Pressure and Data Acquisition: A Comprehensive Guide
Pressure Problem 7
Pressure (Pascal or N/m2) = Force (N) / Area (m2)
Air pressure decreases with increasing altitude
Static pressure measurement= NO MOTION Dynamic pressure measurement= WITH MOTION. Absolute pressure is measured relative to a vacuum (0 N/m2). Gauge pressure is relative to ambient atmospheric pressure. Differential
Pressure is like gauge pressure, but a specific reference pressure is used instead of the atmospheric pressure.
Locations to measure differential pressure: ACROSS FANS, ACROSS PUMPS(to tell if functional/working or not), ACROSS FILTERS(to tell how dirty it is), IN FLOWING WATER STREAM(used to calculate flow rate of air/water)
Air Pressure
The pressure of air is made up of static pressure plus additional pressure associated with speed of air. If we subtract the static number from total number, able to calculate the pressure caused by wind speed. Differential pressure sensor measures total component and static component and does the subtraction. The sensor measures the pressure associated with wind speed. Thus,can calculate how fast the wind speed is.
Pressure Sensor/Capacitance Pressure Transducer
Pressure is converted to intermediate quantity like displacement, before convert to electrical output like voltage/current. Most used common pressure transducers uses strain gauge, variable capacitance, piezoelectric crystals. Capacitance = Absolute permittivity of insulating material X (Surface area / Distance). A variable capacitance pressure transducer measures change in capacitance between metal diaphragm and fixed metal plate. The transducers are very stable with rapid response, but sensitive to high temp and more complicated to implement.
Signal Conditioning for Capacitance Sensors
A typical signal conditioning circuit for capacitance sensors is shown.Capacitors C1 & C2 form a voltage divider in AC bridge.Fixed AC voltage is derived
from the right side of the bridge.The voltage at point X is a function of the capacitance of C1 & C2.The voltage at point Y is normally half of the supply voltage.The capacitive reactance of C1 & C2 will depend on the frequency of the oscillator.Output voltage can be easily determined based on the values of C1 & C2.Normally in the absence of any pressure, the bridge will be balanced and its output is zero.
Piezoelectric Pressure Transducer
It capitalized on electrical properties of naturally occurring crystals like quartz. When the crystals are strained, and electrical charge will generate. They do not need presence of excitation voltage and are very rugged. Since they respond to strain, they are sensitive to shock and vibrations. Charge amplification circuitry is required.
Potentiometric Pressure Transducer
Electric output can be obtained from mechanical pressure gauge by using potentiometric pressure sensor. The wiper arm of a precision potentiometer is connected to a bellow element that expands and contracts based on pressure. Change in resistance in respond to pressure can be detected with aid of wheatstone bridge. the sensors generate strong output voltage without additional amplification, and can be used in low power applications. Potentiometric sensors can be used to measure pressure of between 35kPa to 70MPa.
Pressure Measurement
Pressure transducers will deliver voltage output. Strain-based pressure transducers generate voltage in mV range. Signal conditioning ciscutis are required to handled small signals. Transducers will output conditioned signal of 0-5V or 4-20mA, where oV n 4mA will correspond to 0 pressure measuremnt, and 5V n 20mA corresponds to full scale caoacity or maximum pressure it is designed to measure.
Signal Conditioning Calculations
When an output voltage is obtained, it must be converted into pressure reading. PRESSURE = (Cfs/Vex)(Vmeas/CF)
Selection Criteria
Accuracy, Pressure Range, Operating Temperature, Types of environment(vibrations,shock, explosive gas, etc), EMI&EMC requirements , Outputs(Voltage or current output? 0-5V, 4-20mA, etc), Isolation(Required when sensor is used in medium which is highly corrosive like acids or other hazardous chemicals)
Applications
-Engine coolant, lubricant & oil pressure monitoring in automotive applications -Used in medical equipment for applications relating respiration, dialysis & infusion. -Heating Ventilation & Air Conditioning -Process Controls. -Pumps & compressors
DAC Problem 9
Applications: A/D & D/A converters have applications in many devices like RF transceivers,measuring instruments and electrical appliances. Since computer process signals in digital form, the most significant use of A/D & D/A converters is to allowcomputers to interact with real world. A/D & D/A converters are widely used in computerized control systems and instrumentation systems for process control.
