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• Performance Testing
• Safety Testing
Need for Medical Equipment Testing • Medical device incidents resulting in patient injury and death• Ensure that the equipment is performing to the expected standards of accuracy, reliability, free of hysteresis and linear (as designed).• Safe and effective devices need to be available forpatient care– Downtime costs money• Regulations, accreditation requirements and standards.
Waveform display devices
Chart recorder: (Recording Oscillographs)
used in a medical instrumentation to make permanent recording of the waveform.
most common type: permanent magnet moving coil (PMMC).
The PMMC is very similar to the Galvanometer movement.
A writing pen replaced the meter point.
Current flowing in the moving coil creates a magnetic field that interact with the magnetic field of the permanent magnet. This will cause deflection of the pen.
The tip of the pen is positioned over a strip of chart paper that is pulled under the pen tip at a constant speed. In this mechanism:
Y-axis is the deflection to the pen and
X-axis is the time base established by moving the chart paper at a constant speed.
This will produce a chart recording of the waveshape of the applied waveform
The pen assembly sweeps an arched manner and so will write in a curvilinear manner.
The pivoted pen motor assembly is a solution of curvilinear action.
The pen is connected through a mechanical link that translates the curvilinear motion of the PMMC to the rectilinear motion at the pen tip.
In this type of recorder the pen assembly is very long compared to width of the chart paper.
The pen tip, therefore travels in an arc the length of which is very short compared with the radius.
The trace will appear to be nearly linear
Thermal, and Direct contact
Both of these types use a special writing stylus (rather than a pen and a knife edge, also called writing edge)
The mark on the paper by the contact of the stylus on the paper along the knife edge,
The stylus tip travels in a curvilinear path, but the resulting trace is rectilinear because the knife edge is straight.
The stylus can write anywhere along its length, so by keeping the knife edge straight under the paper, we obtain the rectilinear recording that shows waveshape as well as amplitude
Several writing systems are commonly used on PMMC recorders:
1- direct contact,
2- thermal pen
3- ink pen
4- ink jet
Recorder that use any type of pen (ink) or stylus (Direct contact and thermal) have a relatively low-frequency response due to the inertia of the pen or stylus assembly (100-200 Hz) Ink jet and optical type
Lighter weight fixtures and writing stylus system have a frequency response of 1000-3000 Hz
The thermal recorder also uses special paper, but in this case it is waxed or treated with paraffin so that it will turn black when treated
The thermal recorder is the most commonly employed in medical instrumentation, especially in cardiovascular instruments such as the ECG and pressure monitors
The stylus in a thermal system is little more than a heated resistance wire connected to a low voltage ac or dc power supply.
Early models formed a U-shaped electrical resistance element
Modern models use a wire inside a cylindrical metal stylus
In both cases a low voltage electrical power supply energizes the element, causing the tip to become heated.
The black mark is made at the points where heated stylus touches the paper
use a hollow pen and an ink supply to write on the chart paper.
In some machines the ink pressured by an atomizer-like hand pump
EEG system use this type of writing system.
More automatic machines use a thick, viscous ink in a special cartridge that is placed under pressure by a spring-driven piston
In multichannel instruments, the ink may be distributed to all pens from the same cartridge by connecting the line from the cartridge to the input side of a special ink manifold and additional pressure is applied to the ink by the solenoid-operated manifold bladder
Higher-frequency response than are the types mentioned previously.
This type is popular on European instrument
Low-viscosity ink is directed to a nozzle mounted on a PMMC galvanometer in place of the pen assembly.
The ink jet produced by the nozzle is directed at the paper.
When the system is properly adjusted, it will produce a recording that is very linearly rectilinear.
Only a small amount of trace fuzziness due to ink splattering is apparent
الانواع التى تستجيب للتردادات المرتفعة اكثر من الانواع السابقة مشهورة فى اوروبا
حبر منخفض اللزوجة يوجه الى فوهة فى الجلفانومتر مكان القلم
الحبر المتدفق من الفاتحة يوجه الى الورقة
عند ضبط النظام تماما سوف ينتج خط تام الاستواء
ينتج بعض التشوش نتيجة بعثرة الحبر
There are two types of optical recorder:
- PMMC type uses small inertia mirror in place of the pen assembly or stylus.
- The other uses a photographic paper that is pulled across a cathode ray tube and is called a CRT camera recorder.
Most of these recorders use wide paper.
On multichannel optical recorder it is possible to examine the time relationships between different traces more easily because the traces can be allowed to overlap each other.
The paper in the optical recorder is often develop by exposure to an ultraviolet lamp as the paper comes out from the recorder.
Unless the paper is either wet developed following the recording session or stored in a light-light box.
In CRT camera, the CRT sweeps only the vertical axis.
The time base is provided by pulling the photosensitive paper in the front of the CRT screen.
The frequency response of the CRT camera recorder is better than that of any of the other types, being limited mostly by the writing speed of the photosensitive paper.
In potentiometeric measurements a three-terminal variable resistor (potentiometer) is connected to produce an output voltage that is a function of both a reference potential and position of the variable resistor’s wiper arm.
A galvanometer will read zero when the unknown voltage and potentiometer voltage are equal.
A servo recorder is a self-nulling potentiometer that records the waveshape of the applied signal on graph paper.
The pen attached to a string that is wound around a pair of idler pulleys and drive pulley that is on the shaft of a dc servomotor.
