How-to-Guide / Robotics Assembly
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How To Select a Robotic Collision Sensor

March 5, 2014
Trans

The Purpose of a Robotic Collision Sensor

Robotic collision sensors are used to help prevent damage to the expensive custom tooling attached to the end of a robotic arm in the event of a collision with another object. These collisions commonly occur during initial robot teaching but can also occur during production for a variety of reasons.

A collision sensor should not only alert the robot that a collision has occurred but also provide a mechanism that can prevent or minimize the damage resulting from this collision. This compliant mechanism is normally held in the working position through the use of compression springs, a pneumatic piston or a combination of both. Units with a pneumatic piston allow you to adjust the force required to signal a collision and are the most versatile.

Cell uptime is critical and collision sensors that reset automatically do not require manual intervention to get the cell up and running again. Also, if the obstruction can be mechanically cleared, no operator need enter the robotic cell adding an additional level of safety.

Choosing the Proper Size Collision Sensor

In order to successfully match a collision sensing device to a specific application, it is critical to consider the loads produced by the static weight of the tooling, the inertial loads imposed by robot motion, and the loads produced by the end-effector when performing its intended task.

Calculating the Loads:

The selection process must begin with the calculation of the static, dynamic, and working loads. Loading will vary with the orientation of the load relative to the collision sensor and acceleration or deceleration of the load. Finding the point in the robots motion that induces the maximum load on the collision sensor and calculating the associated loads is the key to selecting the appropriately sized collision sensor.

Static: The load applied by the weight of the tooling and any attached part(s) while the robot arm is stationary. Moment, axial and torsional loads must be considered. Moment and torsional loads consist of a weight times a distance.

Dynamic: The inertial force imposed at the center of gravity of the tooling due to acceleration of the robot arm. Dynamic forces are additive to static forces and often account for the largest forces acting on the collision sensor.

Working: Forces are generated by the process under normal working conditions. If these forces and their location are known, they can be converted into loads on the collision sensor using the same technique.

Selecting an Appropriately Sized Model:

Manufacturers of collision sensing devices usually provide either graphs or tables and formulas to convert the moment, torque, and axial forces applied to the end-effector tooling into the amount of air pressure required to support these loads on each model.

The sum of these pressures must fall within the operating pressure range of the unit. Since it is usually difficult to accurately calculate all forces acting on the device, it is advisable to choose a unit where the calculated operating pressure does not fall near the upper or lower end of the range. For example if the required pressure is 80 psi on one size unit, 40 psi on the next larger size unit and the maximum pressure rating of both units is 90 psi, it is best to choose the larger unit. The exact pressure required can be determined experimentally during system integration.

Conclusion

Robotic collision sensors can save money by protecting expensive tooling and fixtures. They also increase run time by facilitating rapid recovery from a collision. Selecting a collision sensor requires collecting information about the loads and forces that will be acting on it. Calculations are then performed and a unit selected. To insure that this is done accurately, have your supplier perform the calculations and select the appropriate model.

For more information, visit ATI Industrial Automation at www.ati-ia.com

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