Transcript ppt - LaDiSpe - Politecnico di Torino

ROBOTICS
01PEEQW
Basilio Bona
DAUIN – Politecnico di Torino
Force/Torque Sensors
Force/Torque Sensors
http://robotiq.com/products/robotics-force-torque-sensor/
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ROBOTICS 01PEEQW - 2015/2016
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Force/Torque Sensors
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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.
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FT Types: Resistor
 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.
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FT Types: Flexible Force Sensor
 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.
 They are adaptable and fit into small spaces,
but only detect forces applied at one point
and in one direction.
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FT Types: Capacitive Flexible Force Sensor
 Capacitive flexible force sensors are a similar option, which can be a bit
more accurate over a smaller range, but are also more expensive
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FT Types: Piezoelectric
 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.
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FT Types: Strain Gauge
 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.
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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.
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FT Types: Load Cells
Load Cells
http://www.sensorland.com/HowPage005.html
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FT Types: Capacitive
 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.
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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
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FT Types: Other Sensing Technologies
 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.
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