Tuesday, May 22, 2012

Force Sensors

Measuring force becomes very important in some cases like surgical instruments, robotic arms, laproscopic devices, etc; where the force applied is very minute.

In such cases, the force can be measured by using various sensors like;
  • Strain Gauge
  • FlexiForce Sensors
  • Piezo Disc
Strain Gauge

It is the most common device for measuring force. It is named so because, it is used to measure force through the strain produced in it. It consists of an insulating flexible backing which supports a metallic foil pattern as shown in the figure. 

This gauge can be fixed (mounted) onto the object, whose strain/force is to be calculated, through a suitable adhesive like silicone or cyanoacrylate. When the object is deformed due to any applied force, the strain gauge experiences tension/compression based on the nature of the force, which causes the resistance to change. This resistance change can be measured by connecting this to a Wheatstone bridge network.
    
GF=\frac{\Delta R/R_G}{\epsilon}
where
\Delta R is the change in resistance caused by strain,
R_G is the resistance of the undeformed gauge,
\epsilon is strain and
GF is the gauge facto.
For metallic foil gauges, the gauge factor is usually a little over 2. For a single active gauge and three dummy resistors, the output v from the bridge is:
v=\frac{BV \cdot GF \cdot \epsilon}4
where
BV is the bridge excitation voltage.
 FlexiForce Sensors
A FlexiForce Sensor is another kind of sensor that is used to measure force. It is shown in the figure. It is an ultra-thin, flexible printed circuit. It is constructed of two layers of substrate (polyester) film. 

On each layer, a conductive material (silver) is applied, followed by a layer of pressure-sensitive ink.

 
Adhesive is then used to laminate the two layers of substrate together to form the force sensor.

The active sensing area is defined by the silver circle on top of the pressure-sensitive ink. Silver extends from the sensing area to the connectors at the other end of the sensor, forming the conductive leads.
     
This can be mounted on the object on which the force is to be measured. When the force sensor is unloaded, its resistance is very high. When a force is applied to the sensor, this resistance decreases. The resistance can be read by connecting a multimeter to the outer two pins, then applying a force to the sensing area.

Advantages of FlexiForce Sensors over Strain Gauges
  • Direct force measurement - measures load directly as opposed to correlating strain of an assembly with load.
  • Larger dynamic range (resistance range from MΩ to KΩ).
  • Simpler Electronics - simply measured with multimeter or integrated with inexpensive circuitry.
  • Lower cost - standard sensors start at $65 for a 4-pack.
  • Easier to integrate - simple mounting process.

 Piezo Disc
A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge.

A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated).

Metal disks with piezo material, used in buzzers or as contact microphones


References

http://en.wikipedia.org/wiki/Strain_gauge
http://www.tekscan.com/flexible-force-sensors
http://en.wikipedia.org/wiki/Piezoelectric_sensor

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