Robotics 2017 11 FT Sensors - polito.it · 2017-02-18 · Strain gauge bases sensors are the most common force sensing technology in robotics. These are either used individually,

ROBOTICS

01PEEQW

Basilio Bona

DAUIN – Politecnico di Torino

Force/Torque Sensors

Basilio Bona 3ROBOTICS 01PEEQW - 2016/2017


http://robotiq.com/products/robotics-force-torque-sensor/

� Many Force/Torque (FT) sensors consist of strain gauges attached to a piece of steel. When a

force is applied, the material bends and the resistance of the strain gauge changes.

� With most modern FT sensors, the internal electronics take care of all the calculation and signal

cleaning and produces a digital force signal for all six axes

� However, strain gauges are not the only sensing technology used in 6-axis FT Sensors.

� Capacitive sensing technology can be hugely effective for robot force sensors; capacitive 6-axis

FT Sensors overcome many of the problems with strain gauge technology.

� 6-axis FT Sensors have two ends: a fixed casing and a floating plate. The fixed casing is usually

attached to the end of the robot arm, while the floating plate is attached to the end effector or

tool. Inside the FT sensor, the plate is attached to the casing by a stiff but compliant metal

structure, which bends slightly when a force is applied.

� Various sensors are fitted to the structure to detect the deflection of the material. In traditional

strain-gauge FT sensors, this compliant structure usually consists of three metal beams, each

fitted with several strain gauges.

� In capacitive sensors, it consists of an intricately machined metal block, which is designed to

deform in a particular way under force. This structure is attached to two metal plates (one at

each end) which hold a set of capacitive sensors to measure the displacement of the plates.



� The most basic force sensor is a simple Force Sensitive Resistor. They

are cheap (some as low as 5 USD), and, in general, they are not suitable

for precision measurements. They are good for detecting if a force has

been applied or not; the error may be as high as 25%, but if that is good

enough for your application, then it is certainly an economical option.


FT Types: Resistor

� Another type of sensors are flexible force sensors. While they may look

similar to the previous option, and work on similar principles, they

generally have a better accuracy and are also more expensive (more in

the range of 20 USD for a resistive flexible sensor).

� For example, the Flexi Force is a popular brand of flexible resistive force

sensors which can have errors of less than 3% and support loads up to

445N.


FT Types: Flexible Force Sensor

� They are adaptable and fit into small spaces,

but only detect forces applied at one point

and in one direction.

� Capacitive flexible force sensors are a similar option, which can be a bit

more accurate over a smaller range, but are also more expensive


FT Types: Capacitive Flexible Force Sensor

� Piezoelectric force sensors are another sensing technology that works

using quartz crystals which generate an electrostatic charge when a

force is applied, creating a voltage which is proportional to the input

force.

� They can be good for dynamic force applications due to their high

responsiveness. However, they are not good for static force

measurements, because the voltage decays quickly, meaning you would

have to employ lot of extra signal processing for use in robotic

applications.


FT Types: Piezoelectric

� Strain gauge bases sensors are the most

common force sensing technology in

robotics.

� These are either used individually, in a

single load cell, or multiple strain gauges

are used together to measure multi-axis

forces, as in a 6-axis FT Sensor.

� The function of a strain gauge is quite

simple – the resistance of a flexible

conductive foil changes when placed

under strain. This resistance change is

usually measured using a Wheatstone

Bridge, which produces a voltage

proportional to the resistance change.


FT Types: Strain Gauge

� The strain gauge is fixed to a material with known mechanical

properties. When a force is applied to the material it will deform

elastically. The strain gauge measures the deformation and this is

multiplied by the Young's Modulus and cross sectional area of the

material (which are both constants) to calculate the force.

� As they usually rely the voltage change, strain gauges are inherently

affected by noise. Various techniques exist to reduce the effect of noise

on strain gauges, which designers implement in the integrated

electronics of most FT Sensors. However, it is something to bear in mind

if using such sensors in environments with high levels of electrical noise

(e.g. near motors, AC power lines, arc welders or relays).

� Strain gauge force sensors of one axis are known as load cells. Unless

the load cell has integrated electronics you will have to perform some

signal processing on the output of the load cell.


FT Types: Strain Gauge


FT Types: Load Cells

http://www.sensorland.com/HowPage005.html

Load Cells

� Capacitive technology is the next step beyond the traditional strain

gauge technology because it overcomes some of the inherent problems

with strain gauges. Capacitive sensors are used to detect the distance

between two parallel conductive elements.

� An alternating voltage is applied to the elements, which produces an

alternating electric current which can be then used to calculate the

distance between the elements.

� A set of specially positioned capacitance sensors is used. These sensors

are attached to two metal plates which are connected to each other

mechanically through a compliant element. The two metal plates act as

the top and bottom of the force sensor. When a force is applied, the

compliant structure squashes (or stretches) and the capacitive sensors

can measure this deflection. We can then use the measurements to

calculate the applied force in all six axes.


FT Types: Capacitive

� There are several advantages to using capacitive sensors over the

traditional strain gauges. Because the sensors use an AC voltage, they

are not affected by noise as strain gauges are, because the provided

signal is inherently digital.

� Capacitive sensors generally produce a stronger output, so less signal

conditioning is needed, which ultimately means better accuracy.

� While strain gauges must be bonded to the metal with an adhesive,

which can be a point of weakness over time, capacitive sensors do not

require bonding


FT Types: Capacitive

� There are many more technologies which are used in force sensing.

� These include pressure-based sensing, using pneumatics or

hydraulics, which have been used in research for micro applications

and can be used in restrictive environments such as fMRI machines.

� There are also less common techniques, like magnetic force sensors.

� There are even new, cheap, tactile sensors which are based on

barometer technologies.

� New force sensors and force sensing technologies are being developed

all the time, especially in the field of micro and nano-electronics, as

used in robotic micromanipulators.


FT Types: Other Sensing Technologies

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Robotics 2017 11 FT Sensors - polito.it · 2017-02-18 · Strain gauge bases sensors are the most common force sensing technology in robotics. These are either used individually,