SIGNALS, SENSORS, and SIGNAL CONDITIONING
All industrial processing systems, factories, machinery, test facilities, and vehicles consist of hardware components and computer software whose behavior follow the laws of physics as we understand them. These systems contain thousands of mechanical and electrical phenomena that are not steady state; rather, they’re continuously changing. The measurable quantities that represent the characteristics of all systems are called variables. The proper functioning of a particular system depends on certain events occurring in time and the parameters of these variables. Frequently, one is interested in the location, magnitude, and speed of the variables, and one uses instruments to measure them. One may assign the variables units-of-measure: e.g., volts, grams, and meters per second, etc.
Most variables must be measured with a device that converts the phenomena into a form that a human can perceive, such as a visual display, a transducer for sound, or vibrations to stimulate physical sensations. The conversion devices are called transducers or sensors, and they translate the physical phenomena to electrical signals (or vice versa) to be measured with electronic instruments. These instruments have traditionally been ammeters, voltmeters, and various other gages, and the variables can be observed in real time. However, an increasing need to record and store (or log) these phenomena -- and , often, analyze them at a later time -- forced engineers to develop data recorders and data acquisition (DAQ) systems.
Variables may be classified in several ways; usually, most experts prefer two classifications:
Variables classified by characteristic include:
Those classified by measurement signal include motion, force, electrical, and time-modulated. Measurement signals for variables often are hard to differentiate from the measuring system. Four factors require close consideration for measurement signals and systems:
(1) the types of transducers available for converting variables to measurement signals
(2) transmission characteristics
(3) data acquisition system input matching
(4) transducers available to convert from one type of measurement signal to another measurement signal
DATA ACQUISITION SYSTEMS (DAQs)
Data acquisition systems (DAQs) have evolved over time from electromechanical recorders containing normally from one to four channels to all-electronic systems capable of measuring hundreds of variables simultaneously. Early systems used paper charts and rolls or magnetic tape to permanently record the signals; however, since the advent of computers, particularly personal computers, the amount of data and the speed with which they could be collected increased dramatically. But, many of the classical (sometimes called legacy) data-collection systems still exist and are used regularly.
PERSONAL-COMPUTER- (PC-) BASED DATA-ACQUISITION EQUIPMENT
Before the 1960s, costly mainframe computers were extensively used for gathering multiple channels of data, mostly in large industrial or scientific applications. They were rarely used in small projects because of their relatively high cost. What changes all this was the introduction of small rack-mounted minicomputers that developed in the 1960’s and later desktop personal-type computers (PCs) that had microprocessors and explodes in use in the 1970’s justified their use for smaller projects. Not long after this, data-acquisition plug-in cards (as well as hundreds of other types of plug-in cards) for these small computers were a common means to collect and record data of all types.
Plug-in cards for computers didn’t always do what the users expected, however. Internal noise from rotating devices such as drives and electromagnetic and electrostatic noise from the computer’s internal busses frequently interfered with the measured variable, notably in data-acquisition cards. Isolation and shielding have really helped to solve the problem in most cases; nevertheless, many data acquisition manufacturers also provide signal-conditioning and signal-processing circuits in small, stand-alone, shielded enclosures. The separate box provides isolation by distance, expansion for hundreds of channels, and portability with laptop computers that desktop personal computers with plug-in cards don't possess.
All PC-based DAQ systems will record very accurate, repeatable, reliable, and error-free data -- but only if they are connected and operated according to the manufacturer’s recommended practices. These practices include:
Other items include choosing the right impedance and using doubled-ended (differential) inputs instead of single-ended where possible. The environment should also be considered, especially for extremes of ambient temperature, shock, and vibration. and here lies the major goal of this discussion -- to inform and guide users of the top recommended practices based upon a fundamental knowledge of the internal workings of DAQ system instrumentation.
After the information about a physical process -- such as temperature or pressure -- is measured by a sensor or transducer, the analog signal heads toward the data-acquisition system (DAQ). Analog signals can take many forms, but may be roughly categorized into type types:
Often, the signal is too "noisy" or "quiet" and must, therefore, be changed to a form suitable for analysis. The process of doing so is called signal conditioning. Signal conditioning includes amplification, filtering, converting, and other processes required to make sensor output suitable for reading by computer boards. it's primarily used for DAQ, systems in which sensor signals must be normalized and filtered to levels suitable for analog-to-digital conversion so they can be read by computers. In most cases, this is done by changing the sensor's output to a voltage (if it isn't already), modifying the sensor's dynamic range to maximize the accuracy of the DAQ system, removing unwanted signals, and limiting the sensor's spectrum. One may also think of signal conditioning as a deliberate attempt to increase signal fidelity or signal-to-noise ratio.
