Force-Balanced Sensors
The force balance sensor is geared towards DC and low-frequency acceleration measurements, such as those found in the motion of vehicles, aircraft, and ships. These sensors have the capacity to measure levels from as low as 0.0001 g up to 200 g across a frequency range from DC to 1000 Hz. Additionally, due to their innate sensitivity to gravity, making specific modifications or introducing special features to the force balance accelerometers makes them excellent instruments for measuring inclination angles. This kind of sensor, generally known as an inclinometer, is practical for applications such as borehole mapping, gun sight control, platform levelling, pipeline levelling and other low-level seismic measurement applications.
The force-balance sensor offers several advantages that produce excellent performance in many applications. Internal displacements within any accelerometer generate inaccuracies and errors typically in the form of disproportionate levels of hysteresis, stickiness, non‐linearity, and non‐repeatability.
LVDT, potentiometric, variable reluctance and similar kinds of sensors all have the potential to cause these errors, which results from the fact that the sensing element must travel over some distance to generate a measurable change in output.
Conversely, the output signal from a force balance accelerometer is not contingent on the displacement of some internal element being a linear function of acceleration. Internal displacements remain comparatively small, generally less than one ten-thousandth of an inch.
In addition to minimizing static error, the minute displacements relative to the force balance sensor contribute to the relatively high natural frequency this type of sensor possesses. A strain gage sensor does not necessitate excessive internal displacements but is vulnerable to effects of temperature, creep, and aging, which can result in instability.
Opposed to other low-frequency accelerometers, which need viscous media, dashpots or similar mechanical damping techniques, the force balance sensor’s dynamic response can be damped and adjusted with ease to a specific value by way of electronic networks.
The damping ratio can be calibrated to near critical for a maximum usable response or to a higher degree, or limited response and sensitivity to high frequencies.
Generally, in open-loop types of transducers, the ratio is either controlled by means of viscous media or uncontrolled, as in the case of piezoelectric devices. In the former case, it is not possible to control the damping ratio by any tight tolerances due to viscosity changes vs. temperature.
Several strain gage, potentiometric or LVDT type accelerometers incorporate thermostatically controlled heaters in an attempt to stabilize damping characteristics.
Additionally, in most cases, the force balance accelerometer is completely self-controlled, which means additional signal conditioning is not necessary, which means they have the capacity to directly interface with spectrum analysers, oscilloscopes, data acquisition systems, digital voltmeters and displays. The full-scale output is typically in the order of several volts and necessities no additional amplification.
Servo Sensors
The difference between an LSOC and an LSI servo inclinometer is only the housing in which the measurement elements are contained. The LSI is lightweight housing while the LSOC is based on a solid metal enclosure.
Force-balanced (servo) precision inclinometers/accelerometers are extremely sensitive, rugged transducers with virtual infinite resolution. The sensor output is a Voltage or Current signal which is not requesting any further conditioning.
The output of a force-balanced (servo) sensor is a sine-based voltage signal. Depending on the connected device (digital display, data acquisition system, control loop) the voltage signal is scaled to the calibrated range.
A force-balanced (servo) sensor is not ageing and the calibration is usually not changing over time. Sherborne, like most sensor manufacturers, is not suggesting re-calibration periods in their product performance specifications. Higher-accuracy measurements require more frequent calibration, ranging from once every six months to once every year. The best way to determine the optimum time between calibrations is from the calibration history of the initial period of life of the sensor in its application. As a guide, where average industrial accuracy is required, no sensor should be used for more than a year without a calibration check. The absolute limit is three years.
Servo Accelerometers
Most accelerometers are calibrated by exiting the sensor together with a reference sensor on a shaker. A force-balanced (servo) accelerometer is calibrated on a turning table, whereby the set angle of the sensor represents the acceleration caused by gravity.
A force-balanced (servo) sensor is most suitable to measure static acceleration due to its gravity-based measuring element.
Wireless Inclinometers
Yes. The wireless signal can be demodulated, with radio telemetry data, to an analog output converter. The analog outputs are configurable to 0-10V, 0-5V, 4-20mA, 0-20mA, +/-10V, +/- 5V. The update rate can be set to up to 2000 per second.
Load Cells
The entry barrier to manufacturing simple loadcells is very low and has attracted many manufacturers in emerging countries. Simple load cells are manufactured at very low prices.
To compete in such conditions, the output volume has to be high to generate an acceptable revenue. Sherborne is not a company for mass production and therefore we concentrate on technical demanding, bespoke loadcells.
Accuracy is the sensor’s or instrument’s degree of veracity – how close a measurement gets to the actual or known value. To check the accuracy means to check how close a reported measurement is to the actual value. Resolution on the other hand is the smallest distinguishable change the sensor can detect and display.