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Biomedical Measurement Systems Biomedical measurements are carried out for
essentially two purposes:
To obtain data about some physiological orbiomechanical event or process for diagnosticor research purposes. EMG measurement to determine if co-contraction in a muscle pair is
occurring, indicating a pathological condition
EMG measurement to understand the action of the locomotor muscle
in walking
To provide sensory data as an element in aclosed loop biomedical control system. Use of an EMG signal to control the application of FES in a Drop
Foot Stimulator.
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Measurement Systems Elements
Measurement systems can be considered to have three
basic constituent elements:
Measured Value Signal Modified signal
Transducer/ Signal Display
sensor Conditioner Element/data
Acquisition
Element
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Measurment Systems
There are three approaches to the implementation of a
biomedical instrumentation system incorporating the
three elements discussed:
Hard-wired instrumentation system
Microprocessor-based instrumentation system
PC-based instrumentation system
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Hardwired Measurement System
In a hard-wired measurement system, the function of
the system is determined by the wiring of the system
there is no software aspects to these instruments.
Thew
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Transducer/Sensing Element
Modified
Measured value Transducer/ Signal Signal Signal DisplaySensor Conditioner Element/Data
Acquisition
Element
The sensing element or as it is frequently called, the transducer, is
the first element.
This produces a signal which is related to the physical quantity
being measured.
Usually this signal is an electrical signal and the transducer is then
referred to as a sensor.
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Transducer Element
For example, one example of a light sensor uses a
resistor, whose resistance changes with light intensity,
as a transducer.
This device is called a LDR or light dependent resistor
A LDR experiences a resistance change when the
incident light changes - the higher the light intensity,
the lower the resistance of the LDR device.
Sensing elements or sensors take information about he
instrumentation system to detect changes in light
intensity.
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Transducer Element - LDR
Light Dependent Resistors Quantelec MPY54C569
OPT149 Cadmium sulphide photoconductive cell inepoxy coated package.
Peak Response: 550 nm
Resistance (10 lux) 20 k W-100 kW
Resistance (1000 lux): 50 W-2kW
Dark Resistance: 20 MW(min.)
Power Disssipation (max) @ 30 C: 60mW Vmax (DC/AC peak): 200V
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Signal Conditioner
Modified
Measured value Transducer/ signal Signal signal Display
Sensor Conditioner Element/Data
Acquisition
Element
The second element in our system is a signalconditioner or signal converter or signal processor.
This takes the signal from the sensing element and
converts it into a form which is suitable for use by thedisplay/data acquisition part of a measurement systemor in the case of a control system, for combining with
the reference signal
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Signal Conditioner
An example of a signal conditioner might be an
amplifier which takes a small signal from the sensingelement and provides it with sufficient power toactivate the display or results in a voltage which will
have a low sensitivity to noise. In the case of the LDR - the resistance change
produced as a consequence of a change in lightintensity may be converted to a voltage change using a
buffered voltage divider circuit.
The voltage output from the buffered voltage dividercircuit can be input directly to the input of a data
acquisition unit.
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Display Element
Modified
Measured value Transducer/ signal Signal Signal Display
Signal conditioner Element
Data
Acquisition
The display or data acquisition element is where theoutput from the measuring system is displayed oracquired for a PC-based or microprocessor basedinstrument
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Display Element
In the case of a LDR-based light intensity measurement
system - the output may be displayed as a voltage on
voltmeter, at its simplest.
The display / data acquisition element takes the
information from the signal conditioning block and
presents it in a form which enables an observer to
recognise it or a software-based instrument to process
it.
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Measurement System
Light Intensity Resistance Voltage
Change resistance Change Resistance to Change Displayelement voltage Acquisition
converter
The various parts of a LDR-based light intensity
measurement system are shown above.
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Measurement system
Digital Control Circuit TimerInterrupt
Input
Instrumentation strobe
Sensor Amplifier
Parallel Micro
Filter s/11 A/D Input Port Computer
Bridge
Display D/A Parallel
Control Input Port
Output strobe
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