupled with dynamic braking can stop the motors immediately.

Video:

The video is available at this link http://www.esnips.com/web/4dofm

Acknowledgement:

1. Balaji R. RBIN

2. Anish M. CMU

Appendix:

1. Power supply:

2. Controller circuit for one PMDC motor:

3. Pictures:

• Wrist

• Shoulder (Pitch)

• Shoulder (Swivel)

• Elbow

• Gripper

• Controller