15-12-2012, 02:18 PM
Robotic Sensing Devices
[attachment=43920]
Abstract
Presented in this report is an overview of robotic sensors, many of which are in experimental stages. Two
main sensor types are discussed: contact and noncontact. Descriptions of the physical measurements. how
they are measured, and operating principles of specific devices are provided for both types of sensors.
Contact, or tactile, sensors comprise three groups: touch, proximity, and slip sensors. Noncontacting sensors
comprise six groups, according to principles of operation: optical, magnetic, capacitive, resistive, ultrasound,
and air pressure, each of which can measure numerous physical properties.
INTRODUCTION
‘Ihc potcntial range of robotic applications requires different types of sensors to perform different kinds of
scnsing tasks. Specialized devices have been developed to meet various sensing necds such as orientation,
displaccmcnt, velocity, acceleration. and force. Robots must also sense the characteristics of the tools and
matcrials they work with. Though currently available sensors rcly on different physical propertics for their
operation, they may be classified into two general types: contacting and non-contacting.
Since contacting sensors must touch their environment to operatc, their use is limited to objects and
conditions that can do no harm to the sensors. For instance, the elastic limit of a dcformable sensor must not
be excecdcd: also, a material such as hot steel would be extremely difficult to measure using contact sensors.
Contact devices vary in sensitivity and complexity. Some can only dctermine whcthcr something is touching
or not, while others accurately measure the pressure of the contact. The most simple contact sensor is merely
a mcchanical switch. The more sophisticated devices can produce a three dimensional profile of an object.
Contact, or Tactile, Sensors
Contact scnsor opcration is bascd on transduccrs. Whcrcas some usc purely clcctrical transduccrs such as
pressurc variablc rcsistanccs, others rcly on mechanical processes that arc changed into an elcctrical signal by
various mcans such as strain gaugcs, optics. or potentiometcrs. Almost all contact scnsors mcasurc one of
thrcc diffcrcnt physical quantitics: touch/force, proximity, and slip. Touch includcs whcthcr somcthing is
touching, thc prcssurc of a touch. and weights and forccs. Proximity sensors measure the ncarncss of objects
and displaccmcnts of the robot or target. Slip rcfcrs to thc motion of an object sliding out of a mechanical
hand or gripper.
Touch and Force Sensing
Touch and forcc scnsors determine whcther thc manipulator is touching somcthing, thc prcssure of the
touch, or how much of somcthing is bcing touched. Thc simplest tactile sensor is a switch that either turns on
or off when pressed. Simplicity and low cost are two of thc benefits of switches, and they are a good
investment for a system that only requires basic infonnation such as whcther an object is being touched. They
have only two states, so they arc ideal to interface with digital equipment. Most switchcs are mechanical,
although onc device uses a pncumaticallp operated switch. Switches may be used singularly or in large arrays
to gain morc information.
Mechanical Switches - The ACM [l]
The Active Cord Mcchanism (ACM) [l] is a snake-like robot with 20 segments in its body. Thc robot can
negotiate twisting mazcs, wrap itself around objects to pull them along, and push off of objccts wlicn starting
to mobc. 7hc tactile sciisors for the ACM consist of 30 on-off switchcs, onc on eithcr side of cadi scgment of
its body. ‘Ihe mcchanical switches make contact when the activc cord mechanism touchcs sorncthing.
Ylcchanical switches are employed as sensors for many robotics applications, such as the ACM, where
complex information is not required.
Pneumatic Switches
Pneumatic switches have been used as tactile sensors for a computer contro!led grippcr that has morc than
100 switches on each finger. The gripper is used in a robotics experiment [2] to insert a peg into a holc. When
the peg conbcts the hole, a computer uses the force distribution on the sensors to calculate thc approximate
position of the hole. The path of the fingers is then adjusted so that the peg can be accurately inserted.
(Figure 2-1 [2] shows an enlargement of the fingers.)