The proper design of the signal-conditioning system is critical in mapping the sensor output to the data-acquisition input. Poor choices can affect the way the DAQ system reacts to input signals. Therefore, it's important to note the changes in the properties of the sensor signal caused by the conditioning circuitry.
In a nutshell, the following signal-conditioning technologies are used to improve the performance and accuracy of data-acquisition systems:
The following table lists the type of signal-conditioning technologies required or recommended for data-acquisition input components:
7: Signal Conditioning and Real-Time Interfacing
2. Elements of a Data Acquisition and Control System
6. Application Software
7. Summary; References
This section presents theoretical and practical aspects of computer interfacing and real-time data acquisition and control. Besides the computer and the real-world system, the remaining devices are sensors, actuators, and a general-purpose input/output interface card that includes the A/D and D/A devices. Signal-conditioning accessories amplify low-level signals and then isolate and filter them for more accurate measurements. Amplifier selection and analog-to-digital conversion techniques are the focus of the analog electronics section. Mechatronics integrates signal conditioning, hardware interfacing, control systems, and microprocessors. Signal processing and data interpretation also are handled using the visual programming approach. The versatility of our visual programming environment allows us to present three popular systems: LabVIEW, MATLAB, and VisSim.
SIGNAL CONDITIONING and REAL TIME INTERFACING
Real-time interfacing is a general term used to describe the aspects of connecting a computer with a real-world process and communicating data between the two. Monitors, keyboards, printers, disks, modems, and CD's are familiar (but specific) examples of real-time interfacing. A more general approach categorizes the interfacing process into four major components: sensors, actuators, the computer, and a real-world process. e.g., the process of entering data into a computer satisfies this categorization if the human operator is viewed as the real-time process. The sensor is the keyboard; it transfers information from the real-time process to the computer. The monitor is the actuator, transferring information from the computer back to the real-time process. In this section, we focus on a broader class of real-time processes which can be human as well as machine based.
Sensors and actuators then become components which transfer information between the electrical computer discipline and others (including electrical, mechanical, fluid, thermal, and human). This section is designed to be self-contained and does not rely on prior programming knowledge.
2. Elements of a Data Acquisition and Control System
A data acquisition (DA) system is a collection of add-on hardware and software components that allow your computer to receive real-world information from sensors. Although sensors can be based on electrical, mechanical, optical, or other principles, they all perform the same function: to convert real-world information (such as motion, temperature, and pressure) into low-power electrical signals which can be read by the computer. Once the data resides within the computer, any of three operations may be performed: plotting, processing, and writing to a file.
A data acquisition system also can be thought of as a monitoring system. It can receive data from a real-world process and display the data. It can also display the features of the data extracted through its processing. In situations where it's necessary to acquire and process data and to also send data back to the real-time process, we make use of a data acquisition and control (DAC) sys tem. A DAC system is a superset of a DA system and requires both sensors and actuators. The purpose of the actuator is to convert low-power computer signals real-world signals, into resulting in motion, heat, pressure, etc. Common actuators include stepper motors, solenoids, relays, hydraulic motors, speakers, and piezoelectric actuators.
As an example that illustrates the difference between a DA and a DAC system, consider the measurement of the speed-flow-pressure characteristics of a variable speed compressor at 20 different speed values. The compressor has been connected to a variable speed motor-the speed of which can be manually varied by turning a knob. Signals from flow, pressure, and speed sensors attached to the compressor outlet and motor are read and plotted by a computer.
The DA system reads and displays the three sensor input values. At each speed, the operator must vary the speed knob until the desired speed is reached and then record the sensor readings for speed, flow, and pressure. In the DA system just described, there is a fair amount of variation in the speed reading due to the speed adjustment at each point by the operator. A DAC version of the same system could be used to remove this speed variation. The DAC application could be created by adding an actuator that moves the speed knob in response to a signal produced by the computer. If the value of this signal is programmed as a function of the error between the actual motor speed and the desired speed, the knob will be moved in the direction that reduces this error. Eventually, the actual motor speed will agree precisely with the desired speed. An application program could be written to automate this process further-to reset the desired speed value to each of the 20 desired values, wait for the flow and pressure signals to stabilize, and record and save a measurement.
The current trend is to use personal computers (PCs) with DAQ hardware for data acquisition in areas of laboratory research, testing and measurement, and industrial automation. The DAQ hardware which act as an interface between the computer and the outside world could be in the form of modules that can be connected to the computer's ports (parallel, serial, USB, etc.) or cards connected to slots (PCI, ISA, PCI-Express, etc.) in the mother board. The newest DAQ devices offer connectivity over wireless and cabled Ethernet for remote or distributed DAQ applications. The DAC system presented in Ill. 1 shows a screw terminal panel with I/O devices.
