Flex sensor:
Flex Sensor 2.2" RoHS Compliant Description: A simple flex sensor 2.2" in length. As the sensor is flexed, the resistance across the sensor increases. The resistance of the flex sensor changes when the metal pads are on the outside of the bend (text on inside of bend). Connector is 0.1" spaced and bread board friendly. Note: Please refrain from flexing or straining this sensor at the base. The usable range of the sensor can be flexed without a problem, but care should be taken to minimize flexing outside of the usable range. For best results, securely mount the base and bottom portion and only allow the actual flex sensor to flex.
Flexion sensors, (from Latin flectere, 'to bend') also called bend sensors, measure the amount of deflection caused by bending the sensor. There are various ways of sensing deflection, from strain-gauges1) to hall-effect sensors2). The three most common types of flexion sensors are:
§ conductive ink-based
§ fibre-optic
§ conductive fabric/thread/polymer-based
A property of bend sensors worth noting is that bending the sensor at one point to a prescribed angle is not the most effective use of the sensor. As well, bending the sensor at one point to more than 90˚ may permanently damage the sensor. Instead, bend the sensor around a radius of curvature. The smaller the radius of curvature and the more the whole length of the sensor is involved in the deflection, the greater the resistance will be (which will be much greater than the resistance achieved if the sensor is fixed at one end and bent sharply to a high degree).
Specifications
A typical bend sensor has the following basic specifications:
§ range of deflection
§ uni- vs. bi-directional sensing
§ uni- vs. bi-polar sensing
§ range of resistance (nominal to full-deflection)
Range of deflection: Determines the maximum angle of deflection that can be measured (as opposed to the maximum angle the sensor can be bent).
Uni- vs. bi-directional sensing: Some flexion sensors increase the resistance when bent in either of two opposing directions, however there is no difference in the measurement with respect to the direction.
Uni- vs. bi-polar sensing: A bi-polar flexion sensor measures deflection in two opposing directions yielding different measurements.
Range of resistance: Bend sensors can vary largely (even the same product) in terms of their range of resistance, measured as the difference from nominal resistance to resistance at full deflection.
Conductive Ink-based
These types of bend sensors are passive resistive devices typically fabricated by laying a strip of resistive ink on a flexible plastic substrate, shaped as a thin, flexible stripe in lengths between 1” and 5”. At rest (when laid flat), the bend sensor is characterized by an intrinsic resistance. As the sensor is bent, the resistive materials inside it are pulled further apart. Fewer adjacent resistive particles come into contact, thereby increasing the resistance. Typically, the nominal resistance lays between 10kΩ and 50kΩ and increases by a factor of 10 at full deflection.
Within the layers of the flex sensor substrate is a printed pattern of conductive ink. To conduct electricity, this ink contains carbon, or silver, particles mixed into a pigmented medium. Typically, the carbon particles are suspended in the ink to avoid fading of the pigment over time. This type of ink can also be safely applied to paper to avoid absorption into the fibers, thus changing the paper’s properties.
Most conductive ink-based bend sensors on the market are unipolar devices, that is the resistance increases as the deflection increases in one direction, and is unchanged if bent in the other direction. Placing two devices back-to-back will allow bipolar measurements for capturing deflections in both directions. Hysteresis and noise in resistance value are small if not entirely negligible.
Flexpoint Sensor Systems Inc. makes the Bend Sensor® in 1”, 2”, and 3” lengths. They are available with connectors that can be interfaced with standard sized headers.
As length increases, the intrinsic resistance goes up, as does the resistance for each sensor at full deflection. As well, there are different lamination and coating options to increase durability and stiffness.
Images Scientific Instruments offers the bipolar flexion sensor FLX-02, which increases resistance in one direction and decreases resistance in the other direction. In addition, Images SI sells a special component-based flexion sensor which can also be used as a pressure sensor. This sensing principle is identical to conductive/thread/polymer-based sensing illustrated below. Worth mentioning is the online tutorial on how to construct a bi-directional (not: bi-polar!) bend sensor using a stripe of resistive material which is sandwiched between two copper clad laminates and sealed using heat shrink tubing.
