Machine Design

Force/torque sensors help industrial robots make the right moves

Knowing what load levels robots exert is key to more repeatable, safer operation.

By Dwayne Perry
Chief Sensor Technologist
ATI Industrial Automation
Apex, N.C.

Edited by Lawrence Kren

Feedback from force/torque sensors (black cylinder mounted on arm end) gives robots the delicate touch needed for operations such as this camera function test. An industrial PC controls the robot and documents test results.

A robot arm fitted with a force/torque sensor checks actuation forces on an automobile cruise-control switch.

Industrial robots perform myriad tasks including electronic and mechanical assembly, product testing, and material handling. On-board force/torque (F/T) sensors help robots verify part insertion, hold force constant during buffing, polishing and deburring, collect force data for lot testing and statistical process control (SPC), and other functions.

Such systems consist of a robotmounted transducer connected by a flexible cable to a sensor interface controller. The transducer converts force and torque loads into straingage signals and transmits them to the sensor controller. The sensor controller performs computations on these force vectors and directs them to the robot.

Some basic guidelines will help weed through available sensor systems. First, characterize the application: How will the robot be used? What force range will it experience? In what environment will it operate (laboratory, assembly line, etc.)? Once these questions are answered, next focus on components.

Calculate expected moments and forces. Moment capacity typically sizes a transducer. A robot end-effector attached to a transducer generates forces when performing a task.

The measured moment equals applied force multiplied by the distance from the transducer origin to the point at which the force acts.

It is important to consider overload conditions as well as normal operating forces and moments. Include all loads the transducer sees including those the application doesn't monitor. Be aware that published robot payloads and the associated positional resolutions are typically maximum values. Robots can handle and create much larger loads, but with some loss of positional repeatability. Moreover, robots are typically overpowered for an application and are capable of exerting loads many times the rated capacity. Deceleration from inadvertent crashes often generate large inertial and impact loads, for instance. Even E-stops can produce 5-g decelerations.

A transducer's factor of safety against damage from overload depends mostly on the type of strain gages used. Transducers fitted with high-output strain gages can be made to withstand higher overloads than designs using lower output types. High-output strain gages can also have lower noise levels because they require less signal amplification. For example, silicon strain gages provide a signal 75 times stronger than conventional metal foil gages.

Next, identify transducer capacity. Select a transducer and calibration based on minimum and maximum orthogonal forces (Fx, Fy, Fz) and torques (Tx, Ty, Tz), weight, and physical size. Sensor makers typically provide tables to cross reference measurement ranges with available transducer types.

Resolution and accuracy are the next considerations. As a rule, transducers with a finer resolution have lower moment capacity and vice-versa. In all cases, transducer output resolution is much finer than absolute accuracy so be sure that the absolute accuracy level fits the application. Multiaxis transducers have an absolute accuracy expressed as a percentage of full-scale load for each axis.

Sensor controllers receive information from the transducer and produce resolved force and torque data. Onboard software multiplies strain-gage vectors by a calibration matrix to form three orthogonal forces and torques. The force and torque data transmit to the robot as control signals. Most commercially available sensor controllers can output six load axes, do tool transformations that move the center-of-origin to a user-specified location, and detect and store peak F/T values. A biasing function subtracts unwanted loads from readings and data filtering minimizes vibrations. Some systems have a programmable threshold monitoring system with high-speed optically isolated I/O that pipes to the robot's discrete I/O panel.

When selecting a sensor controller consider output format, output resolution, and available interface software. Common output formats include RS-232, analog voltage, and computer bus (ISA, PCI, CompactPCI, VMEbus, PCMCIA, USB, and IEEE-1394). Resolution, noise rejection, and interface software vary by model and manufacturer so check system specs carefully.

There are two basic sensor controller types: Stand-alone and computer bus. Standalone sensor controllers are self-powered and self-contained. They typically communicate with the robot controller via an RS-232 serial port, by analog voltages, or by a combination of the two. The sensor controller's discrete I/O connections ease connections to PLCs and other industrial equipment.

Computer bus sensor controllers target a specific computer backplane architecture and plug into the robot or computer mother-board. Communication is through software drivers such as ActiveX for Windows platforms, or directly to I/O mapped registers. The computer bus sensor controller can locate inside the robot system and therefore has a much cleaner appearance than the standalone type. Typically, software provided by the sensor controller manufacturer can display the F/T information for all six axes simultaneously, allowing users to modify various measurement parameters and examine robot loads.

How the F/T information will be used often dictates the sensor controller type. Such data can be collected and analyzed to ensure process consistency or provide real-time force control and threshold detection.

For data collection, a computer bus sensor controller tends to be the easiest to integrate with PCs. It communicates directly with offthe-shelf software such as LabView and Visual Basic. However, data collection speeds can be influenced both by computer speed and the Windows operating system.

Real-time force control is possible by integrating software drivers with an ISA bus sensor. All F/T data is available on the computer bus, giving control software instant access. When users are not working in a PC environment, analog outputs from stand-alone sensor controllers can interface to any analog input card.

Force and torque threshold or limit detection are available on some types of sensor controllers. These features allow the sensor controller itself to monitor transducer loads for specific loading conditions and then notify the robot controller when these conditions exist, thus relieving the robot controller of the task. For example, the sensor controller can check for dangerously high loads and use its discrete output to trigger the robot E-stop circuit.

A manufacturer-supplied cable typically connects a transducer to the sensor controller. The cable should reach from the sensor controller to the transducer in any robot position. It is best to err on the side of a longer cable rather than risk breaking it and possibly damaging the system. There are several ways to mount a transducer to a robot such as quick disconnects, standard or customized interface plates, and others. Most F/T transducers are ruggedized for industrial use, but this factor should still be considered in the selection process. Customized sensor systems can be built to tolerate extreme temperatures, nuclear radiation, and intense magnetic fields associated with MRI.

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