Carbon Fiber Sensor
This sensor [3] is made of carbon (graphite) fibers 7 to 30 microns in diameter. When pressure is exerted on
a single carbon fiber its resistance changes: but the resistance change over a useful range of pressures is not
usehl for sensing. The area of contact between two fibers is what is important for sensing. When two fibers
come into contact the area of the junction is approximately .5 rnm by Smm, and its resistance is about 2
kilohms [3]. As pressure is applied, the fibers press together and the area of contact increases by clastic
deformation. The conductivity of a junction increases with increasing area of contact. As increasing pressure
is applied the resistance and the noise level of a junction both decrease (table 2-1).
Carbon fibers are produced in a flat ribbon approximately half a millimeter across and a tenth of a
millimeter thick. Two of these ribbons placed across each other form the basic multifiber junction. An
effective way to utilize carbon fibers is to make a matrix of many fibers: a multifiber junction makes an ideal
matrix with a nominal thickness of about 1 mm. Researchers make sensor elements by forming a sandwich of
one or more matrices between or across foil electrodes. A single 1 cm2 matrix 1 mm thick has a resistance of
about 20@ ohms [3]. The matrix is flexible and can be custom shaped for any application.
Conductive Silicon Rubber Sensors
The silicon rbbber sensor consists of two electrodes, one or both made of electrically conducti\e silicone
rubber in a convex shape resembling a rod. The rounded component can be the metal, the rubber, or both.
When no pressure is exerted on the device the rubber-metal junction area is at a minimum corresponding to a
maximum resistance. As the pressure is increased, the contact area increases, giving a current more parallel
paths to flow through decreasing the resistance. The most common way to connect the sensor is in series with
a fixed resistance as a voltagc divider (figure 2-4). The output voltagc (figure 2-5) varies rapidly for small
pressures and then changes more slowly for higher pressures. The voltage shown in figure 2-5 is for a 1
kilohm series resistor. A series resistance higher than 1 kilohm would shift the whole curve downward and
make the device very sensitive to very low pressures, cg.less than 50 grams per junction. Operation in the low
pressure range is not always reliable because the metal electrode can slide off to one side instead of being
grabbed by the rubber. Another problem with the rubber is that its voltage output changes slowly when a
pressure is applied. The output is insensitive to the radius of the cylindrical electrode: Even a flat electrode
shifts the output curve by only about .25 volts.
[attachment=43920]
Abstract
Presented in this report is an overview of robotic sensors, many of which are in experimental stages. Two
main sensor types are discussed: contact and noncontact. Descriptions of the physical measurements. how
they are measured, and operating principles of specific devices are provided for both types of sensors.
Contact, or tactile, sensors comprise three groups: touch, proximity, and slip sensors. Noncontacting sensors
comprise six groups, according to principles of operation: optical, magnetic, capacitive, resistive, ultrasound,
and air pressure, each of which can measure numerous physical properties.
INTRODUCTION
‘Ihc potcntial range of robotic applications requires different types of sensors to perform different kinds of
scnsing tasks. Specialized devices have been developed to meet various sensing necds such as orientation,
displaccmcnt, velocity, acceleration. and force. Robots must also sense the characteristics of the tools and
matcrials they work with. Though currently available sensors rcly on different physical propertics for their
operation, they may be classified into two general types: contacting and non-contacting.
Since contacting sensors must touch their environment to operatc, their use is limited to objects and
conditions that can do no harm to the sensors. For instance, the elastic limit of a dcformable sensor must not
be excecdcd: also, a material such as hot steel would be extremely difficult to measure using contact sensors.
Contact devices vary in sensitivity and complexity. Some can only dctermine whcthcr something is touching
or not, while others accurately measure the pressure of the contact. The most simple contact sensor is merely
a mcchanical switch. The more sophisticated devices can produce a three dimensional profile of an object.
Contact, or Tactile, Sensors
Contact scnsor opcration is bascd on transduccrs. Whcrcas some usc purely clcctrical transduccrs such as
pressurc variablc rcsistanccs, others rcly on mechanical processes that arc changed into an elcctrical signal by
various mcans such as strain gaugcs, optics. or potentiometcrs. Almost all contact scnsors mcasurc one of
thrcc diffcrcnt physical quantitics: touch/force, proximity, and slip. Touch includcs whcthcr somcthing is
touching, thc prcssurc of a touch. and weights and forccs. Proximity sensors measure the ncarncss of objects
and displaccmcnts of the robot or target. Slip rcfcrs to thc motion of an object sliding out of a mechanical
hand or gripper.
