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12. DISCRETE SENSORS
• There are a number of system conditions that need to be detected when doing control. These conditions are most commonly true/false detections, having discrete states.
12.1 INTRODUCTION
•There are a few important concepts when connecting to sensors, including,
-The physical phenomena that allows detection
-The actual wiring of the sensors
12.2 SENSOR WIRING
•There are a variety of basic methods for connecting sensors to devices. These include,
-Plain Switches - normally open or closed to allow current to flow.
-TTL (Transistor Transistor Logic) - for low voltage logic using 0V and 5V.
-Sinking/Sourcing - DC current is drawn through the sensor.
-Solid State Relays - These are used for switching AC outputs.
12.2.1 Switches
• Plain switched outputs usually accompany relays for outputs, or similar devices.
12.2.2 Transistor Transistor Logic (TTL)
•This method switches between two logical DC voltages.
•Typically the low voltage is 0V and it indicates a false condition.
•The true state is normally indicated with a high voltage. 5V is quite common, but other voltages can be used for certain sensors.
•The voltage levels have certain tolerances built in. For example 0V indicates false, but a voltage up to 1.2V might still be considered false. This also means there is an ambiguous zone where the voltage will be judged neither true or false. This zone can be eliminated using schmitt trig-
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gers.
12.2.3 Sinking/Sourcing
•These scheme is used for a variety of reasons.
-they can be made to appear like normal switches.
-they can be used to switch low current devices and hence replace controllers for simple applications.
•The two types indicate which way the current and voltage are switched to the output. The methods also refer to transistor types because of the similar behavior (Note: in fact they do use transistors inside).
•The two main differentiation is,
-Sinking (NPN) - In this case, when actuated, the sensor will connect to ground, or pull the input low. If this is the case you need to use an output that normally stays high, true, floats high, etc. The figure below shows an approximate representation.
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V+ |
The sensor responds to the physical |
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V+ |
phenomenon. If the sensor is |
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inactive (nothing detected) then |
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physical |
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sensor |
the active line is low and the |
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phenomenon |
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transistor is off, this is like an |
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output |
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open switch. That means the |
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NPN |
NPN output will have no current |
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Sensor |
in/out, and hence no voltage. |
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When the sensor is active, it will |
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Active Line |
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make the active line high. This |
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will turn on the transistor, and |
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effectively close the switch. |
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V- |
This will allow current to flow |
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V- |
into the sensor to ground (hence |
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sinking). The voltage on the |
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NPN output will be pulled down |
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to V-. Note: the voltage will |
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always be 1-2V higher because |
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of the transistor. |
- Sourcing (PNP) - In this case, when actuated, the sensor will connect to V+, or pull the input high. If this is the case you need to use an output that normally stays low, flase, floats low, etc. The figure below shows an approximate representation.
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V+ |
The sensor responds to the physical |
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V+ |
phenomenon. If the sensor is |
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inactive (nothing detected) then |
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physical |
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the active line is low and the |
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phenomenon |
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transistor is off, this is like an |
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Active Line |
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open switch. That means the |
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PNP output will have no current |
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Sensor |
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in/out, and hence no voltage. |
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PNP |
When the sensor is active, it will |
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make the active line high. This |
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sensor |
will turn on the transistor, and |
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output |
effectively close the switch. |
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V- |
This will allow current to flow |
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V- |
from V+ through the sensor to |
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the output (hence sourcing). The |
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voltage on the PNP output will |
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be pulled up to V+. Note: the |
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voltage will always be 1-2V |
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lower because of the transistor. |
• To directly connect devices to these sensors we can use the arrangement below.
sensor V+
NPN
V-
sensor V+
PNP
V-
V+
power supply
V-
V+
power supply
V-
Note: remember to check the current and voltage ratings for the sensors.
• It is worth stating the obvious - The output of sensors will be the inputs for other devices, such as PLCs - this will lead to confusion when specifying PLC input devices. Some manufacturers
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indicate what the input type is, others specify what it is for. This basic result of this is that you must look at the electrical connections of the card, and not just the designation.
Manufacturer |
Sensor Type |
Name of Required Card Type(s) |
Allen Bradley |
NPN |
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PNP |
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Aromat |
NPN |
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PNP |
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• Note, if using these sensors with PLCs, care is required to select the appropriate cards and connections. The figure below is for NPN sensors.
PLC Input Sourcing Card (2 DC inputs)
Electronics Card Internal
PLC Data Bus
+V |
current flow |
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+V |
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+V |
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power |
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NPN |
supply |
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NPN |
-V |
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sensor |
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00 |
-V |
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01 |
Note: When we have a PLC input card that has |
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a V+ (not a common) then we can use NPN |
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sensors. In this case the current will flow |
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External Electrical |
out of the card (sourcing) and we must |
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switch it to ground. |
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ASIDE: This card is shown with 2 optocouplers (one for each output). Inside these devices the is an LED and a phototransistor, but no electrical connection. These devices are used to isolate two different electrical systems. In this case they protect the 5V digital levels of the PLC computer from the various external voltages and current.
• The figure below is for PNP sensors. These are generally more common combinations.
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PLC Sinking Input Card (2 DC inputs)
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+V |
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00 |
PNP |
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PNP |
current flow |
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sensor |
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Internal |
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-V |
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Card |
01 |
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+V |
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Electronics |
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power |
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supply |
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-V |
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com |
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Note: When we have a PLC input card that has |
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a common then we can use PNP sensors. In |
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this case the current will flow into the card |
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and then out the common to the power sup- |
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ply. |
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•It is quite common for manufacturers to offer PLC output cards that will handle PNP and NPN sensors. In this case the card will require both V+ and a common connection, and the each output must be set for either NPN or PNP.
•Two wire sensors are also common because they reduce the wiring.
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PLC Sinking Input Card
PLC Sourcing Input Card
V+ |
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+V |
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two wire |
00 |
sensor |
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-V |
01 |
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+V |
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power supply
-V
com
Note: These sensors require a certain leakage current to power the electronics.
V+
00 |
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+V |
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two wire |
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sensor |
01 |
-V |
+V |
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power |
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supply |
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-V |
com