The pen assembly is also linked to a potentiometer (R1) in such a way that the position of the wiper arm on the resistance element is proportional to the pen position.
The potentiometer element is connected across a reference potential Eref, so potential Eis the electric analog of pen position.
The pen position is controlled by the dc servomotor, which in turn driven by the output of the servo amplifier.
The amplifier has a differential inputs: Ein (unknown)is connected to one input, and the position signal E is connected to the other input
The difference signal (Ein – E) represents the error between the actual pen position and the position the pen should be in for the applied voltage.
If the error is zero , meaning that the amplifier output is also zero and the pen is correctly positioned, then the motor remain turned off.
If Ein# E then the amplifier creates an output signal that turns on the motor
The motor drive the pen and potentiometer in such a direction as to cancel the error signal
When the input signal and position signal are equal, then the motor turns on the motor.
A paper drive motor forms time base because it pulls the paper underneath the pen at constant rate.
Most high-quality servorecorders use a stepper motor to drive the paper supply.
Such motor will rotate only a few degree every time a pulse applied to its windings
A few models use a continuously running motor that drive the sprocket through a speed-reducing gear box.
The stepper motor system is capable very good accuracy because a crystal oscillator or the ac power mains are used to drive the pulses used to advance the motor.
Digital integrated circuit frequency dividers (counter circuit) can be used to reduce the clock frequency to the frequency required to drive the motor at desired speed.
The reference potentiometer used to measure the pen position may be any of the following devices:
- Slide wire - Rectilinear, or - Rotary.
The slide-wire system is often used because it can be built with less friction and no mechanical linkage.
A resistance wire and a shorting wire are stretched taut parallel to each other and the direction of the pen travel.
A shorting bar on the pen assembly serves as a wiper on the resistance element and also connects the shorting wire.
A,B, and C in Figure refer to the potentiometer terminals
the shorting wire serves as terminal B of the potentiometer (the wiper)
The X-Y recorder uses two servo mechanisms connected to the same writing assembly, but at right angles to each other.
The X-axis servomechanism moves the pen-bar assembly back and forth across the paper in the horizontal plane, while the Y-axis servomechanism moves the pen vertically up and down along the bar.
The paper itself does not move. It is held in place either by clamp or, in high-quility instruments by a vacuum pump that is used to evacuate a hollow chamber below the paper platform.
Holes in the platform create the negative pressure needed to keep the paper in place.
One advantage of the X-Y recorder is that almost any type of paper may be used.
1- A dead band signal: largest signal to which the recorder will not respond due to pen assembles mass (inertia). Sufficient preamplification of the signal is required
2- Overshooting and undershooting of the recorded trace.
3- undercritically damped reader will overshoot the correct point and then hunt back and forth across the correct point for a few cycle until it hones in and settles properly
4- over critically damped recorder is sluggish the pen approaches the correct position very slowly .
Pen assembles are often damaged if they strike the limits-of-travel steps at a high speed.
A pair of Zener diodes connected back to back across the PMMC coil are sometimes used to accomplish the same job.
The Zenner potential of the diodes is selected so that the bodies break over and conduct current only when a voltage greater than the normal full-scale potential is applied to the input of the amplifier
Figure shows how to remove an ink blockage from an ink pen recorder that has been allowed to stand too long without being used.
As a general rule, such recorders should be run for about five minutes or so once a week when not in regular service.
Fill a 3- t0 10-cc syringe with water (or acetone for certain type of ink)
Insert end into the ink inlet
The pen has to removed from the machine for this operation.
Quickly, and with a single sharp motion, drive the plunger “home” so that a high pressure jet of water or acetone is forced into the pen. The ink clot should be forced out the other end.
Always wear protective-goggles and protective clothing.
Make sure that the pen is aimed downward into a sink
As always when dealing with needles, be careful not to stick yourself
The thick high-viscosity ink strains every thing it touches and a nearly impossible to remove.
Ink pen tips are designed to operate parallel to the paper surface
If the pen is worn or when a new pen is installed, it is necessary to lap the tip in order to reestablish the parallelism.
The sign that lapping is needed will be either (or both) of the following:
A- a blob f ink when the machine first stats a waveform, or
B- a too-thick trace
To lap the pen, place a piece of fine Emery cloth (sandpaper) under the tip.
Work the pen tip back and forth 5 to 10 times to sand the tip parallel to the paper
The pressure of the stylus or pen is also important.
If the pressure is not correct, then the waveform may be distorted
In medical equipment it is possible to make a normally healthy lead-1 ECG as though the patient has.
Suitable stylus ECG gages can be purchased from ECG machine manufactures.
The stylus pressure adjustment is made using a screw that is usually located on the rear of the stylus or the assembly that holds it in place.
The original dot matrix printers used a 5 X 7 matrix of dots to form alphanumeric data.
The dot matrix machine used a print head to cause the correct dot element
Two different methods:
A- Thermal based machine:
Some of the earliest machines were thermally based.
The dots were thermally connected to heating coils and could be heated when needed.
Special sensitive paper was used to receive the text.
This method is no longer widely used.
B- an array of seven print hammers (pins).
The pins would either extend or retract depending whether or not that particular data was active for the character being printed.
An advantage of the pin method is that ordinary paper can be used
The pen impacted at inked ribbon to leave the impression
Higher resolution models are now valuable with 9, 18, 24 pins