The PC-based DAQ system depends on each of the following system elements.
Data acquisition and analysis hardware Signal conditioning Transducers Personal computer Software
ILL. 1 THE TYPICAL PC-BASED DAQ SYSTEM
Overview of the I/O Process
The input/output (I/O) process is the means by which a computer communicates with real-world phenomena via the DAQ device. The performance of the I/O process therefore is dependent on the available computer, selected DAQ device, and the bus architecture. Today's computer (with high speed processor coupled with high-performance bus architecture) has the capability of transferring data by any of the following methods.
• Direct Memory Access (DMA): With this mechanism data is transferred between the DAQ device and computer memory without the involvement of the CPU. This mechanism makes DMA the fastest available data transfer mechanism. Also, the processor is not burdened with moving data, and hence, it can engage in more complex processing tasks.
• Interrupt Request (IRQ): IRQ transfers rely on the CPU to service data transfer requests.
The device notifies the CPU when it's ready to transfer data. Hence, the data transfer speed is tightly coupled to the rate at which the CPU can service the interrupt requests.
• Programmed I/O: This is a data transfer mechanism in which a buffer is not used-instead the computer reads and writes directly to the device.
• Memory Mapping: it's a technique for reading and writing to a device directly from the program, which avoids the overhead of delegating the reads and writes to kernel-level software.
However, the data transfer mechanism the computer can use depends upon the selected data-acquisition device and its bus architecture. e.g., while PCI and FireWire devices offer both DMA and interrupt-based transfers, PCMCIA and USB devices use interrupt-based transfers.
The available hard drive is the limiting factor for real-time storage of large amounts of data.
Hard drive access time and hard drive fragmentation can significantly reduce the maximum rate at which data can be acquired and streamed to disk. For systems that must acquire high-frequency signals, high speed hard drive with large memory space is needed.
In general, the overall communication speed is directly proportional to:
• The clock frequency of the processor chip.
• The bit length of the bus (i.e., 8 bit, 16 bit, 32 bit, ...) and inversely proportional to.
• The bit length of the processor (i.e., 16 bit, 32 bit, ...) Ill. 2 presents these major components and their interconnections in a four-sensor/two actuator DAC system. The number of I/Os varies, depending on the manufacturer, as does the functionality. Some screw terminal panels have resistors that can be cut or soldered to change the gain range of a single or group of channels. The screw terminal is attached to the GPIO card via ribbon cables. Ill. 2 shows two cables; however, the number may vary depending on the type of the card. On some cards four cables are required, two for the analog channels and two for the digital channels. The application software function is to provide the engineer with an easy means of reading sensors, writing to actuators, and processing data (plotting, control algorithms, saving data, and data manipulation).
ILL. 2 COMPONENTS OF A DAC SYSTEM and INTERCONNECTIONS FOR A SYSTEM WITH FOUR SENSORS and TWO ACTUATORS Computer I/O card Ribbon cables Screw terminal panel 2 wire actuator lines Actuators Real-world process Sensors 2 wire sensor lines Computer bus
General Purpose I/O Card (GPIO)
The necessary ingredients for the general I/O process are the PC and operating system soft ware, general purpose I/O (GPIO) card and software driver, and the proper termination panel(s) and cabling for the GPIO card. The GPIO card is installed into a free expansion slot in the PC bus. Its address is specified both on the card (using micro-switches) and in its driver software.
The termination panel is connected to the GPIO card by one or more cables. At this point, the system is ready to operate, except for the application software.
Whatever application software is selected for the mechatronic programming tasks, it must pro vide the programmer with the ability to create open-loop as well as closed-loop applications. Most GPIO card manufacturers (including Computer Boards, Inc., Advantech, Data Translation, and Metrabyte) offer their own Windows-based software for controlling their cards. An important limitation of this type of software is that it only works on cards provided by one manufacturer, making multi-card, multi-brand applications impossible.
An alternative to board-manufacturer software is a general Windows application software pack age designed to work with many GPIO cards of different brands. The success of this approach is evident by the popularity of such well-known packages as LabTech Notebook, LabVIEW, and Snapshot. Most of these packages are aimed at the data acquisition market and , as such, are often not suitable for closed loop applications. Listed packages capable of both open- and closed-loop operation tend to be more oriented towards control systems and include LabVIEW, MATRIXX, Simulink, and VisSim. Table 1 presents some of the most popular graphical application software.
