Sensors improve robot performance

Force/torque (F/T) sensors are critical components of robot systems used for assembly, test, and other applications. Sensor systems help robots verify part insertion, apply a constant force during assembly and test operations, and collect test and quality data. With the right F/T sensor system, you can improve a robot's performance.

A robot's sensor system consists of the transducers mounted on the robot, the cabling, and the controller. The strain gages in a transducer convert force and torque into electrical signals and transmit them to the sensor controller. The sensor controller collects transducer strain-gage vectors, performs computations, and outputs F/T data directly to the robot.

Before selecting the components of a sensor system, you must first consider your application: Determine how much force the robot will experience in operation, and study the environmental conditions in which the robot will operate.

• Calculate expected moment and forces. Moment capacity, or torque, is usually the most important specification of an F/T sensor. The moment is the force applied to an end effector, such as a gripper or manipulator, multiplied by the distance from the transducer to the point at which the force is applied. Be sure to account for overload conditions as well as normal operating forces and moments the transducer will experience.
  Next, calculate the total load on the transducer, including all loads that the robot will not monitor. A robot's specified payload is typically the maximum load the robot can handle and still achieve the specified positional accuracy. Normally, a robot can handle and create much larger loads than specified, but with some loss of accuracy. Robots are typically overpowered for an application, and the robot is capable of exerting loads many times its rated loading. During a crash, the inertia of the sudden deceleration can generate large loads, not to mention the force of impact.
• Identify transducer capacity. Once you've calculated the loads, compare them to transducer specifications, including minimum and maximum force (Fx, Fy, Fz), minimum and maximum torque (Tx, Ty, Tz), weight, diameter and height. Values for typical transducers are shown in Table 1.
• Verify resolution and accuracy. A fine resolution requirement can conflict with a transducer chosen based on moment capacity. Transducers with larger ranges have coarser resolutions. The output resolution of a transducer is much finer than its absolute accuracy—be sure the absolute accuracy fits the application. As for single-axis load cells, transducers with six degrees of freedom have their absolute accuracy expressed as a percentage of their full-scale load for each axis.
• Compare transducer specifications with the application specs. After completing the three steps above, it should be clear which transducer to choose. For example, if you calculated the maximum load to be 55 lbs of force and your end-effector is 8-in. long, the maximum torque needed is 440 in.-lbs. Comparing these requirements to the values in Table 1, the obvious choice would be Model E.

To properly control a robot's end effector during a test or assembly operation, you must accurately measure the force and torque on the effector.

Two types of sensor controllers are available—stand alone and computer bus. Stand-alone controllers are self-powered and self-contained. To communicate with the robot controller, stand-alone controllers may have an RS-232 serial interface or an analog interface. This type of controller normally has I/O connections that let you easily connect it to PLCs and other industrial equipment.

Computer-bus controllers plug into a computer motherboard, including the computer that controls the entire robot system. These controllers communicate with a robot controller or other computer controller over the bus. To use this type of controller, you will need to have a software driver for the controller that works with the operating system of the host computer.

The type of sensor controller you select will often depend on how you plan to use the F/T information. You can use this data in several ways, including data collection and analysis, real-time force control, and threshold detection. If you must collect the F/T data, selecting a computer bus controller may be your best bet. Installed directly in the PC, it can communicate with standard applications, such as LabView and Visual Basic.

Computer bus controllers can also be used when you need real-time force control. Since F/T data is available on the computer bus, the control software will have access to the data and can make control decisions based on it.

Some F/T sensor controllers have force and detection capabilities that allow them to monitor transducer loads and notify the robot controller when the loads exceed the specified limits. When the controller detects a dangerous load condition, it can trigger the robot's E-Stop circuit. By moving this monitoring function to the sensor controller, the robot controller is relieved of the monitoring task, which should improve overall performance.

Whatever transducer you select, remember that you'll have to connect it to the controller. Keep in mind that the cable must be long enough to reach from the sensor controller to the transducer in any robot position. Also, make sure that the transducer is rugged enough for your application. While most transducers are durable enough for the majority of applications, you also have to consider how you will mount the transducer and what will be attached to it.

Finally, working closely with sensor manufacturers will help you choose the right sensor for your application. Contact a company that has specially trained technicians who understand your application. They will recommend either standard sensors or custom sensors that will meet the requirements of your application.