|Editor's note: this section will be covering output interface selection and applications of the most frequently used interfaces in coming newsletters, starting with analog in this issue.
Analog interfaces are an obvious choice for design engineers needing to connect a position or angle sensor with a pre-selected control device that accepts standard current and/or voltage inputs. These sensors are generally available with one or more of three analog outputs: percentage of supply, defined voltage and defined current outputs.
A voltage output option that is a percentage of supply voltage, say 10 to 90% of a 10 V supply would result in a sensor output range 1.0 to 9.0 V full range.
An example of a defined voltage range output, independent of a 24 V supply, could be stated as a 0 to 10 V output range representing 0 to full scale of stroke length (or angular range). The actual usable range must be a small minimum voltage e.g. 0.2 V, making the actual output range 0.2 to 10 V. This output is sometimes available with a decreasing value representing an increasing value in stroke or angle. In that case, +10 to 0 V could represent, for example, 0 to +100 mm change in stroke position.
There are generally two defined current output ranges. These are 0 to 20 mA and 4 to 20 mA. These current values represent a minimum to maximum stroke range or angular range.
Voltage output sensors are easy to implement as most PLCs have modules accepting a voltage input. Sensors with current outputs are used for longer cable runs and where electrical noise is present in the environment, as current is inherently unaffected by noise and its level is sustainable over a long cable run with a sufficiently sized conductor wire gauge.
Sensor current outputs of 4 to 20 mA have the additional benefit of making it possible to detect a broken or disconnected wire condition because the output would drop to 0 mA. Since this value is outside of the operating output range, it would indicate a fault condition.