TBL: POPULAR GRAPHICAL-BASED APPLICATION SOFTWARE
Name Description Labtech Notebook General purpose DAC with analysis LabWindows General purpose DAC with analysis Workbench PC General purpose DAC Snap-Master General purpose DAC with analysis and display EasyEst General purpose DAC with analysis Unkel Scope High speed DA Snapshot High speed DA Acquire General purpose DAC LabVIEW General purpose DAC with analysis Hyperception High speed DAC with analysis and display MATRIXX High speed DAC with analysis and display Simulink High speed DAC with analysis and display Visual Designer General purpose DAC with analysis XANALOG High speed DAC with analysis and display VisSim General purpose DAC with analysis and display
Installation of the I/O Card and Software
Before finalizing the data acquisition card, some thought should be given as to how frequently the I/O must be sampled. Most GPIO cards operate in the 1 to 3 kHz range, which means that you can expect anywhere from 1000 to 3000 samples per second, depending on the card. Application soft ware based on Windows may suffer a slight degradation in this rate, but the difference is small and often can be regained by modifying the algorithm or moving to a more powerful processor. Each GPIO card takes up one slot and provides inputs, outputs, or a combination of both. Most cards come with digital I/O, which can be configured either as input, output, or a combination of both.
Analog I/O is harder to come by because it requires a D/A or A/D converter, but most card manufacturers offer many configurations that combine digital and analog I/O.
Once you have selected a manufacturer and identified the potential card(s) which could be used, you must determine the type and precision of the I/O channels needed. I/O channels may be either inputs or outputs and are classified based on the type of data transferred. This leads to three channel types:
Precision of the I/O channel pertains to the accuracy and transient characteristics of the D/A and A/D converter employed. Accuracy of the converter is a function of bit length. Most converters have 12-bit resolution, which is ample for most applications; however, this depends on your application and should be a consideration prior to purchasing. A list of some of some popular I/O cards is presented.
In mechatronic applications, the I/O typically operates in either of two modes, open loop or closed loop. Open-loop operation exists when inputs are read and /or outputs are written, but no relationship or dependency exists between the inputs and outputs.
EXAMPLE: A manufacturer of compressors may use the open-loop method for automatic compressor testing. A time based sequence is often employed. At time , a signal is sent to the compressor motor which resets the motor speed to the specified value. Since this signal is sent from the computer, it's called an output. After waiting a preselected time, T seconds, for the equipment (motor and compressor) to hopefully stabilize, the pressure, temperature, flow, and speed are read and saved in memory. Because these four signals are read into the computer they are called inputs. The process then is repeated with a new motor speed command. This approach is called open loop because the output signal (motor speed command) occurs at T second intervals regardless of the values of the input signals. Had stabilization not been reached in the T second period, it would not be known. Also note that this process reduces to a classic data logger if the output signal is omitted.
In a closed-loop operation, the output signal(s) are dependent on the input signal(s). The same example can be made to operate in closed loop by changing the time-based logic on when a reading is taken to event based logic-the event being stabilization of the four input signals. Clearly, this requires a little more programming effort, because the time differential of each of the four signals must be computed and combined in such a way that the read input command is initiated at the instant when all four at the derivatives are less than a pre-specified magnitude. Aside from the extra programming, the closed-loop approach will automatically vary the time between measurements-each measurement is taken at a precisely controlled operating condition, which is something that the open-loop approach can't claim.
In mechatronic applications, the I/O typically operates by either, an open-loop or closed loop operation. An open-loop operation exists when inputs are read and /or outputs are written, but no relationship or dependency exists between the inputs and outputs. In a closed-loop operation, the output signal(s) are dependent on the input signal(s).
= = =
Important requirements for Analog-to-Digital Conversion (ADC)
As stated above, the main reason for analog signal conditioning is to change the sensor output into a form that can be optimally converted to a digital data stream by the DAQ system. Important input requirements of most DAQ systems include:
Other Reasons for Signal Conditioning:
Signal pre-processing: Often, it's desirable to perform pre-processing on the sensor signal before data acquisition. Depending on the application, this can help lower the required computer processing time, lower the necessary system sampling rate, or even perform functions that will enable the use of a much simpler data acquisition system entirely. e.g., while a transducer system can output a voltage proportional to amount of change in the physical phenomena, it may be desired to only tell the computer when the change of the phenomena is greater than a certain amount. This can be done using analog signal-conditioning circuitry. Therefore, the DAQ system is reduced to only having a single binary input (no need for an ADC (analog-to-digital conversion)).
Removal of undesired signals: Many sensors output signals that have many different components to them. It may be desirable or even necessary to remove such components before the signal is digitized. Other added signals may actually corrupt the sensor output. This "noise" can also be removed using analog filter circuitry. e.g., 60 Hz interference can distort the output of low-output sensors. Signal-conditioning circuitry can remove this before it's amplified and digitized.
When Shopping for Signal-Conditioning Devices, look for:
Also look for:
Further reading : Signal Conditioning: Buy vs. Do-it-Yourself