The major uses of sensors in industrial robotics and other automated manufacturing systems can be divided into four basic categories :
- Safety monitoring
- Interlocks in workcell control
- Part inspection for quality control
- Determining positions and related information about objects in the robot cell
One of the important applications of sensor technology in automated manufacturing operations is safety or hazard monitoring which concerns the protection of human workers who work in the vicinity of the robot or other equipment.
The second major use of sensor technology in robotics is to implement interlocks in workcell control. interlocks are used to coordinate the sequence of activities of the different pieces of equipment in the workcell.
In the execution of the robot program, there are certain elements of the work cycle whose completion must be verified before proceeding with the next element in the cycle.
Sensors, often very simple devices, are utilized to provide this kind of verification.
The third category is quality control. Sensors can be used to determine a variety of part quality characteristics. Traditionally, quality control has been performed using manual inspection techniques on a statistical sampling basis.
The use of sensors permits the inspection operation to be performed automatically on a 100 percent basis, in which every part is inspected.
The limitation on the use of automatic inspection is that the sensor system can only inspect for a limited range of part characteristics and defects.
For example, a sensor probe designed to measure part length cannot detect flaws in the part surface. Many applications of automated inspection are accomplished without the use of robotics.
The reason for including this category in our discussion of robotic sensors is that robots are, in fact, often used to implement automatic inspection systems by means of sensors.
The fourth major use of sensors in robotics is to determine the positions and other information about various objects in the workcell (e.g., workparts, fixtures, people, equipment, etc.).
In addition to positional data about a particular object, other information required to properly execute the work cycle might include the object’s orientation, color, size, and other characteristics.
Reasons why this kind of data would need to be determined during the program execution include:
- Workpart identification, and
- Random position and orientation of parts in the workcell
Accuracy requirements in a given application exceed the inherent capabilities of the robot. Feedback information is required to improve the accuracy of the robot’s positioning.
An example of workpart identification would be in a workcell in which the robot processes several types of workparts, each requiring a different sequence of actions by the robot.
Each part presented to the robot would have to be properly identified so that the correct subroutine could be called for execution.
This type of identification problem arises in automobile body spot-welding lines where the line is designed to weld several different body styles (e.g., coupes, sedans, wagons).
Each welding robot along the line must execute the welding cycle for the particular body style at that station.
Simple optical sensors are typically used to indicate the presence or absence of specific body style features in order to make the proper identification.
An example of the part position and orientation problem is where a robot would be required to pick up parts moving along a conveyor in random orientation and position. and place them into a fixture.
To accomplish the task, the exact location of each part would have to be sensed as it came down the line. In addition, for the robot to use a mechanical gripper to grasp the particular workpart, the orientation of the part on the conveyor would have to be determined.
All of this information would have to be processed by the workcell controller (or other computer) in real time in order to guide the robot in the execution of its programmed work cycle.
Vision systems represent an important category of sensor system that might be employed to determine such characteristics as part location and orientation. We consider machine vision to be a topic in robotics which is significant enough to merit its own chapter. That chapter follows this one.
In some applications, the accuracy requirements in the application are more stringent than the inherent accuracy and repeatability of the robot.
Certain assembly operations represent examples of this case. The robot is required to assemble two parts whose alignment must be very close, closer than the accuracy of the robot.
One possible solution that might be used is a remote center compliance (RCC) device.
All four categories of sensor applications (safety monitoring, interlocks, inspection, and positional data) are instances where the sensor constitutes a component of a control system used in the robot work cell to accomplish some specific control function.
That control system, in turn, is a component of a larger control system which we are calling the workcell control system. All of the control functions which takes place in the workcell are coordinated and regulated by this larger system.