Sensors and Data Acquisition Flashcards
What is a sensor
A device that measures a physical quantity and converts it into a signal which can be read by an observer of by an instrument
What is the signal most sensory convert to
voltage
What are 5 sensors used in biomechanics
- Strain gauge
- Force platform
- Accelerometer
- Potentiometer
- Motion capture
What does a strain gauge do?
Converts forces along one axis to volts
What does a force platform do?
Converts force along three axes to volts
What does an accelerometer do
converts acceleration to volts
What does a potentiometer do
Converts angular displacement to volts
What does motion capture do
converts light to volts
A strain gage is really a…
specially designed resistive circuit that is attached to a malleable object
How does a strain gage work?
When a circuit is physically bent (in tension or compression) the resistance to the flow of electricity changes and therefore the voltage changes
In a strain gauge the chain in voltage is related to…
the physical change
If the strain gauge is in tension the area _____ and the resistance _____
narrows
increases
If the strain gauge is in compression the area _____ and the resistance _____
thickens
decreases
If a signal is analogue it means that there are…
No separations based on time between points in the signal
If a signal is digital it means that there are
Separations between points in the signal
What is sampling
the process of converting a continuous signal into a digital signal
most physical signals are inherently ______ but when we use sensors to quantify the data we must convert them to a _____ signal
continuous
Discrete digital
What is the sampling frequency
the number of samples in one second and is expressed in Hertz
What is calibration
The validation of specific measurement techniques and equipment
A comparison between measurements, one of a known magnitude and another of an unknown magnitude
How is a sensor calibrated
Apply various know forces to the device. Then we could graph the value of the force and the voltage that the sensor outputs related to this force. We get a slope. with the slope and the offset we can determine the force for any voltage output and the sensor is said to be calibrated
The offset is also known as the ____
y-intercept
If the relationship between applied force and voltage is non linear, we need…
more than just the slope and offset, we need the coefficients that describe the polynomial
The ____ can be used to convert from a measured voltage to a force in Newtons
Calibration equation
What is hysteresis
The influence of the previous history of a system on its subsequent response to a given stimulus
When do hysteresis occur
when a different force to voltage relationship exists when the force the force is incrementally increased compared to when it is incrementally decreased
Why is hysteresis undesirable
Because it would require different calibration coefficients for loading and unloading
What is drift
a gradual change in a transducer’s voltage output without an accompanying change in the applied force
What can drift occur as the result of
a transducer warming up or as an effect of continued use
If the drift is ___, the slope can be determined and the voltage output can be adjusted
linear
What 3 things does a sensor normally include
- The physical sensing unit
- a power supply
- An operational amplified
What is an operational amplifier
A dedicated circuit that amplifies the voltage from the sensor and can perform operations on the signal (from simple to complex math)
How is data collected from a sensor
a break out box
What is a break out box
A plastic or metal box with one or more rows of terminals (or connectors). A wire for each sensor connected to the breakout box is bunndled into one large cable that connects to the rest of the data acquisistion system
What is multiplexing
Allows the breakout box to record from more than one sensor at a time.
Each connection to a multiplexor is called a ____
channel
How does a multiplexor work?
The DAQ draws the first sample from the first sensor; the multiplexor then switches the circuit to receive a sample from the second sensor, once all connected sensors have been read, the multiplexer switches back to the first sensor
What are two problems with multiplexing
- The effective sampling rate is decreased (by a factors of the number of channels being multiplexed)
- Samples are draw sequentially, so there is a delay between each one. No simultaneous
What is the sampling rate equation
number of samples divided by number of channels
What is sampling skew
The sequential pattern of the samples
Sampling skew increases as…
The number of channels being sample increases
What is a gain
the number of times that a signal’s amplitude is increased
How do you limit the amount of noise amplified
a preliminary amplifier located close to the sensor is often used
What does an A/D do
Works to divide the analog signal into discrete values. The A/D tried to determine the number that most closely corresponds to the applied voltage
What does the sample-and-hold circuit do
freezes the voltage signal while the A/D determines the correct numerical value. The voltage input must not change much or the number produced by the A/D will not be an accurate reflection of the voltage. The sample-and-hold circuit helps accomplish that
What 3 things are AD converteers limited by
- the range of voltages that they can read (window and amplification)
- The number of magnitude options (Magnitude resolution or steps)
- The number of time options (temporal resolution)
If a voltage is applied beyond the upper and lower limits of an AD the digitized signal will be ___
truncated or clipped
The magnitude resolution of an AD is usually expressed in units of -__
bits
What is bits short for
binary digits
an 8-bit A/D has _____ levels. this means that.
- That 8 digits of either 1 or 0 are required to represent 256 individual numbers
An A/D’s range is divided into…
a specific number of equal steps
How do you find the amplitude between each step
take the voltage range and divide it by the number of steps
What is resolution
the number of steps that divides an A/’s range
How does the amplifier facilitate sampling
by ensuring that the voltage input extends throughout as much of the range of the A/D as possible, this increases the resolution of the sampled signal by allowing the signal to be divided into more divisions
What is the ideal gain of an amplifier
One that maximizes the range of the A/D but that doesn’t result in cliipping
What occurs to data after it is converted to digital,
it is sent to a PC and is stored on a long-term memory device
What is the aim of any software package used to process and analyze data
to perform many operations on digital signals very quickly
