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Interfacing Pressure Sensor to logic device input pin
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A G
Inc.
Your comfort level with your responses to the following 2 design tasks assessment tool should indicate if the presentation that follows will increase you knowledge base on the topic outlined by the questions in this tool.
Pre-presentation Self Assessment Activity
Pre-presentation Self Assessment Activity
Your task is to design an interface circuit that connects a strain gauge based pressure sensor to a TTL AND device so that an action will occur if the pressure sensor and a temperature sensor are active at the same time. Assume the temperature sensor is an active low temperature sense high device however;
(a) Make the pressure sensor behave like an active high PSH.
(b) Make the pressure sensor behave like an active low PSL.
Comparator application Limit Sensor interface
+
-
“non-inverting input”
“inverting input”
output
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Vstrain
Vtotal
Rpull up
Vpull up
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
As the pressure changes the strain gauge bends and gauge resistance changes.
Strain gauge
Note:
A strain gauge is a thin flexible membrane with a resistor embedded in the film.
Vout
Vstrain equals the voltage drop across the strain gauge. Vpull up equals the voltage drop across the pull up resistor, R pull up. Vtotal equals the Vstrain plus V pull
up.
Vpull up
Vstrain
+Vtotal=
Vout= V
total- Vstraino
r 0 Volts +
Vpull up
Vout=
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
Strain gauge
Vstrain
Vtotal
Rpull up
Vpull up
The signal from this terminal;
Needs to be monitored to determine if the pressure has changed from steady state value.
May trigger a process change when the pressure exceeds its allowed limit value.
Vout
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
Strain gauge
Vstrain
Vtotal
Rpull up
Vpull up
Needs to be monitored to determine if the pressure has changed from steady state value.
Vtotal
+12 V
-12 V
Power terminal for device
+
-
Rtune
Power terminal for device
If strain gauge resistance chances from steady state value, op amp output voltage will continue to increase (decrease) until;
Vout
Gauge resistance returns to steady state value. Output signal reaches maximum possible value.
May trigger a process change when the pressure exceeds its allowed limit value.
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
+12 V
-12 V
Power terminal for device
Power terminal for device
Strain gauge
Vstrain
Vtotal
Rpull up
Vpull up
+
-
Needs to be monitored to determine if the pressure has changed from steady state value.
May trigger a process change when the pressure exceeds its allowed limit value.
Vtotal Rtune
5 V clamped between 0 V and 5 V
Vout
is now
an input to a logic device
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
+12 V
-12 V
Power terminal for device
Power terminal for device
Strain gauge
Vstrain
Vtotal
Rpull up
Vpull up
+
-
When the pressure goes down the op amp output goes to 0 volts and action signal remains passive.
Vtotal Rtune
5 V
TSH Temperature sensor is from a different part of the process.
action
PSH
Vout is now an active high high pressure limit sensor
When the pressure goes up the op amp output goes to 5 volts. If TSH is active, action signal become active.
Pressure Limit Sensor interface
Operational Amplifier’s only Rule of Operation
If the two inputs signals are equal, then the output signal is zero volts.
Comparator application
+12 V
-12 V
Power terminal for device
Power terminal for device
Strain gauge
Vstrain
Vtotal
Rpull up
Vpull up
+
-
Vtotal Rtune
5 V
TSH Temperature sensor is from a different part of the process.
action
Vout is now an active low low pressure limit sensor
When the pressure goes down the op amp output goes to 0 volts. If TSH is active, action signal become active. When the pressure goes up the op amp output goes to 5 volts and action signal remains passive.
PSL
Post Presentation Self Assessment Activity
Your task is to design an interface circuit that connects a strain gauge based pressure sensor to a TTL AND device so that an action will occur if the pressure sensor and a temperature sensor are active at the same time. Assume the temperature sensor is an active low temperature sense high device however;(a) Make the pressure sensor behave like an
active high PSH.
+12 V
-12 V
Vstrain
Vtotal
Rpull up
Vpull up
+
-
Vtotal Rtune
5 V
TSH
action
PSH
(b) Make the pressure sensor behave like an active low PSL.
Post Presentation Self Assessment Activity
Your task is to design an interface circuit that connects a strain gauge based pressure sensor to a TTL AND device so that an action will occur if the pressure sensor and a temperature sensor are active at the same time. Assume the temperature sensor is an active low temperature sense high device however;
+12 V
-12 V
Vstrain
Vtotal
Rpull up
Vpull up
+
-
Vtotal Rtune
5 V
TSH
action
PSL
End of Presentation
A G
Inc.
