FISIOPATOLOGIA DE LA HEMORRAGIA POR VARICES

Principle of operation: The following descriptions of distance measuring ultrasonic sensors are all based on the principle of measuring the echo transition time.

As the evaluation of the echo is performed at the same point, form which the ultrasonic wave was transmitted, this is referred to as direct detection.

An ultrasonic transducer transmits, at a point in time t

0, a short train of pulses of length Dt, which are propagated in the surrounding medium with the speed of sound c. If the train of pulses hit an object a part of the waves will be reflected and reach the sensor after a period of time 2 (see diagram 4.10).

The echo, which is returned to the sensor at time t₁, will be detected by the same or a second transducer and in a following amplifier amplified to a signal, which can be evaluated.

The evaluation electronic ,which determines the distance of the object, measures the transit time of the echo, in that at time t

0 the time measurement is started and at time t

1 the arrival of the echo is stopped.

When a single ultrasonic transducer is used for transmitter and receiver, this is referred to as a single head system, when two separate transducers are used for transmitter and receiver this referred to as a duel head system.

The single head system has the disadvantage that after transmitting a number of ultrasonic pulses a dead period must pass, during which the transducer oscillations decay, before it is possible to receive an echo.

As a consequence of the dead period ultrasonic transducers with single head operation have an unusable close range, within this limit echoes cannot be detected.

The build-up of the transducer oscillations is influenced by various factors, such as the total oscillating mass, inner damping, the decoupling material and the mechanical suspension of the transducer. The close range for P+F-ultrasonic sensors for object detection range of 1m and 6 m is in the region of 0.2 m and 0.8 m; this corresponds to a decay time of about 1ms for a 1 metre system and 5ms for a 6 metre system.

The close region can be greatly reduced when a two head system is adopted, that is two different ultrasonic transducers are used one for the transmitter and the other for the receiver.

In the construction care must be taken to ensure that the maximum transmitting sensitivity of the transmitter and the maximum receiving sensitivity of the receiver are at exactly the same frequency.

The variation of voltage with time for a single head system on a ultrasonic transducer is shown in diagram 4.10.

Echo

clock Ultrasonic Transducer

Receiver electronic

Transmitter Amplifier

Output Board Control

Electronic

Diagram 4.11 Block diagram of a Single head system Ultrasonic Sensor Interface

Diagram 4.10: Variation of transducer voltage with time for a single head system

∆

Diagram 4.12 Block circuit diagram of transmitter and receiver stages Switch Oscillator Amplifier

TRANSMITTER AMPLIFIER

Limiter Control Selection Rectifier Amplifier Comparator Amplifier Amplifier

Block circuit diagram: The principle of the electronic circuit of a direct detection mode ultrasonic transducer is shown in diagram 4.11

The transmitter output circuit is switched on with a trigger pulse, a train of pulses, of approx. 250 V peak to peak, is delivered to drive the ultrasonic transducer. The train of pulses is applied at the same time to the input of the receiver amplifier overrides it. After switching off the transmitter the receiver amplifier requires a recovery time of approx.

300 µs, in order to come out of saturation and be ready to receive signals again. The recovery time is generally smaller than the decay time and therefore does not effect the close region.

When an object, with a sufficiently large echo, is the detection zone a high frequency echo alternating voltage is produced, after the echo transition time. The signal is

amplified by the receiver amplifier, rectified and with a comparator converted to a square pulse. The evaluation electronic produces the trigger pulse, determines the period

between trigger pulse and arrival of the echo and controls functions such as the control of switched or distance proportional outputs. Following the arrival of the first echo pulse the evaluation electronic must delay further transmission until echoes from more distant objects can no longer be expected (Time out).

Interference Suppression: Following echoes can lead to false information and must be suppressed. For this reason the gain of the receiving amplifier with increasing time following the trigger pulse is continuously increased by means of a control voltage.

By this means the following is achieved, immediately after transmitting a pulse the echo, of the last but one pulse, arrives from a long distance at an insensitive receiver amplifier and is not registered.

The control voltage has another function to counteract the large decrease in the echo amplitude with increasing distance of the object.

The control voltage is generated in the receiver amplifier and is synchronised by the clock (see diagram 4.12).

Variable Cycle time: A further measure to suppress multi-echoes and background echoes is to increase the trigger pulse width and therefore the transmitted pulse length. Use is made of the fact that on applying a square wave voltage pulse the surface amplitude does not rise suddenly but increases over the build-up time. The relationship between the transmitter pulse length and the maximum generated sound pressure is used to match the transmitter energy to the distance of the object.

