FORCE TORQUE SENSORS

INTRODUCTION TO

Updated 2014-09-09

What is a force torque sensor?

What are the typical specifications for these sensors?

IN THIS LESSON

In robotics, a force torque sensor is used between the robot tool and the robot flange for force controlled applications.

Robotic Force Torque Sensor

Robot tool

Robot flange

FT sensor

As the Force Torque Sensor is located between the robot flange and the robot tool, it measures the applied force and torque at this point.

Robotic Force Torque Sensor

Fx, Mx

Fy, My

Fz, Mz

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There are several different ways to describe this sensor, they all have the same meaning:

Force torque sensorMulti-axis force torque transducer

6-axis load cellFT sensor

Robotic Force Torque Sensor

A 6-axis Force Torque sensor will measure forces along the x-y-z axes, as well as the torque applied around these axes.

Most robotic force controlled applications require feedback from all 6 components of force and torque.

Robotic Force Torque Sensor

The efforts (force and torque) are measured in the reference frame of the sensor.

To be useful in the robot control loop, it is necessary to know the position and orientation of the sensor reference framerelative to the robot tool reference frame. These parameters are defined in the robot controller.

Robotic Force Torque Sensor

The following specifications are the most relevant for typical force control applications:

• Signal Specification– Force and torque measuring range– Precision (resolution and accuracy)– Repeatability– Drift– Noise level– Output rate

• Mechanical– Physical dimension

Each of them are introduced in the following slides.

FT Sensors Specifications

Measuring Range

What does it mean?The range of force and torque that the sensor can measure.

Why is it important?Sensor range needs to cover the effort that will be applied by the tool on the robot flange. For example, if a sanding procedure requires the application of 100 N, the sensor needs to be able to sense more than 100 N.

Fmin Fmax

Resolution

What does it mean?The minimum change in force that the sensor can measure.

Why is it important?The resolution needs to be graded sufficiently to distinguish between events that are pertinent to the robot application.

F

Accuracy

What does it mean?The sensor measures precisely the applied force and torque. If you apply 50 N, do you read 50 N?

Why is it important?Robot control software sometimes use dynamic models in which the actual force value is important.

Repeatability

What does it mean?How repeatable is the sensor when measuring force and torque? If you apply exactly the same force twice, will you read the same value each time?

Why is it important?Some processes need to apply a repeatable level of force with the tool. Think of finishing applications for instance.

Drift

What does it mean?The signal is drifting when the sensor is reading a changing force even if no real force is being applied to the sensor.

Why is it important?The force measurements are used in the control loop. A drift will generate false values that will cause unwanted behaviour. For example, a sensor drift in hand-guiding will cause the robot to move even if nobody touches it.

Noise Level

What does it mean?How much noise is generated by the sensor. This noise is added to the force signal.

Why is it important?A high level of noise will require signal filtering, reducing the effective bandwidth of the sensor and adding delay to the reaction time of the robot.

Output Rate

What does it mean?It is the number of force readings per second that the robot can get from the sensor.

Why is it important?The output rate needs to be sufficient to feed the control loop, so that it remains stable.

What does it mean?The size of the sensor.(diameter, thickness)

Why is it important?It is needed to look at tooling reach and to use as a parameter in the robot controller.

Physical Dimension

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