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:

1. What is capacitance
   1. 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
   2. 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.
   3. The formula for the Capacitance of the system can be shown as:
      1. 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
      2. For Air, Er=1 and E0 is a constant equal to 0.00854 picoFarad/mm
      3. 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
      4. If the gap between the plates changes to 0.17 mm then the capacitance increases to 4.9 pF
      5. That is a change of 1.5 on 3.4 or 44.11%
2. How resistive load cells operate
   1. 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
   2. 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
   3. The strain affects the resistivity of a strain gauge
   4. 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.
   5. This resistivity change is measured using a balanced wheatstone bridge network
   6. 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!
   7. 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
3. The mechanical advantage of using capacitive sensing techniques
   1. The sensitivity of a capacitive sensor can be tuned for the application
   2. 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!
   3. 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.
   4. 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.
4. Form factor advantages of capacitive sensors
   1. 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
   2. They can constructed out of common metals or out of dielectric material with coated conductive areas to create capacitive zones
   3. They can rigid or flexible in nature
   4. They can be built using conventional fabrication techniques without need for sophisticated MEMs or other semi-conductor fabrication methods.
5. Electronic Measurement advantages of Capacitive sensors
   1. The fundamental measurement circuits to measure capacitance changes are similar to the ones used to measure resistance changes.
   2. 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.
      
   3. This gives us two advantages:
      1. 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”.
      2. 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.
      3. 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
      4. 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
6. Versatility of Capacitive Sensors
   1. These sensors are very versatile because the basic fundamental unit changes in response to three parameters – Area, Distance and the Dielectric Constant
   2. One can vary one or more of these parameters to measure various physical quantities
   3. 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.
   4. By varying the area of overlap, once can create a Torque sensor or rotary encoder
   5. 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.
   6. 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.
   7. 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.
7. Cost of manufacture
   1. 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
   2. 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.
   3. 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
   4. 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
8. 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.

USB Load Cells have a quick plug-and-measure capability. They work with a personal computer (PC) and typically have easy-to-use, small footprint software to allow engineers to read, log and plot forces, weights and pressures.

Setting up a USB load cell only requires a connection to a computer with a USB port. The USB port provides the power required to activate a load cell as well as the communications link for the software to talk to the load cell.

ASCII commands can be sent to the USB load cell via the software to display force readings. This could be the standard HyperTerminal software offered within Windows XP or other similar Terminal Emulators offered by other 3rd party software developers.

Load Cell Options

Load Cells with USB output can be made of conventional strain gauge technology or with capacitive force measurement technology.

Resistive Load Cells with USB output

Loadstar Sensors offers DI-100U and DI-1000U digital load cell interfaces that work with most any conventional Wheatstone bridge circuits with mV/V outputs. They accept a load cell, torque sensor or pressure sensors with analog millivolt outputs and output a USB signal.

Optionally the DI-1000Z and DI-1000ZP offer wireless output using the IEEE 802.15.4 communications protocol. The DI-1000ZP offers a battery backup option as well for totally cable free operation of the load cell.

Capacitive Load Cells with USB output

Loadstar offers the iLoad Series capacitive load cells with fully integrated signal conditioning and USB communications protocols in a single package - for customers who prefer the fully integrated solution instead of using a separate external interface device. The capacitive devices are smaller in size, lower in profile and have a single cable with USB connectors attached to the load cell. You just plug in the USB load cell to a PC and start measuring!

One can also easily integrate these digital load cells into custom programs written using NI LabVIEW or Matlab, or one can write customer programs in C/C++/VisualBasic/Visual Studio or any other development platform.

Loadstar Sensors is a leading provider of digital load cells, digital torque sensors, digital displacement sensors and other convenient solutions to capture data from sensors and make it available on PCs, Tablets and Mobile Smart Phones.

We provide a variety of conventional sensing technologies including piezo-resistive load cells, torque sensors and displacement sensors.  We offer a variety of interfaces with USB, WiFi, Bluetooth and 0-5V DC Analog outputs that are compatible with these resistive sensors and make it easy to incorporate into your application.

Our patented capacitive sensing technology offers an integrated digital processing and communications capability in a compact yet rugged sensing package. We can adapt this capacitive technology to enable a variety of applications at an attractive price point making them economically feasible for the first time!

