Figure 1. Sensors in the robot grippers.
Figure 2. Calibration curve
PINHEIRO BECK, Joao Carlos
Av. Ipiranga, 6681, predio 30/169, CEP 90619-900, Porto Alegre, RS, Brasil, PUCRS - Faculdade de Engenharia/Departamento de Engenharia Mecanica e Mecatronica, dem_feng@pucrs.br
Abstract: The present contribution shows the complete description method of robot grippers force determination when it realizes a work operation. A sensor was constructed and coupled in the grippers of the robot with strain-gauge adequated instrumentation. By the system calibration was possible to verify all forces during the manipulation with good sensitivity. At the moment doesn't exist similar arrangement in robots enterprises. The values founded in measurements was compatibles with the theoretical previous in the project of the sensor.
Keywords: robots, instrumentation, automation, forces
The word robot was created by Czechoslovakian dramatist Karel Capek, and the word robot is a derivate of the czech word robota, which means work.
Every mechanical point on a robot, except in the gripper or tool, at which some form of drive induces motion in a robot part is called a degree of freedom. The motion can be of a pivoting nature or a reciprocal motion as is produced by a pneumatic or hydraulic cylinder. In a general manner a robot have six degrees of freedom:
The various degrees of freedom and their types of motion determine the physical configuration of the robot (Ray Asfahl, 1992).
In the present work our preoccupation was the determination the forces surrounded in the grippers.
Three types of sensors in the robot are very important for all operations:
In the present work we make measurements in the gripper force.
Was utilized transducers in steel blades: in these dispositives the force action have an output proportional to the input in the present case we apply strain gauges in the tree manner.
First stage: the force in the load cell makes elastics deformations in dependence of geometry and nature of materials used.
Second stage: the elastics deformations are transformed in electric resistance variations by the straing gauges bonded in the metallic structure.
Third stage: the elastics deformations are very small µm/m, and this manner its necessary an amplification system with low noise.
The proposition in the present instrumentation was a construction of the one force sensor with the objective to measure the force in the robot gripper in all movement and aperture when in hold an object in the space or make a position transference. In this way we constructed two sensors in stell ABNT 8660 with 30mm lengh by 10mm width. They had the c letter form with the extremity jointed and to screw on the robot grippers (Robot Scorbot ER-V made by the enterprise Eshed-Robotec). One strain gauge was bonded in the front part (where suffer compression), and the other was bonded in the back part (where suffer traction). The strain gauge connection was made in wheatstone half bridge and the calibration with dead weights was used, in this form we have good exactitude and sensitivity in the measurements.
Characteristic of strain gauges: electric resistance: (119,8 ± 0,2) W
Temperature coefficient: 0,8±0,5%/100oC
Gauge factor: 2,12±1,0%
Fabrication: KIOWA
Material: stell
After a great number of experiments the values founded had the next characteristics (see figure 1)
Repetitivity: 3%
Sensitivity: 27g/mV/V
Histeresis: 3%
Exactitude: 2%
The calibration curve we saw in the figure 2.
In this work we have made measurements in different situations, another and new work is automatically make the control these forces.
Figure 1. Sensors in the robot grippers.
Figure 2. Calibration curve
After the instrumentation of the robot gripper we have excellent characteristics in terms of: sensitivity, exactitude, histeresis, construction facility and low cust for measure the forces in the grippers. By this arrangements was possible to verify the forces on the grippers when it hold the most different body with various geometry and weights. This information are fundamentals when automatized operations are made since this forces, if strongs area damaging to a sensitive operation.
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