What You Should Know about Arduino Pressure, Force, and Load Sensors
Three closely related kinds of Arduino sensors are commonly confused: pressure, force, and load sensors. These three sensors are actually extremely different in how they behave and what data they can give you, so it’s important to know the difference so that you pick the one that’s right for your situation.
Consider the following as you plan:
Complexity: Complexity increases depending on how accurate you need to be:
Pressure pads are designed to detect when pressure is applied to an area, and they come in a variety of quality and accuracy. The simplest pressure pads are often the equivalent of big switches. Inside a pressure pad are two layers of foil separated by a layer of foam with holes in it. When the foam is squashed, the metal contacts touch through the foam and complete the circuit.
For more precision, you may want to use force sensors, which measure the force applied by whatever is put on them within their range. Although force sensors are accurate enough to detect a change in weight, they are not accurate enough provide a precise measurement.
Force sensors are usually flexible. Force-sensitive resistors, resistors that are made on a flexible PCB, change their resistance when force is applied. The resistor itself is on the flexible circuit board, and although it can tolerate extremely high forces and loads, protecting it from direct contact is a good idea to prevent it from bending, folding, or tearing.
Load sensors can accurately measure weight up to their limit. They work in much the same way as force sensors by changing resistance as they bend. In most cases, a load sensor is fixed to a rigid piece of metal and monitors changes as the metal is put under strain. The changes are so minute that they often require an amplification circuit known as a Wheatstone Bridge.
Cost: The cost of each sensor is relatively low, even for the most sensitive ones. A cheap pressure pad can set you back $3 (£2), for all the materials to make a DIY one, to $12 (£8), for an inexpensive, entry-level pressure mat available from most electronics stores and suppliers.
Force sensitive resistors range from $8 to $23 (£5 to £15), but cover a much smaller area than a pressure pad, so you may need quite a few of them to cover a large area.
Load sensors are also relatively cheap at around $11 (£7), most likely because they are so widespread that mass production has knocked the price down. There may be an extra cost in time to plan and make the additional circuitry.
Where: The real challenge with sensors is housing them to prevent damage. In the case of the pressure pad and the force-sensitive resistors, placing a good layer of upholstery foam on the side that the force is coming from is a good idea. Depending on the density of the foam, it should dampen enough of the force to protect the sensor but compress it enough for a good reading.
Underneath the sensors, you want to have a solid base to give you something to push against. This could just be the floor that the sensor is placed on or a sheet of MDF/plywood could be attached to the underside. It’s a good idea to protect the exterior of your pressure sensor, so consider something more sturdy than foam on the exterior. Upholstery vinyl is a great option.
If you plan to have people walk on the surface, for example, a layer of wood on the top to sandwich the foam is a good option to spread the load and can easily be replaced, if needed. In the case of load sensors, they require very little movement and should be connected to or placed in direct contact with a ridged surface.
Picked a sensor? Now you need to figure out how to use it:
Pressure pads are an extremely simple circuit, the same as for a pushbutton. The hardware of a pressure pad is also easy enough that you can make one yourself using two sheets of foil, a sheet of foam, a cover, and a couple of wires. As an alternative to foil, a great material to use is conductive fabric or conductive thread, which are a lot more flexible than foil.
Force sensors are also relatively easy to use and can take the place of other analog sensors, such as light or temperature sensors, in simple Arduino circuits. The ranges of force may vary, but you can scale the force to your needs quite simply in the code.
Load sensors are probably the most complex sensor if they are being used for accurate reading, as with a set of weight scales. They require extra circuitry and an amplifier for the Arduino to read the minute changes in resistance.