Monday, May 11, 2009

Accuracy vs. Resolution


the often confusing but important difference with digital display inclinometers



In mechanical testing and measurement, it is often important to understand how an object reacts to various forces. An inclinometer is commonly used to measure tilt, or the amount of inclination of an object in relation to gravity. When using an inclinometer, you mount the sensor to the device, apply tilt, and measure the incline by detecting changes in a certain output.

One type of output, via LCD digital display, provides an interpretation of angle based on the sensor output being sampled by a microprocessor - which in turn calculates an angle and presents it (typically) by a series of numbers (00.00º). With digital displays, these numbers are what is considered the resolution or number of decimal places the angle is represented. If the LCD is displaying 25.1º, this indicates an angle resolution in tenths of a degree (25.01º represents an angle resolution in the hundredths of a degree). This is not what determines the accuracy specification of the device.

Ideally, we would like the output of the inclinometer to change only in response to the tilting action. However, the sensor also responds to changes in temperature. Temperature related effects are the most common causes of error.

Accuracy depends on the expected temperature range of the installation. It is a combination of initial sets of sensor zero offset and sensitivity, sensor linearity, hysteresis, repeatability, and the temperature drifts of zero and sensitivity. Typically in room ambient conditions the accuracy is limited to the sensor linearity spec. So a digital inclinometer with LCD display indicating 0.1º resolution does not equate to a 0.1º of accuracy (a very tight accuracy spec) - this should be considered a major question when looking at digital display inclinometers.

If you have any questions on this subject, feel free to contact me directly.