For small object distances the pulse width is shortened, by doing this background echoes from long distances are reduced.

Electronic Assembly of transmitter stage and receiver amplifier:(diagram 4.12). The transmitter comprises an electronic switch, an oscillator and an amplifier output stage, which delivers the necessary 250 V required to drive the piezoelectric ceramic. The oscillator is set, only once, to the resonant frequency of the ultrasonic transducer in order to obtain the best possible efficiency. The resonant frequency of P+F-ultrasonic transducers depends on the sensor type; it lies between 70 kHz for 6 m transducers and 170 kHz for 1m transducers. The electronic switch switches the oscillator on and off, depending on the trigger pulse width, enabling the generation of short transmission pulses.

Since the transmitter electronic and the ultrasonic transducer are limited in their energy conversion, the ratio of on time to off time must be maintained at a maximum of 1:50.

The Receiver consists of a limiter, a controllable amplifier, a selective amplifier, output amplifier and a comparator.

Since the received ultrasonic signal can be between a few microvolts and a few volts, it is limited to +/- 0.7 V by the limiter circuit; this also protects the amplifier from too high a value of peak voltage. The background echoes, referred to above, are suppressed by means of the controllable amplifier and the control voltage generation, which in turn counteract the reduction in echo amplitude with increasing distance.

The function of the selective amplifier is to filter out stray sound signals of other frequencies and allow only the useful signal to be further processed.

In order to produce a low cost amplifier it is necessary to have a low frequency signal; for this reason the ultrasonic signal is demodulated and rectified and only the resulting envelope further amplified. The amplitude of the envelope voltage is compared with a preset threshold voltage by the comparator; when the threshold voltage is exceeded a pulse appears at the echo output, equal in magnitude to the supply voltage, for processing by the evaluation electronic.

Evaluation unit: in addition to the transmitter and receiver, which have been described, a complete ultrasonic sensor also requires an evaluation unit to control the timing and the output function of the sensor. As the evaluation electronic has to perform complex control tasks it is advantageous to use a microprocessor system. A further advantage of such a system is that the evaluation algorithm is not hard wired but is flexible in the form of a program, which can be stored in a program memory. The same electronic circuit can be used control various output stages or carry out changes in the evaluation algorithm.

Functions which are performed by the evaluation electronic are the generation of the, already mentioned, clock rate, control of the transmitted pulse width, determination of the echo propagation time recognition of noise, control of the output signal and circuit self-test. In addition a microprocessor control system can communicate with a central computer via a suitable interface.

Correction of measured values: with the determination of the echo propagation time there are slight differences in the measured distance of the object from one measurement to the next due to changes in the air. A more accurate measurement can be obtained by taking the average of a number of measurements; however the results will not be very often repeatable.

The effect of interference echoes can be suppressed by comparing each value with the momentary average and rejecting measurements, which exhibit a large difference.

In applications where a higher rate of measurement is required a different algorithm can be used to suppress interference. The difference in the last two measurements is taken and stored. If the difference is zero, then the object is stationary, a constant difference indicates an object moving at constant speed and a changing difference indicates an object, which is accelerating.

A measured distance is accepted as valid when the last two measured differences are almost identical, in this way accelerating objects can also be detected with certainty.

Mechanical construction: The housing is in three parts: the transducer assembly with the analogue transmitter and receiver stages, the housing part with the evaluation unit and output stage and the base of the housing with the terminal compartment for the electrical inputs and outputs. The part of the housing, which contains the ultrasonic transducer can be plugged into the main housing at different angles, so that with the base fixed different directions in space can be monitored. The plug-in connection between the main part of the housing, which contains all the electronic, and the base enables the easy interchange of sensors, without the need of installation.

Diagram 4.13: Mechanical assembly of an ultrasonic sensor (UJ 2000+U1+H12+P1, Pepperl + Fuchs)

The sensor can easily be recognised in diagram 4.13. The section shows the piezo ceramic, the decoupling layer, the P+F-oscillator as shown here and the suspension of the transducer in integral foam. The function of the integral foam is to support the transducer so that the oscillating system is damped as little as possible. At the same time the internal parts of the housing are sealed against the effects of humidity by the integral foam.

Housing for the evaluation unit and output stage

Housing base with

terminal ompartment for the electrical connections

Rotatable Housing for the analogue transmitter stage and receiver stage Piezo Ceramic

Integral foam Decoupling layer Transducer Assembly