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Need for Capacitive Load Cells?
One of the questions we get asked frequently is why use capacitance? Resistive sensors have been used for a long time…so why change? Why now?
Here is an attempt to answer this question:
- What is capacitance
- It is the ability of a system to store charge. So if you take a set of parallel plates and apply a potential difference to it (Voltage), then for a brief time interval, electrons move from one plate to the other (a small current is generated) until it reaches a state of equilibrium (the current stops). The charge gets stored between the two parallel plates
- The amount of charge that can stored (its Capacitance) depends on the area between the plates, the distance between the plates and the dielectric constant between the plates.
- The formula for the Capacitance of the system can be shown as:
- C = Er*E0*A/d where Er is the dielectric Constant, E0 is the Permittivity of free space, A is the area of overlap between the plates and d is the gap between the plates
- For Air, Er=1 and E0 is a constant equal to 0.00854 picoFarad/mm
- So if you have two small overlapping discs of radius=5.6 mm and the gap between them equal to 0.25 mm, then the Capacitance of the system- C - is given by C = (1.0)*(0.00854)*(3.14159)*(5.6)2/(0.25) or 3.4 pF
- If the gap between the plates changes to 0.17 mm then the capacitance increases to 4.9 pF
- That is a change of 1.5 on 3.4 or 44.11%
- How resistive load cells operate
- Generally a load cell is made in the shape of a cantilever, pancake or S-beam type of mechanical member with several strain gauges attached to the member
- When a force is applied to the member – it deflects by a few thousandths of an inch, in response to the applied load creating a strain on the strain gauges
- The strain affects the resistivity of a strain gauge
- For a typical 350 Ohm strain gauge, the change in resistance for full scale loading at full deflection is of the order of 0.7 Ohms! Just 0.2% of full scale.
- This resistivity change is measured using a balanced wheatstone bridge network
- The circuit used converts the full scale deflection into an output change of just 20 millivolts which need to converted into 5000 divisions to achieve 0.02% accuracies!
- In order to achieve measurement accuracies of this magnitude, one needs to carefully condition the input voltage and resolve the output signal with at least 2 micro-volt resolution several times a second at least. Typical response rates of 100 Hz needs very high quality Analog to Digital converters
- The mechanical advantage of using capacitive sensing techniques
- The sensitivity of a capacitive sensor can be tuned for the application
- For e.g. if there is a need for very high sensitivities, then the starting gap between the plates can be decreased. So for e.g. if you start with 0.10 mm gap and still change by about 0.5 mm in response to applied loads of say just 1 lb, then you still get 100% change in capacitance!
- The change in resistivity is very small – and there is a limit to how small the strains can be as the device gets smaller – and still achieve reasonable resistivity change to still be usable as a sensing system! The mechanical deflections need to be increased in order to achieve greater strains – but this means the sensor becomes less rugged and more delicate in practice, leading to damage to the sensor and/or need for repeat calibrations.
- This is one of the most important advantages offered by a capacitive sensing device – it allows one to achieve very high sensitivities in very small packages. It allows capacitive sensors to offer high sensitivity in very rugged, tough packages that can withstand much higher peak loads than a similar resistive sensor with similar sensitivity.
- Form factor advantages of capacitive sensors
- Capacitive sensors can be built in any shape – it doesn’t have to be square/circular/oval – it can literally be built in any shape or form
- They can constructed out of common metals or out of dielectric material with coated conductive areas to create capacitive zones
- They can rigid or flexible in nature
- They can be built using conventional fabrication techniques without need for sophisticated MEMs or other semi-conductor fabrication methods.
- Electronic Measurement advantages of Capacitive sensors
- The fundamental measurement circuits to measure capacitance changes are similar to the ones used to measure resistance changes.
- However, instead of using analog DC voltages of the micro-to-millivolt range outputs, we use the charge discharge frequency of a capacitor – converted into a square wave output – as the fundamental measurement circuit.
- This gives us two advantages:
- Noise immunity: Since we are not trying to measure the level of a tiny analog DC signals (0 to 20 milliVolt) in the midst of ambient noise (of the order of +/- 1 millivolt) our noise levels are much lower. We measure the number of low to high transitions of the square wave (frequency) of the output signal – and so at a very low analog level – our signals are “digital”.
- We don’t need an ADC to convert the signal into a digital format. By counting the pulses of the square wave (or low to high level transitions) we get a “count” of the wave in a defined time interval. Generally speaking, this counting function is easily available on a Digital I/O pin of most commonly available micro-processors.
- By reducing the noise levels and allowing the change to be measured with fewer, less stringent components, we reduce the size, cost and complexity of end user application designs
- This reduction allows us to pack more features into the sensors itself – in the form of built in digital communication modules with USB/WiFi/XBee Wireless/Serial protocols, algorithms to convert raw signals into readily usable calibrated data and easy to use ASCII command set to access the finished data
- Versatility of Capacitive Sensors
- These sensors are very versatile because the basic fundamental unit changes in response to three parameters – Area, Distance and the Dielectric Constant
- One can vary one or more of these parameters to measure various physical quantities
- For e.g. One can use a capacitive sensor to sense position (by varying the gap or varying the area). Such position sensors can be used to detect the thickness of films, or detect earthquakes or be used to create micrometers.
- By varying the area of overlap, once can create a Torque sensor or rotary encoder
- By varying the dielectric property of the substance between the plate, one can detect things such as variations in humidity or the change of material properties (such as the quality of motor oil in an engine) with time and use.
- A proximity or touch sensor can be created by the capacitive coupling of a finger coming close to a capacitive zone. Such "touch sensors" are now commonly used in applications such as smart phones, appliance control panels, car dashboards and other interactive panel applications.
- One can combine various sensing elements into a single unit to make a multi-purpose sensor with the basic capacitive sensing platform we have created.
- Cost of manufacture
- Because capacitive sensors can be built with a variety of materials and one does not need to carefully attach strain gauges to a smooth, clean surface with carefully chosen adhesive materials, the manufacturing process is simpler and can be easily automated
- When produced in volume, the cost of manufacturing capacitive sensors are an order of magnitude cheaper than producing similar volumes of resistive load cells manually.
- This cost difference is magnified several fold as the devices get smaller and smaller. It becomes impossible to build tiny force or load cells based on resistive techniques because of the difficulty of bonding tiny strain gauges to mechanical members
- If MEMs techniques are used to build resistive devices, the signal to noise ratios are not adequate to get reliable performance at the strains achievable in the system and are sometimes prohibitively expensive in small quantities
- Therefore capacitive sensing techniques offer a great combination of three key parameters: high sensitivity, small size and low cost – a hard to beat combination!
However, note that, despite these advantages, capacitive load cells may not always be appropriate for monitoring applications that require high accuracies over long periods of time and when used in environments with changing temperatures and moist . We recommend using the capacitive load cells for applications that require a quick measurement where the initial conditions can be zeroed out and where the environment is maintained at steady room temperature and humidity conditions. Please consult Loadstar Sensors for advice on the most appropriate sensor for your application.