Properties
§ hysteresis/noise neglectible3)
§ resistance is function of radius of curvature, not angle at one point
§ high temperature and humidity-tolerance
§ relatively low cost
§ customizable (coatings, laminating materials)
Applications
§ automotive applications
§ industrial applications, e.g. safety switches, shipping, machine control
§ medical applications (e.g. “SmartBed”)
§ gaming devices
§ measuring devices
§ assistive technology
§ robotics (e.g. floor mapping, collision detection)
Fiber-optic
Fiber-optic bend sensors (also called optical goniometers) consist of a light source, a plastic optical fibre (POF) with an abraded section and a photosensitive detector4). Light is emitted into the POF at one end and sensed at the other end. Bending the optical fibre results in a loss of light (intensity). The loss of light is often enhanced by cutting, polishing or abrasing a part of the POF5). Due to the sensing principle single fiber-optical bend sensors are uni-polar devices.
The image on the right shows a fiber-optic bend sensor with abraded section as used in TWEND.
A bend sensor can also be made from a length of fiber optic cable with an LED and a photodiode placed at both ends of a section of cable;
Properties
§ mostly custom-built or developed in the lab
§ high repeatability, hysteresis neglectible
§ can be bent in any direction
§ usually unipolar measurement (global flexion)
§ can be expensive
Applications
§ medical applications
§ educational studies (e.g. the CARESS project)
Conductive Fabric/Thread/Polymer-based
Conductive fabric-, thread- or polymer-based flexion sensors typically consist of two layers of conductive material with a layer of resistive material (e.g. Velostat) in between. It is mostly sandwiched inbetween layers of more rugged material, e.g. Neoprene. As pressure is applied (directly or by bending) the two layers of conductive material get pushed closer together and the resistance of the sensor decreases. This sensing mechanism is similar to force-sensitive resistors. Strictly speaking, these types of sensors are pressure sensors which also sense deflection (pressure as a function of deflection): bending the sensor across an angle of a rigid structure results in stretch of the sensor material which exerts pressure onto the sensor. It is this pressure that is measured. Foam/Polymer-based sensors decrease their nominal resistance as the material is compressed. These sensors are known to have poor accuracy, repeatability and hysteresis6). External links to distributers and manufacturers of conductive fabric/thread can be found at the bottom of this page.
"The impedance buffer in the [Basic Flex Sensor Circuit] (above) is a single sided operational amplifier, used with these sensors because the low bias current of the op amp reduces errer due to source impedance of the flex sensor as voltage divider. Suggested op amps are the LM358 or LM324."
"You can also test your flex sensor using the simplest circut, and skip the op amp.
"Adjustable Buffer - a potentiometer can be added to the circuit to adjust the sensitivity range."
"Variable Deflection Threshold Switch - an op amp is used and outputs either high or low depending on the voltage of the inverting input. In this way you can use the flex sensor as a switch without going through a microcontroller."
"Resistance to Voltage Converter- use the sensor as the input of a resistance to voltage converter using a dual sided supply op-amp. A negative reference voltage will give a positive output. Should be used in situations when you want output at a low degree of bending."
Properties
§ quasi-linear behaviour for pressure-sensing
§ slow response, due to physical deformation (internal energy)
§ high hysteresis, poor accuracy & repeatability
§ high temperature- and humdidity-tolerance (can be used underwater)
§ highly customizable
§ cheap
Features –
· Angle Displacement Measurement –
· Bends and Flexes physically with motion device –
· Possible Uses –
· Robotics –
· Gaming (Virtual Motion) –
· Medical Devices –
· Computer Peripherals –
· Musical Instruments –
· Physical Therapy –
· Simple Construction –
· Low Profile
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