Rodrigo Romo Becerra: CRS A255 robotic arm

The project objective is to create and design a manual control stage for a robotic arm CRS A255, the robot has 5 degrees of freedom. Originally the robot was controlled by a computer, working as power stage and control stage with his own programming language. The computer was damaged some years ago, the robotic arm has been useless for a long time in the manufacture laboratory at the university.

The idea is to create a power and control stage through a source power, control box, with their own circuits and a connections box to get the robot in function, and this way it can be used for future projects.

Robot Components

The A255 arm has five joints or axes, providing five degrees of freedom. This allows the arm a forceps or other tool to spatial Cartesian coordinates and orientation move defined by X, Y, Z, Z-rotation, Y-rotation, X rotation.

The A255 robotic arm has 5 degrees of freedom (joints). Each joint except the former has a brake which prevents movement of the robot by gravity when the motors are not running, in other words the brake prevents the robot moves by its own weight. Before moving joints 2,3,4,5 should disable their brakes and brake must be activated at the end of the movement.

As an initial step we proceeded to remove the casing of the robot with an Allen wrench, to review the components and connections thereof. motors that were exposed on the lower image correspond to the joints 2 and 3. In the rear of the robot motors you find joints 4 and 5. The motor of the joint 1 is in the inside of the base.

We can see in below image the components of the joint number 3, from right to left:
Encoder, motor and brake.

The movement of the joints is caused by a servomotor, between higher the voltage applied to the greater will be the motor torque and motor speed. By changing the polarity connections in the motor rotates to the opposite direction.

The encoder is a device that allows us to measure the degree that it has moved the motor. It is a very precise device. The operation of the encoder is outside the scope of this project however may be used in future projects.

CHECKING THE ELECTRIC MOTORS OF ROBOT

The objective of this stage is to ensure that each robot servo function correctly. For this robot housing removed exposing its internal connections, then with an external power source is energized each of the cables with different voltages (V).
The following conclusions were obtained:

1. The brakes have no polarity
2. The brakes release at least 10 volts.
3. If the same 10 volts applied to the motor, it move too fast.
4. Agreed to manipulate the motors with a voltage of 5 volts for testing.
5. A cable of each motor and each brake are connected in common.

As a result the number 5 in the previous section, the following problem is created. How to activate the brakes and the motor 10 volts to 5 volts having a common cable?

As first approach it was thought to introduce a new cable in order to have the connections separated and working independently, this is a challenge because you would have to change the physical structure of the robot, without mentioning that would completely disarm which is very difficult.

A subtraction of the voltages was proposed as solution. The Brake does not have polarity does not matter if we apply what part of the positive voltage must be applied at least 10 volts to deactivate the brake, depending on the applied voltage an polarity the motor will rotate at different speeds and directions.

Two current sources were used to feed independently the motor and brake. A H bridge is used to change the polarity to the motor supply and thus the sense of direction of the motor as shown in the figure below.

A H bridge is an electronic circuit that enables a voltage to be applied across a load in either direction. These circuits are often used in robotics and other applications to allow DC motors to run forwards and backwards.