Sensor manufacturers often provide selection tables that cross-reference measurement ranges with available transducer types.
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Sensors and transducers measure and analyze changes off a system norm. Force and torque transducers generate output signals from vertical, lateral, and longitudinal forces and camber, steer, and torque moments. That’s handy on robots that rely on such information to successfully manipulate the surrounding environment. Also called F/T sensors, these devices sense up to six degrees of freedom and are critical to electrical and mechanical assembly, product testing, material handling, and other robotic applications. They verify part insertion, hold constant force during buffing, polishing, and deburring, and collect force information for lot testing and statistical process control. Utilizing strain gages or optical sensors, the sensor controller collects transducer strain gage vectors, performs computations, and outputs F/T data directly to the robot.

F/T Sensor selection

Many sensor systems for robotic applications are available, so selecting the right one is sometimes a challenge. Before choosing it’s important to fully define the application. How will the robot be used? What range of force will it experience? What is the environmental condition of the application — laboratory, assembly line, or someplace else? Once the application is clearly identified, components for the sensor system can be specified. Following a few steps and comparing transducer specifications to those of the application ensures optimized performance.

To select a transducer, first calculate expected moment and forces. Attached to the transducer is an end-effector that generates forces as it performs its task. Coupled with this force, the distance of the applied force from the transducer results in a moment. (Moment is the applied force multiplied by the distance from the transducer origin to the point at which the force is applied.) It is important to consider both overloaded and normal operating forces and moments affecting the transducer. In fact, moment capacity is usually the determining factor when choosing the best transducer for an application.

When calculating load on the transducer, be sure to include all loads the transducer will experience, including those loads the application does not monitor. Be aware that the published payloads of robots are typically the maximum loads for a published positional resolution. Because they’re typically overpowered, robots can actually handle and create forces many times greater than their load rating. During a crash, the inertia of the sudden deceleration can generate large loads, not to mention the force of impact. Typical robots can handle these conditions, generating 5 gs of deceleration during emergency stops. Handling greater forces does come with some loss of positional repeatability.

A robotic sensor system consists of a transducer mounted to a robot, with a sensor interface controller connected to the transducer by a high-flex cable.

Strain gage sensing technology can also influence the transducer’s factor of safety. Transducers using high-output strain gages are designed to withstand higher overload conditions than those of lower output strain gages. High-output strain gages can also have lower noise levels, since they require less signal amplification. For these results, silicon strain-gages provide signals 75 times stronger than conventional foil gages.

Identify transducer capacity. Minimum and maximum force and torque, weight, diameter, and height must be known to select the correct transducer model and calibration. Typically sensor manufacturers provide selection tables that cross-reference measurement ranges with available transducer types.

Verify resolution and accuracy. Fine resolution requirements can conflict with moment capacity requirements. Transducers with larger ranges have coarser resolutions. A transducer’s output resolution is much finer than its absolute accuracy, so be sure the absolute accuracy fits the application. Like single- axis load cells, the absolute accuracy of six-DOF transducers is expressed as a percentage of rated full-scale load for each axis.

Sensor controller selection

Sensor controllers receive information from the transducer and produce resolved force and torque data. Onboard software then calculates the output data by multiplying the strain-gage vector by a calibration matrix to form the F/T data — consisting of three orthogonal forces and torque. After this step, force and torque data can be transmitted to the robot and serve as instructional signals to help the robot perform its intended function.

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