Force Sensing Resistors
A Force Sensor
resistor (FSR) is a special material that changes resistance with the application of force. Force sensing resistors are typically very thin in profile – which offers advantages when force-sensing abilities are needed in tight spaces. A FSR is usualy a mixture of suspended conductive particles, than when compressed / forced together, increase the conductivity of the material – and hence can sense force.
Pressure and force transducers include a large grouping of sensors. Some return a digital (on/off), or analog (varying voltage feedback) signal and can be used to detect position, force & pressure, or obstacles to avoid.
Many times these sensors are classified as “tactile” feedback devices because in order for them to return useful information, they must contact the element under investigation (touch, push, squeeze, etc.). Robots that utilize these pressure or force feedback sensors use this data to gather more information about their interaction with the environment. An example of their application might be a gripper with pressure sensitive pads integrated into its gripping fingers. As the gripper squeezes an object to pick up, the pressure sensitive pads will tell the robot how hard it is gripping and can be mindful to not exert too much force (such as when gripping fragile objects). Pressure sensitive information can also be used to determine if the robot can properly handle/lift objects without compromising its safety, or provide critical information about balance, motion, or even drive-train efficiency.
Simplistic Pressure feedback and Pressure Switches:
Pressure and force data can be sensed in a variety of ways. One of the most simplistic ways is to incorporate a spring and a potentiometer combination. As the spring deforms with force, the potentiometer feedback is used to determine the displacement of position - and thus allow the controller to deduce the force upon the object. Bumpers used on the outside of robots such as the Roomba
rely on similar setups, except they use a digital switch; When the force reaches a certain level it toggles a switch which alerts the robot of an obstacle.
More advanced pressure and force sensors rely on material properties to deduce force. Force transducers for example are based upon the principle that all materials deform when a force is applied. Even though this deformation might be too small to see with the naked eye, all materials deflect when a force is exerted on them - and as more force is applied, the material will deform to a greater extent. If you can detect this change in deformation and know how much the material will deform per unit of force applied (based on material properties), you can easily determine how much force is being applied to the object. This is very similar to the “potentiometer and spring” concept noted above, except on a much smaller scale. To detect this material deformation, force transducers use what are called strain gauges .
A strain gauge is a very flat resistor that is cemented to the surface of the deforming material. The resistor is patterned into a long flattened coil-looking shape than when deformed - causes a slight fluctuation in resistance. This miniscule change in resistance can be amplified with proper circuitry and then detected by a micro-controller to provide force feedback data. Depending on their size and form, force transducers can be used to detect almost any level of force - ranging from a few ounces to many thousands of pounds. They are used in a host of different situations, from detecting forces within gearboxes to digital scales and are used in places that you might not even expect!
Other types of Tactile Pressure Sensors:
Another type of pressure sensor takes advantage of the “piezoelectric effect”. A few select materials display this particular quality, however the effect occurs when these materials are quickly placed under stress / force. As these materials are “squished” they generate a voltage. This voltage can be detected and will vary based on the force applied. A great example of the piezoelectric effect in action are BBQ lighters. When you push the button, it releases a spring that quickly strikes a piezoelectric crystal - and causes many thousands of volts to be generated, leading to a spark. Piezoelectric material are placed in sensors, however only supply accurate data for very brief periods of time (called dynamic forces) and thus only useful for detecting quick and sudden changes of force.
One more common type of sensor employs special resistors that change resistance when force is applied to them (called a force sensing resistor - FSR). Capacitive sensors rely on object deformation, but measure capacitance instead of resistance to determine deflection. Diaphrams are integrated into sensors to detect gas pressure, and even light can be used to detect pressure. Light, when directed at an angle to a surface will bounce back to a certain location. As the surface deforms with pressure - the light shifts in reflected location and force can be determined.
Anyway you look at it, pressure and force can be detected in many different ways. The main takeaway is that most all force sensors are based on the idea that a force will result in material deformation. There are different ways to sense this deformation and can be utilized to determine forces or pressures.