Applications of DAC
Video: Video signals from digital sources must be converted to analog form if they
are displayed on an analog monitor.DAC is normally integrated with some RAM which contains conversion tables for brightness, contrast,etc,making a device called RAMDAC (Random Access Memory Digital-To-Analog Converter).RAMDAC can be found in the VGA controller chip that maintains color palette and converts data from memory to a type that is suitable for
the monitor.
Signal Processing: DACs are widely used in analog signal processing to replace potentiometers. Small adjustments can be made to a circuit with the use of a software instead of using potentiometers or trimmers. Some types of DAC have non-volatile memory to enable them to remember their last settings before the power is turned off.
Resolution: Resolution of the DAC is the number of bits in the input digital word. Each combination of digital input value will have a unique analog output voltage which is associated with it.
Offset Error (Zero Error):Output voltage that exists when inputdigital code is set to give an ideal output of zero volts. All the digital codes in the transfer curve are offset by the same value.
Gain Error: It is the difference between the actual and ideal gain points without the influence of offset error. Gain error is measured at the rightmost vertical jump. The difference between the output voltage (or current) with full scale input code and the ideal voltage (or current) that should exist with a full scale input code.
DAC Monotonicity: DAC is said to be monotonic if output increases or at least remains the same for an increase in the input digital code.Monotonicity is a critical characteristic in digital control applications, where non-monotonicity can generate oscillations in the system. A monotonic function has a slope whose sign does not change. Monotonic DAC has an output that changes in same direction (or remains
constant) for each increase in the input code. The converse is true for decreasing
codes.
DAC Accuracy: A comparison of actual output of DAC with expected output. Expressed as percentage of full-scale/maximum output.
Conversion Speed & Settling Time: Time which the DAC takes to provide an analog output when the input digital word is changed is termed as the conversion speed. Settling time of the DAC is the time it takes for the output to settle to within a given margin of error (usually ±0.5 LSB) due to oscillations. Loading & switching speeds can affect settling time of DAC.
Settling Time: The time from a change in input code until a DAC’s output signal remains within LSB of the final value.
ADC PROBLEM 10
DAQ Applications: ENVIRONMENTAL STUDIES(Temp, Humidity, Acidity, Pollution Index),PROCESS CONTROL(Temp, pressure, level, flow measurements,displacement, velocity), MEDICAL RESEARCH(Blood Pressure, Respiration, Heart Rate,Weight), QUALITY CONTROL & TESTING (Vibration, Shock)
Basic Components of DAQ
Analog to Digital conversion(ADC): Once conversion is complete the digital information can then be processed by the computer, or transferred and stored in memory.
Digital to Analog conversion(DAC): These converters convert the stored data back to their analog equivalent for display or control purposes. The output from the D/A converter can be used to drive external devices which accepts analog input (e.g. pump motor, heating coil, sound systems).
Digital Input Output: These are primarily used for control purposes between the host computer and a digital device which accepts digital input.
Analog Signals
Analog signals are continuous signals of varying magnitude. They are normally represented by continuously changing physical quantities.
Digital Signals
Digital signals are essentially discrete signals.They are represented by “bits” (binary digits).Each “bit” is represented by one of two possible states:ON or OFF, TRUE or FALSE, 1 or 0.Digital signals can represent physical quantities in binary codes (1’s or 0’s).
PC Based DAQ
There are many ways in which an instrument can exchange data with a computer, one of itis with the use of a serial port. Normally a DAQ module will have an ADC (Analog-to-Digital Converter) and DAC (Digital-to-Analog Converter) to allow the input & output of analog and digital signals as well as Digital I/O (Digital Input/Output).
Communucation Standards
GPIB (General Purpose Instrumentation Bus) : Used to allow instruments like oscilloscopes and spectrum analyzers to exchange data with a PC or laptop.This is a standard bus to control electronic instruments with the aid of a computer.
Serial Communication:
The most common serial connection in the industry. Supported standard include RS232/ RS422 / RS485 which varies on the total number of devices connected and the transmission distance.
PCI/PCIE:
Very popular hardware interface used on many computer platforms. They are ideal for data acquisition (DAQ) requirements because of its high speed and high performance throughout.
USB port:
Allow easy and efficient Plug & Play where many instruments, data acquisition devices, cameras, and other hardware can connect to a computer through a USB cable. There is no need to open up the computer chassis to install boards. Just plug in the module, and then get the data via the software provided.
