Tactile and Touch Sensors
Tactile sensors can be used to detect a wide range of stimuli, ranging from the presence or absence of a grasped
object, to discovering a complete tactile image. The principle component of a tactile sensor is the touch sensitive
sites on the sensor that are capable of sensing and measuring a variety of different properties. Some properties
include the ability to measure contact forces, which can be used to identify the state of grip (successfully pick
up object, or failed to grasp object). Texture, impact, slip and other contact conditions generate specific force
and position patterns. This information again can be used to identify the state of manipulation. In other words, the
objects size, shape and mass can be used to test whether the object has been rotated ninety degrees and flipped
vertically, for example.
Since there is no complete theory available that outlines the sensing requirements of a robotic system, much of the
knowledge in this field is drawn from the investigation of human tactile and touch senses, along with the analysis of
human grasping and manipulation. After careful study of human tactile senses, researchers have discovered that a
gripper incorporating tactile sensing would require a wide range of sensors to fully determine the state of grip.
Along with that, the gripper would need, stored in memory, the proper tactile signature of the manipulation of the
object. (The gripper needs to know what it feels like when the object has been manipulated correctly so it can
compare its current state to the completed state). Currently there aren't any detailed specifications for a touch
or tactile sensor. However, the following few points are an excellent basis for a working tactile or touch sensor
for the majority of industrial applications:
1) A touch sensor should be a single-point contact. Although the sensory area can be any size, an area of 1-2 mm2 is considered the ideal size considering a compromise between constructing the sub-miniature size and the need for a large number of sensors.
2) A sensitivity within the range of 0.4 to 10 N.
3) A minimum sensor bandwidth of 100 Hz.
4) Since the sensors are to be used in industrial applications, they need to be protected from surrounding objects that could damage them.
The simplest form of touch sensor is one where the applied force is applied to a strategically placed micro-switch to
form a binary touch sensor (either on or off). The force required to operate the switch will be determined by the type
and size of the material to be sensed. If one wanted to sense large objects, the weight to activate the sensor would
be set to a greater amount, visa versa for a lighter object. Other approaches are based on a mechanical movement
activating a secondary device such as a potentiometer or displacement transducer.
Resistive based sensors
The use of materials that have defined force-resistance characteristics have received considerable attention in touch and
tactile sensor research (material that will only bend when a specific amount of pressure is applied). The basic principle
of this type of sensor is the measurement of the resistance of a conductive elastomer, or foam, between two points. The
majority of the sensors use an elastomer that consists of a carbon-doped rubber.