Understanding Sensor Specifications
Operating voltage, linearity, dimensions, polarity protection, range… When it comes to sensors, every situation calls for understanding these specs in detail during design, purchasing, and installation.
Why Are Sensor Specifications Important?
Sensor specifications are not something to be overlooked when considering the purchase of new sensors. Whether the sensor is purchased to repair existing equipment or for use in the design process of a new or upgraded automated system, a quick consultation of the specifications can help ensure the correct usage of the sensor.
However, knowing what a datasheet is saying can be difficult without some background knowledge. This article will look at some of the common information found in spec sheets and the meaning behind it.
Figure 1. Sensor specifications found on datasheets will vary based on the type of sensor. Image used courtesy of Adobe Stock
Sensor Datasheets / Specification Sheets
Specification datasheets for sensors give specific information that includes voltage ratings for the sensor itself, operating range, min/max values, voltage drop, and many others. Some information is displayed for all types of sensors, but other specifications are specific to the type of sensor being used. For example, a pressure sensor will not have any specifications for light wavelength like some laser sensors might.
In addition to physical constraints, there are limits imposed on the sensors based on the electronics inside of the sensor. These can also be found in specifications and are useful in determining the accuracy of certain sensors. Examples of limitations based on the sensor's electronic capabilities include nonlinearity, response time, and hysteresis.
Common Specifications and What They Mean
Looking at a specification sheet for a sensor can be intimidating because of the amount of data available, especially since most of it is labeled with technical language that can be difficult to grasp. Even though the language may be difficult to understand at first, most of the concepts are straightforward. Looking at common terms with a simple explanation of what they are can be useful.
Figure 2. A sensor’s range determines the value of inputs a sensor can process. Image used courtesy of AdobeStock
The range of a sensor determines the value of inputs that the sensor can process. For example, a laser distance or proximity sensor might have a range of 0.2 m - 10 m. This means that the laser can be used to measure distance or the presence of an item within the range of 0.2 m at a minimum to a maximum of 10 m. The values 0.2 m and 10 m are this particular sensor's min and max values, respectively.
An additional connotation that often accompanies range is span. The span of a sensor is simply the total of the range. In the case of the laser distance sensor example, the span would be 9.8 m because 10 m - 0.2 m = 9.8 m. Range is very important and often one of the main determining factors in choosing a sensor since a sensor is useless outside of its operating range.
Each sensor has its own needs when it comes to power supply, consumption, and protection. Looking at the electrical specifications is necessary for ensuring the sensor will have the correct operating voltage, current, and protective measures necessary. The following are typical examples of useful electrical specifications and their meanings:
Operating Voltage - This is the voltage range the sensor can operate normally without damage, for example, 10-30 VDC or 90-130 VAC.
Current Consumption - This is the maximum amount of current the sensor will use, for example, <100 mA, meaning the sensor will not exceed a current usage of 100 mA.
Reverse Polarity Protection - Whether or not a sensor is protected, should the polarity of the circuit be switched. This is useful to keep the sensor from being damaged should it be wired incorrectly upon installation.
Short Circuit Protection - Short-circuit protection protects the sensor's output from damage should the output be short-circuited. Without this, the sensor would become permanently damaged from a shorted output.
Typical Lifetime - The amount of time the sensor can be expected to function correctly under normal circumstances, for example, 50,000 hours. This equates to just over five years of continuous use. For electromechanical devices, lifetime might be measured in switching cycles.
Figure 3. Diagram of sensor nonlinearity showing ideal curve, actual measured curve, and maximum error. Image used courtesy of NI
Nonlinearity - Expressed as a percentage that the sensor reading deviates from the actual measurement curve. It is often represented graphically as an ideal curve with the measurement curve transposed over it. The maximum difference between the two is the maximum nonlinearity. This is important because nonlinearity may create a data difference larger than the maximum amount of error to be tolerated in the system, making the sensor unusable for certain applications.
These are the specifications that are useful for the physical constraints of the sensor. These include temperature ranges, physical size, and weight.
Temperature Range - Temperature range can be split into the operating temperature range and storage temperature range. Operating temperature range is the maximum range the sensor can function normally in, for example, 0-100 °C. Storage temperature range is the range of temperature the sensor can be stored in when not in use without causing damage to the sensor.
Dimensions - Dimensions are often given in length, width, and height.
Weight - The weight of the sensor without any added components. It is important to know which sensor is being purchased for these final two specifications since many vendors use stock images. For example, all the other specs between a 22 mm and 30 mm diameter sensor may be identical.
Figure 4. Laser distance sensor on a roller conveyor. Image used courtesy of Adobe Stock
Sensor Accuracy / Deviations
Accuracy and deviations are determining factors in how precise a sensor is in its measurements. These specifications are highly dependent on the type of sensor being used. For example, a laser distance sensor may have a large list of specifications that determine accuracy based on the measured color or material. Still, a flow meter may have only a few specifications on accuracy based on temperature and material measured.
I/O specifications define the amount, type, and specifics of input and output signals for a sensor. Many different types of information can be displayed in this section of a datasheet, but here are a few of the most common:
Outputs - The number of outputs the sensor can produce can be further broken down into analog and digital types. If the signal is analog, a range will also be given for the analog signal range. The output will also have a specification based on whether the output is normally open or normally closed, NPN/PNP, or both.
Inputs - the number and type of inputs available for the sensor to use and whether they are analog or digital. Sometimes an input signal can be used to program or ‘teach’ a sensor.
Output Current - often expressed as a range of available current for analog outputs.
Sensors provide data necessary for the correct operation of all types of automation. Choosing the right sensor is vital to the proper operation of the cell. The specification sheet is full of useful information to help make an informed choice when selecting a sensor.
While some information is sensor-specific, much of the information on one spec sheet, such as operating temperature or operating voltage, will be found on all. Understanding these specifications can save time and money from incorrect sensor applications and the downtime they can create.