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konan
01-30-2012, 07:45 AM
Greetings to all!

Please excuse me if this seems like a trivial question, but I am new to the field gathering information for an upcoming project.

What sort of sensors are available for directly measuring force? Something like a small digital scale that outputs a voltage proportionally to the force applied. I need to be able to have a robotic arm pushing on a target, and measure the force as a function of time, so I need to be able to sample the data at at least 100 times a second. It could either simply output a small voltage that can be measured, or it can have a USB interface.

The reason why I would like this is to be able to measure the strength curves associated with various choices of actuators, and account for them pragmatically.

I'd appreciate any input that you might have, and relevant segways are welcome as well :D
Konan

parallax
02-06-2012, 12:53 PM
konan,

There are multiple ways to accomplish what you are seeking to do. Three types of sensor come to mind immediately which I will outline briefly below:

Force-sensitive Resistors (FSR)

Essentially a variable resistor, a FSR is constructed as two plates of conductive substrate separated by a non-conductive material (a common design utilizes air). As pressure is applied to the working area of the sensor, the non-conductive material is displaced and the two conductive plates begin to make contact. As more of the plates’ surface area comes into contact, conductivity is increased. The amount of conductivity is proportional to the amount of force that is applied, and this relationship turns out to be *roughly* linear. Therefore by measuring the conductivity it is possible to parse the amount of pressure on the sensor. The primary benefit of using a FSR is that FSRs have a very small footprint. This would minimize the amount of space required for the sensing mechanism. The primary drawbacks of using FSRs are that they are subject to saturation, require additional circuitry to be usable and can exhibit error as large as 25% in some situations.

Strain Gauges

Another possibility is a device known as a strain gauge. A strain gauge is a small sensor that utilizes material properties to measure strain. Strain is defined as the amount of deformation per unit length of an object when a load is applied. By understanding the physical properties of a material under load, it is possible to derive the amount of pressure (force) being applied to the object. Strain gauges vary significantly in their form factor, design and deployment. However they all measure strain via the same principle. As a solid is deformed, the cross-sectional area of the solid changes. Strain gauges are composed of very thin conductive wires mounted on a base. This base is then mounted solidly to the object to be measured. As the object comes under load, it deforms slightly and subsequently deforms the strain gauge. When this occurs, the cross-sectional area of the gauge’s wire varies, and the conductivity of the wire changes slightly, but measurably. Due to the minute changes in conductivity, a specialized circuit commonly known as a “Wheatstone Bridge” is used to parse the output. Also, the geometry of the gauge makes it only possible to measure changes in the object along one axis; for each axis you need to assess, you need another strain gauge. By their nature, strain gauges are ideal in high impedance situations. Strain gauges have the advantages of being readily accessible and can be found in a very small form factor. They have the disadvantages of requiring specialized circuitry in order to implement, and generally work best in high impedance situations.

Potentiometers

Potentiometers (sometimes called rheostats) are useful in that they transpose physical position directly to voltage on a large scale (relatively); unlike strain gauges which allow only for minuscule changes (and thereby require amplification). Potentiometers are able to accomplish this due to their geometry. They are three-terminal resistors with a sliding wiper; this essentially forms a variable voltage divider. Many kinds of potentiometer exist for many applications, and they are commonly found all around us. A complete discussion of potentiometers is outside the scope of this post, and therefore only a small subset which are appropriate to the problem model will be considered (specifically, linear potentiometers). Potentiometers used in simple applications generally do not need specialized circuitry to operate. Furthermore, they are available with different output “curves” (known as tapers); some examples of which are log, anti-log, commercial log, and linear. Finally, due to the fact that it requires very little force to change their output, they are appropriate for low impedance situations. These can be used to measure force when coupled with a spring for which the spring constant is know. Because the amount of force a spring applies is linear (and directly corresponds to the amount of deformation it undergoes- look up "Hooke's Law"), measuring the deformation of the spring with a linear potentiometer means you can directly derive the amount of force applied to it. Potentiometers have the advantages of being plentiful in supply, have a wide range of output, and are simple to implement (requiring no additional circuitry). Their primary disadvantage is that they tend to be far larger than the previous two devices, which can make them unwieldy to implement in compact designs.

I hope this helps point you in the right direction for researching your project!

konan
02-07-2012, 08:30 PM
Thank you very much for your detailed reply. It did not even occur to me to use pots in that way. I did go ahead and buy a few FlexiForce sensors, but using a linear pot (with spring) sounds like a really good way for measuring the a force as it travels over a distance, etc.

It also gave me a few ideas as to how to use pots to measure the joint angles. Probably a trivial thing to the experienced electrical engineer, but new and exciting for a software developer who is getting is hands dirty with some hardware :D .

So in short, if I wanted to measure any kind of displacement, be it linear or angular, then should potentiometers be the first thing to consider?

Thanks again!
Konan

parallax
02-07-2012, 10:58 PM
Glad to be of help!

Pots are indeed a fantastic way to measure displacement and are certainly my personal first "go to" option, but there are certainly other options out there that will do the job as well; it really is an application specific choice that depends upon your problem statement and your design specifications. For instance, rotational pots are generally limited to a certain amount of rotation. If you have an application which requires that you read a joint angle which has the capacity to travel beyond that limit, an encoder is probably a better option.

But generally, pots are a great choice as they are easy to implement and cheap to purchase. In contrast, a decent magnetic encoder may cost upwards of $50...