Quiz 1 Flashcards
What are the three parts of a measurement system?
A signal chain consists of:
A sensor (to generate raw signal)
A data acquisition device (signal conditioning and ADC)
A computer (driver or application software)
What are the sources of error?
Offset error: a consistent difference between the actual and detected values of the input characteristic. Offset error does not change the shape of the sensor’s graph from what is ideal, but merely translates it up or down.
Offset can refer to a baseline value output when the input property value is actually zero, which causes all the output values to be offset by a value.
Offset can mean the difference between the actual property value and the output value due to a specific environmental condition that conflicts with the calibration conditions.
Integration error: is the error when the components of a signal chain are not compatible. Reduce this risk of error by combining measurement chains into a single system.
what is used to measure temperature
thermocouples, RTDs, thermistors
what is used to measure strain/force?
strain gauge
what is used to measure position/ displacement?
potentiometers, encoders
what is used to measure fluid flow?
flowmeters
what is used to measure light?
photosensors
what is used to measure vibration/ acceleration?
accelerometers
what is used to measure sound?
microphones
what is used to measure pressure?
piezoelectric transducers
What are the functions of a DAQ?
DAQ devices perform signal conditioning and ADC.
Signal conditioning:
Electrical signals from the sensors often cannot be utilized directly and need modification. This modification is necessary because the signals often contain noise or might be so weak that the DAQ system cannot measure it.
The signal conditioner uses filter circuits to separate the noise from the real signal and uses an amplification circuit to strengthen weak signals.
ADC:
An analogue to digital converter takes a snapshot of an analogue voltage at one instant in time and produces a digital output code which represents this analogue voltage.
What are the types of signal conditioning?
Amplification
Amplifiers increase voltage level to better match the analog-to-digital converter (ADC) range, thus increasing the measurement resolution and sensitivity.
Attenuation
Attenuation decreases the input signal amplitude so that the conditioned signal is within the ADC range. Attenuation is typically necessary when measuring voltages that are more than 10 V.
Excitation
Excitation is required for many types of transducers. External power supply to start the measurement.
Filtering
Filters reject unwanted noise within a certain frequency range. Lowpass filters are used to block out noise in electrical measurements. Anti-alias filters are a form of lowpass filter characterized by a flat passband and fast roll-off.
Isolation
Isolation is usually required in conjunction with attenuation to protect the system and the user from dangerous voltages or voltage spikes.
Linearization
Linearization is necessary when sensors produce voltage signals that are not linearly related to the physical measurement. Linearization can be implemented either with signal conditioning or through software.
Cold junction compensation
Cold-junction compensation (CJC) is required for accurate thermocouple measurements. Thermocouples measure temperature as the difference in voltage between two dissimilar metals. Based on this concept, another voltage is generated at the connection between the thermocouple and terminal of a data acquisition device. CJC improves measurement accuracy by providing the temperature at this junction and applying the appropriate correction.
Bridge completion
Bridge completion is needed for quarter- and half-bridge sensors to form a four-resistor Wheatstone bridge. The completion resistors offer a fixed reference for detecting small voltage changes across the active sensor(s).
What are the characteristics of sensors?
Important Characteristics of Sensors:
Sensitivity
Sensitivity is the ratio between the output signal and the physical phenomenon being measured. In other words, it is the slope of the sensor’s graph.
Range
The range of a sensor is the minimum and maximum values of input property that it can measure. The term dynamic range refers to the magnitude of the difference between the minimum and maximum values.
Precision, Accuracy, and Resolution
Precision refers to how reproducible or consistent a sensor’s measurements are. Accuracy refers to how correct or truthful the sensor’s output is to the input values. Resolution is the smallest measurable change of input property that a sensor can detect and convey through a change in output signal.
Offset
Offset describes a consistent difference between the actual and detected values of the input characteristic. Offset error does not change the shape of the sensor’s graph from what is ideal, but merely translates it up or down.
Linearity
Linearity describes how much a sensor’s graph deviates from the ideal curve. Linearity deals with error in terms of the shape and slope of the graph.
Hysteresis
Sometimes, sensors output different values for the same input depending on whether they approach that input from a higher or lower value. Hysteresis is a measure of how much a sensor’s outputs differ when approaching an input value from different directions.
Response Time
A sensor’s response time is the amount of time it takes to change to and settle on a new output value after a change in input.
What are the key considerations when building a signal conditioning system?
Integration
The ability of the signal conditioning system to integrate easily with the rest of the system is important.
Connectivity
A best-in-class signal conditioning system should offer a wide range of connectivity options, including thermocouple plugs, screw terminals, and BNC connectors.
Expandability
By designing your system in a modular way, you have more flexibility to change and expand both channel count and signal mix. If you design in a modular fashion, you can swap out your sensor module with relative ease.
Isolation
When the measured signal is either a high voltage or a voltage subject to spikes, you should isolate those signals from the rest of the system. Inadequate isolation compromises the safety of the operator as well as the integrity of the entire data acquisition system.
Bandwidth
When designing or specifying data acquisition systems, make sure the systems’ bandwidth is high enough to handle the data throughput you need and accommodate future channel-count growth. To determine the necessary minimum bandwidth of the system, multiply the total number of expected channels by the per channel maximum sample rate.
Software
A large portion of the total cost of a test and measurement system is application development when you account for the necessary engineering and time resources for setup, development, and testing.
Calibration
To make the most accurate measurements possible, you must periodically calibrate the entire data acquisition system. Most measurement devices are calibrated at the factory, but the accuracy drifts over time.
What are the characteristics of signals?
Sensor Signal Characteristics:
Level/State
Level describes the steady-state magnitude of an analog signal at a particular point in time. The digital equivalent is state, which refers to the value of a signal at a certain point in time (usually one of two binary values, either 1 or 0).
Shape/Pattern
The signal characteristic shape is exactly what it sounds like: the profile, or form of the graph, of the signal over a certain period. Some common shapes even have special names, such as square, triangle, and sawtooth.
Frequency/Rate
The frequency of a signal is the number of times a recurring waveform repeats in one second.
