EE 452/EE 830
Assignment 6, 1999
Solutions
36. Fmin = 3.9 mV x 1000 Hz/V = 3.9 Hz
The minimum sampling period must be chosen such that a single pulse is guaranteed for the
minimum resolvable input signal amplitude.  Tsampling = 256 ms.
Fmax = 10 kHz.  2560 pulses within the minimum sampling period.
211 < 2560 < 212 a 12-bit counter is needed.
38. 12-bit ADC  0.024% accuracy
we can use 4.5 s as a reasonable estimate of the minimum required acquisition time,
since that gives us 0.01% accuracy.
the minimum total time to acquire and convert a sample value is 10 + 4.5 s = 14.5 s.
The droop error is 5 mV/ms x 10 s = 50 V, or 0.001% error on 5 V.
The gain error is 0.02%, and the acquisition error is 0.01%.
the maximum total error is 0.031% or 1.25 LSB.
45. Possible causes:
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Electrical interference directly onto the signal lines from lights, heavy equipment (i.e.
electric motors requiring large currents), or from arc welding equipment. Since the noise
is worst during normal working hours, the latter two sources are most likely.
Ground loops: these would be an additional means to pick up the above. Either directly
coupling onto the signal lines (if they are not shielded) or ground loops are the most
likely things to look for. Note: that shielding will only prevent electrostatic coupling of
noise, whereas arc welding and heavy current equipment will generate magnetic field
interference. Since there are 16 strain gauge bridges on the truss, which is electrically
conductive, there is a high probability that at least one, and possibly, several ground
loops exist. The strain gauges will be capacitively coupled to the truss, and the truss is
almost certainly grounded through the test equipment that is creating the mechanical
loads on it.
If the excitation voltage is floating and the preamplifier inputs are ground referenced,
then this will tend to increase the presence of interference in the output.
Temperature: It is possible that there are thermal effects causing changes in the observed
voltages. However, these are not likely to have periods of a few seconds or less.
Corrective Actions:
These are given in approximately ascending order of cost and time to implement.
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Check the ground connections on the measuring equipment. If possible, we want to have
a star ground. This may not be feasible, particularly as the first law of noise reduction is
to ground the electronics at the transducer signal source. This would lead to the need for
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isolation techniques, covered later. However, checking the grounds of all the equipment
including whatever is providing the bridge excitation and the computer ground may lead
to some improvement in the situation, and is the easiest to do and probably the cheapest
to fix. This may mean checking the impedance on the neutral and ground wires of the
mains distribution system, as well as checking for different potentials between grounding
points.
Check grounding on other equipment to make sure that neutral wires and ground returns
wires are intact. These may be inducing large potential differences between ground
points on the measurement system.
Increase the sampling rate of the ADC and do signal averaging or digital low pass
filtering on the signals. This is likely quite easy to accomplish. Since the ADC system is
capable of 20 ksamples per second, there should be some user interface that allows this
to be changed. This should likely be done even if the ground check fixes the problem.
Makes sure the measurement at the bridge is differential. This also should be done
regardless of the first two. If it isn't already differential, you may need to check carefully
as to why it wasn't connected differentially. If the pre-amps aren't capable of differential
input, then this might take time and money to correct.
Where is the regulation happening for the bridge excitation voltage. It should be as close
as possible to the bridges themselves. Otherwise, interference can be coupled on to the
so-called regulated power supply. This will look like a differential signal across the
bridge.
Twisted pair conductors might reduce the direct pickup of the electrical interference onto
the signal lines. This might be checked first on one signal line before doing a full
change over.
Lowpass filters could be added just in front of the ADC. This is more of a bandaid
solution as it doesn't remove the source or presence of the interference.
The AD conversion process could be moved closer to the transducers. This may require
a ruggedized industrial computer or a remote data logger that can send a multiplexed
digital signal to the computer. Consideration could be given to using a different type of
ADC such as dual slope to get some inherent filtering.
Use ac excitation of the strain gauge bridges and a tuned amplifier. This would allow
rejection of any interference that is coupled in prior to the pre-amplifier. If the problem
is after the existing pre-amp, then this won't help.
As mentioned above, the strain gauge bridges are capacitively coupled to the truss and
therefore to each and to ground. It would therefore be much better practice to ohmically
ground the bridges at the truss or at the input to the pre-amps. The pre-amps would then
have to be isolation amps or be followed by isolation amps. Otherwise, ground loops
would be set up.
Use current as the signal down the 30-meter cable rather than voltage. This can be done
with 4-20 mA converters. This is usually done with digital signals. Current sensing uses
a relatively low resistance at the load (ADC) to recover the signal. Since most noise
sources have high effective source impedances, they cannot provide much power (or
current). Thus, there is substantial attenuation of the interference signal that is directly
coupled onto the signal lines. Some ADCs in fact have current inputs rather than (or as
well as) voltage inputs for this type of application.
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Fibre optic transmission of the signals from the truss to the computer. This is probably
more feasible if the signals are digitized at or near the truss and then multiplexed onto
the fibre optic cable. This will only improve the situation if the interference is being
coupled onto the existing 30-meter cable.