Some of the key benefits of our capacitive technology are:

High sensitivities in small packages.

Capacitance based devices can offer much higher sensitivities and resolutions than other measurement techniques such as resistive strain gauges. In comparison to resistive devices the capacitive signals are ten to a hundred fold higher for the same level of displacement.  Because of this, they do not need to be strained or deformed as much in order to obtain a robust signal.  As a result, very rugged sensors can be made in comparatively small sensor packages in a variety of sizes and shapes using standard mechanical and electronics fabrication techniques.

Versatile sensing platform.

Capacitive devices are very versatile and can be used to measure parameters such as: acceleration, vibration, humidity, proximity, torque, turbidity and temperature. One can change the gap between capacitive plates, the area of overlap or the dielectric material between the plates to create sensors for various applications. We have developed an end-to-end solution to harness this advantage in a easy to use manner.

Robust electromechanical device.

Loadstar sensors are not based on delicate strain gauges or piezo components and thus are less susceptible to shock loading and can offer several times the overload protection offered by resistive load cells. This can be a huge advantage for certain market segments such as consumer products where OEM's want to avoid damage to sensors in the field.

Flexible output options.

Our capacitance measurement technique output frequency as the basic signal proportional to loads. Optionally, we provide higher level analog (0 to 5V DC) or digital (USB, Serial, XBee Wireless) outputs.

Low Profile. Flexible shapes.

Our sensors can be built with very small gaps between capacitive plates while maintaining very high sensitivities. Because of this we can build sensors that are very small in diameter and height, and can be built in diverse shapes and patterns to suit the end use application.

Low power consumption.

The sensors draw as low as 10 mW. This can be an important advantage for mobile/unattended battery based applications.

Easy connectivity.

Sensor information can be accessed from a computer or PDA using a wired or wireless gateway. Information can also be accessed via the Internet on a graphical Web page.

Scalability.

Loadstar sensors can be scaled up or down to meet your measurement range requirements. We have already developed sensors that can handle 10, 50, 250, 1200, 2000 and 10,000 pound loads with the same underlying technology.

Lower cost.

Our patented technology enables us to build these sensors at a significant reduction in cost. We are able to pass on these savings to you by charging 50% to 90% lower prices than our competition.

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.

Traditional resistive load cells are typically made from strain gauges arranged in a balanced wheatstone bridge configuration.  When a voltage is applied to them and a strain is induced due to an applied load, they typically output a millivolt signal proportional to the strain felt by them.  This signal is represented as a mV/V number i.e. if a 10 V DC power supplyis applied, a 20 mV total change in signal can be seen in response to applied loads. A user would need a signal conditioner to condition this signal, a data acquisition system to digitize the signal and application software to view and process the signal! Loadstar Sensors offers you a much more elegant and simpler solution!

Our iLoad Digital load cells based on a capacitive sensing technology. converts a load into a reading on a PC or a microcontroller without need for any additional signal conditioners, data acquisition systems or special software. Connect a iLoad Digital load cell to your PC's USB port and start measuring.

Alternatively, our DI-100U or DI-1000U load cell interfaces offers a similar digital output for resistive load cells to connect to a PC via std USB port.  The USB port itself provides the power needed to operate the system without need for any external power source.

Our DI-1000ZP wireless load cell interface enables a resistive load cell to connect to a PC via std. IEEE 802.15.4 wireless protocol.  The unit can be powered from a wall adapter or run on the internal rechargeable battery for truly cordless operation.

This simplicity of use is what we refer to as "Plug and Sense Simplicity". This technology enables end users without any knowledge of load cells to start measuring forces and loads within minutes. It enables software developers to build great new applications that incorporate force and load measurement. You don't need to be an electronics engineer to use one!

The optional LoadVUE load cell software can provide an elegant interface to make it even more convenient to measure loads, and to log the data to a comma separated text file that can be easily opened using Microsoft Excel, Matlab or other statistical analysis packages.

Our iLoad Analog load cells convert a load into a signal in the 0-5 VDC range. These load cells are ideal for applications that need to input signal directly into an existing PLC/DAQ - there is no need for a signal conditioner or linearizer. Our load cells output a linear signal: 0.5 V at No Load and 4.5 V at Full Load, to make it easy to incorporate our load cells into your custom applications.