Sensor Performance Specifications

Measurement How important is measurement? Consider this: Everything you buy in a grocery store is measured. A pound of cherries, a gallon of milk, and a dozen eggs. Just as eggs are measured by counting them, the cashier will measure the money you hand him by counting it. Everything is measured in this transaction. Besides commerce, which is impossible without measurement, we need measurement for numerous everyday tasks. We measure our speed to avoid tickets (and accidents), and we measure the gasoline in the fuel tank so we don’t run out. Cooking without measurement usually doesn’t work out very well. Carpentry is another example: Measure twice and cut once. Clocks for measuring time are everywhere. Engineering and science rely on measurement to an even greater extent. Without measurement, engineering would be impossible, and science would be philosophy. But, as alchemy shows, measurement alone doesn’t make good science or engineering. To make measurements, we need to recognize dimensions. There are really only four physical dimensions, namely, length, mass, time, and charge. We could argue that apples are different from bananas and introduce dimensions such as “pound of apples” or “pound of bananas.” Although this distinction is useful if you’re eating dinner, it isn’t relevant to dynamics where inertia is more important than chemical composition. In addition to dimensions, we need units. For length we can choose inches, centimeters, miles, or some other unit. Without units, measurement is meaningless, and no one, including ourselves, will know what we’re talking about. Units are usually set by agreement so that two people who have never met can exchange measurement data. Although there are many shoe sizes, there is only one foot. A measurement is information about a physical quantity, and to make measurements you need a sensor. A ruler is a good example. By merely placing the ruler next to an object, you can measure its length. Using your eye, you “read” the ruler, and the measurement is transmitted into your brain, where it is stored for as long as you can remember it or as long as it takes to record it on a piece of paper or in a computer. The role of a sensor is to facilitate the transfer of information about a physical quantity into a display, computer register, or onto a piece of paper where it can be accessed. Data acquisition is the process of collecting and storing measurements. But measurement is never perfect. The ruler may be slightly bent, the object may have a rough edge, the ruler might slip, or the object’s edge might fall between two ruler markings. These and other problems can contribute to measurement errors. A more subtle problem with measurement is that the information the sensor conveys about a physical quantity requires a transfer of energy. This is not a problem in measuring length by a ruler. In that case, you merely shine light on the object, which heats it slightly and changes its length by an inconsequential amount. When measuring the flow rate of a gas or liquid, your sensor may disrupt the flow and thereby obtain an erroneous reading with respect to the undisturbed flow. In control system engineering, sensors play a critical role by providing measurements for feedback. The control system engineer must determine which quantities must be measured and how well they must be measured to achieve desired control system performance. In the following sections I explain sensor specifications, which quantify the ability of a sensor to provide measurements of physical variables. Sensor manufacturers typically provide information about sensor specifications in the form of a “spec sheet.” The concepts described here will help you interpret the information on a spec sheet. The concepts discussed below apply to all branches of science and engineering, not just control systems. However, the systems approach of control engineers provides a unique perspective on these topics and associated issues.