Sampling
DAQ modules acquire data through a process called sampling.A sample of the analog signal is taken at discrete time intervals. The rate at which the signal is sampled is termed as the sampling frequency or sampling rate. The quality of the converted analog signal depends on the sampling frequency. Nyquist sampling criterion requires the sampling frequency to be at least twice the maximum frequency of the analog signal to be converted.
Analog to Digital Converters
Once the analog signal has been sampled, it is converted into a digital code. This process is called analog-to-digital conversion. DAQ modules have different sampling frequency depending on the application. Normally the cost of the boards increases with sampling frequency. Most DAQ modules have a multiplexer which behaves as a switch between different channels and the ADC. Through this way it is possible to have a multi-channel input board while using only one ADC.
Resolution of ADC
The precision or the accuracy of the conversion depends on the number of bits of the ADC. The accuracy of the converted signal is a function of the number of bits the ADC uses to represent the analog data in its equivalent digital form. Resolution of the ADC can either be expressed in volts (smallest measurable signal) or bits (number of 1’s and 0’s at the output). The number of division of the voltage range depends on the resolution ofthe ADC in bits. A 8-bit ADC have 256 levels 28, while a 10-bit ADC have 1024 levels 210. Therefore a 10-bit ADC is able to measure a smaller change in analog input voltage as compared to a 8-bit ADC. RESOLUTION (V) = RANGE OF INPUT/ (2resution in bits– 1)
Linearity: In an ideal condition when the input voltage to the ADC is increased linearly, the output digital code will increase in the same manner.
Differential Non-Linearity: Most commercial DAQ modules have ADCs with some degree of nonlinearity.
Sampling Rate VS Conversion Time
The sampling rate of an ADC is the speed at which it acquires the analog input data. This is normally expressed in terms of samples per second (sps). For a multi-channel DAQ module, the sampling rate is normally given as the speed of the A/D Converter. The time between the start and end of the conversion is known as the conversion time. The maximum conversion time = 1/minimum sampling rate
VI FOR LEVEL CONTROL SYSTEM PROBLEM 11
Level Control System: Used in the petrochemical, pharmaceutical, food and HVAC industries. In HVAC industries, Level Control Systems can be applied at the cooling tower to compensate the water loss due to evaporation. Many level monitoring applications make use of continuous level sensing, where the information on liquid level is fed into a DAQ system at a given time interval. In the event when the set value is larger than the actual value ha, the supply valve is opened to increase ha. The drain valve is opened if the set value is less than the actual liquid level ha.
MINI INSTRUMENT PROJECT 1 PROBLEM 12
Bill Of Material(BOM): A bill of materials or bill of material describes a product in terms of its assemblies, sub-assemblies, and basic parts. Basically consisting of a list of parts, a BOM is an essential part of the design and manufacture of any product. Often, BOMs contain hierarchical information with the master, or top level, BOM describing a list of components and subassemblies. Take a PC, for example: the top level BOM might list the shipping box, manual, packaging, packaging labels and the actual PC. The BOM for the PC itself is referenced in the top level BOM and would contain its own list of sub-assemblies like power supply, motherboard, case, etc. This increasing level of detail continues for all sub-assemblies until it reaches its constituent parts (like resistors or processors), or modules that are out of the scope of the BOM (like the parts that make up a fan that is bought in as a module from another manufacturer). BOMs are important, since without a basic knowledge of how many parts a product needs, there is no way of knowing how many units of that part you need to buy. A bill of material can define products as they are designed, as they are manufactured, as they are ordered, as they are built, or as they are maintained. There are different types of bills of materials dependent upon the discipline that generates them and the purpose for which they are intended. It is important to ensure the type of bill of material that you have and its intended use prior to working with a bill of material.
Material Cost
Some vendors might not have the whole list of components; sourcing from a few vendors is the usual practice. Pricing varies from different vendors too. A good example will be the comparison between RS components and Element 14. Choices of vendors also depends on lead time on delivery or available of stocks.
MINI INSTRUMENTATION PROJECT 2 PROBLEM 13
Inputs & Outputs: First, identify the inputs and outputs of the VI. Inputs: Output voltage from Pressure Sensor, Pressure limit in bar, Atmospheric pressure in bar. Outputs: Gauge pressure reading in bar, Atmospheric pressure reading in bar, Absolute pressure reading in bar, High pressure indicator, Low pressure indicator
Interfacing with Pressure Sensor
Absolute pressure = Gauge pressure + Atmospheric pressure
Measured pressure in bar = Output voltage/ 5V x 1bar