biomedical measurement systems lecture 1

7/28/2019 Biomedical Measurement Systems Lecture 1

1/13

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.


2/13

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


3/13

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


4/13

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


5/13

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.


6/13

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.


7/13

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


8/13

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


9/13

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.


10/13

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


11/13

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.


12/13

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.


13/13

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

biomedical measurement systems lecture 1

Documents