Touch and Force Sensing
Touch and forcc scnsors determine whcther thc manipulator is touching somcthing, thc prcssure of the
touch, or how much of somcthing is bcing touched. Thc simplest tactile sensor is a switch that either turns on
or off when pressed. Simplicity and low cost are two of thc benefits of switches, and they are a good
investment for a system that only requires basic infonnation such as whcther an object is being touched. They
have only two states, so they arc ideal to interface with digital equipment. Most switchcs are mechanical,
although onc device uses a pncumaticallp operated switch. Switches may be used singularly or in large arrays
to gain morc information.
Mechanical Switches - The ACM [l]
The Active Cord Mcchanism (ACM) [l] is a snake-like robot with 20 segments in its body. Thc robot can
negotiate twisting mazcs, wrap itself around objects to pull them along, and push off of objccts wlicn starting
to mobc. 7hc tactile sciisors for the ACM consist of 30 on-off switchcs, onc on eithcr side of cadi scgment of
its body. ‘Ihe mcchanical switches make contact when the activc cord mechanism touchcs sorncthing.
Ylcchanical switches are employed as sensors for many robotics applications, such as the ACM, where
complex information is not required.
Pneumatic Switches
Pneumatic switches have been used as tactile sensors for a computer contro!led grippcr that has morc than
100 switches on each finger. The gripper is used in a robotics experiment [2] to insert a peg into a holc. When
the peg conbcts the hole, a computer uses the force distribution on the sensors to calculate thc approximate
position of the hole. The path of the fingers is then adjusted so that the peg can be accurately inserted.
(Figure 2-1 [2] shows an enlargement of the fingers.)
Carbon Fiber Sensor
This sensor [3] is made of carbon (graphite) fibers 7 to 30 microns in diameter. When pressure is exerted on
a single carbon fiber its resistance changes: but the resistance change over a useful range of pressures is not
usehl for sensing. The area of contact between two fibers is what is important for sensing. When two fibers
come into contact the area of the junction is approximately .5 rnm by Smm, and its resistance is about 2
kilohms [3]. As pressure is applied, the fibers press together and the area of contact increases by clastic
deformation. The conductivity of a junction increases with increasing area of contact. As increasing pressure
is applied the resistance and the noise level of a junction both decrease (table 2-1).
Carbon fibers are produced in a flat ribbon approximately half a millimeter across and a tenth of a
millimeter thick. Two of these ribbons placed across each other form the basic multifiber junction. An
effective way to utilize carbon fibers is to make a matrix of many fibers: a multifiber junction makes an ideal
matrix with a nominal thickness of about 1 mm. Researchers make sensor elements by forming a sandwich of
one or more matrices between or across foil electrodes. A single 1 cm2 matrix 1 mm thick has a resistance of
about 20@ ohms [3]. The matrix is flexible and can be custom shaped for any application.
Conductive Silicon Rubber Sensors
The silicon rbbber sensor consists of two electrodes, one or both made of electrically conducti\e silicone
rubber in a convex shape resembling a rod. The rounded component can be the metal, the rubber, or both.
When no pressure is exerted on the device the rubber-metal junction area is at a minimum corresponding to a
maximum resistance. As the pressure is increased, the contact area increases, giving a current more parallel
paths to flow through decreasing the resistance. The most common way to connect the sensor is in series with
a fixed resistance as a voltagc divider (figure 2-4). The output voltagc (figure 2-5) varies rapidly for small
pressures and then changes more slowly for higher pressures. The voltage shown in figure 2-5 is for a 1
kilohm series resistor. A series resistance higher than 1 kilohm would shift the whole curve downward and
make the device very sensitive to very low pressures, cg.less than 50 grams per junction. Operation in the low
pressure range is not always reliable because the metal electrode can slide off to one side instead of being
grabbed by the rubber. Another problem with the rubber is that its voltage output changes slowly when a
pressure is applied. The output is insensitive to the radius of the cylindrical electrode: Even a flat electrode
shifts the output curve by only about .25 volts.