BETABLOQUEANTES

Diagram 5.14 shows clearly that an opto-electronic system can be placed in a very hostile environment. Good signal processing should be able to suppress or eliminate the interference effectively. For a closer examination it is an advantage to separate optical from other interference mechanisms.

Interference from variable light-intensity source

Interference from constant light source

Voltage variations

Out of adjustment

EMI Defect Dirt

Incorrect setting optical

switch

Object

Diagram 5.14: Many interference factors effect an optical sensor

The main source of optical interference are light sources which can be subdivided into constant and variable light sources. The constant light sources include the Sun, which radiates light close to the infrared region and artificial light sources (incandescent lamps).

They induce a direct photocurrent in the receiver element, the size of which can be many times greater that the useful signal itself. In addition this direct current produces noise in the receiver component, which has the effect of reducing the signal/noise ratio. Incorrect operation of the optical switch can not be ruled out under very

unfavourable conditions.

Fast artificial light sources (fluorescent lights), lightning, welding arcs and neighbouring optical switches are examples of interference from variable light-intensity sources. These produce a photocurrent in the receiver with a very small direct current component but with a high alternating component, which as before can be many times larger than the useful signal. The frequency spectrum of these interference sources is unlimited and can lead to incorrect operation of the switch.

Diagram 5.15 depicts the induced photocurrent for different interference sources as a function of distance d; as a comparison the useful signal of a direct detection

photoelectric sensor is also shown in the diagram.

Diagram 5.15: The level of induced photocurrent for different sources.

Sun

Incandescent lamp 100W

fluorescent Tube 40W

Useful Signal

In diagram 5.16 the variation in light intensity with time for an incandescent lamp and a fluorescent tube is shown. The incandescent light source shows a relatively large direct current current component and a very small alternating current. For this reason it can be considered in general as a source of constant light interference. The opposite is true in the case of a fluorescent lamp, the intensity curve of which has superimposed phase related impulse spikes, due to fast gas discharge in the in the tube. The frequency spectrum of these impulses is very large and can, as will be seen later, interfere with the sensor sensitivity.

Attenuation in the optical path, due to dirty optics and reflectors can also be counted as optical interference. The resulting loss in the receiver power can be so great as to cause the optical switch to drop out.

An important non-optical interference variable are temperature changes which above all effect the efficiency of the optoelectronic components. The result of these temperature effects are changes in the sensing range of direct detection sensors and in the case of reflex photoelectric sensors a loss of reserve signal power.

Variations in the supply voltage can have a similar effect.

In critical applications, for example, a reflex photoelectric sensor is used when a strong background reflection must be taken into account, in these conditions an incorrect sensitivity adjustment, so that the error signal lies close to the decision threshold means that only another very small interference effect is required to cause incorrect switching.

External electromagnetic radiation can induce interference in the signal processing circuit which leads to incorrect operation.

Diagram 5.16: Graphs of photocurrent against time produced by Incandescent lamp and Fluorescent Tube.

Incandescent Lamp

Fluorescent Tube

5.3.2 Stages in Interference Suppression

5.3.2.1 Interference suppression by optical modulation at the Emitter.

As opposed to continuous operation the emitter diode of the optical sensor is supplied with a time variable current i

LED and therefore optically modulated. In most cases a rectangular pulse is chosen (Diagram 5.17). This simple measure offers three immediate advantages.

Diagram 5.17: Square-wave modulation of diode current and therefore the light.

One result of the optical modulation is the noticeable difference between interference from extraneous constant light and the alternating voltage pattern of the useful signal. In this way interference from extraneous constant light can be eliminated (Diagram 5.18).

First the useful signal is is increased by the constant interference current id. In the subse-quent signal processing unit, which has a high-pass characteristic, the continuous

current component can be eliminated. The alternating component is (useful signal) remains, this can the be unambiguously interpreted by further signal processing.

A further advantage of optical modulation is the possible increase of the emitter power Φ_s. The receiver power increases by the same amount, all other conditions remaining unchanged, also the signal/noise ratio is similarly increased. In pulse mode operation light emitting diodes can be driven with a high current i

LED.

Using the simple assumption that for the maximum power dissipation given a constant diode current i

LED is permissible; then for optical modulation:

T i_LED = - · I_LED.

ti

The radiated power Φ_s is increased by the same ratio, for the same operating conditions, also the signal /noise ratio of the receiver signal.

Finally the third advantage of the optical modulation should be mentioned, that it is he requirement for so called "blanking", which is described later.

Diagram 5.18:Filtering out the current due to constant light using optical modulation.

Constant light interference

No Object Object No Object Object

Without constant light interference

With constant light interference

5.3.2.2 Interference suppression after reception by band-pass

The band-pass characteristic reduces the frequency range of the total system. In the upper frequency range the noise and interference alternating light are attenuated, in the lower frequency region interference constant light (e.g. Day light), low frequency inter-ference light (e.g. 50Hz modulated light from incandescent lamps) and noise are also attenuated.

Due to the optical modulation at the emitter the receiver current i_r has a pulse width of t_i. The pulse shaped receiver current ir is transformed into a voltage U_a by the high and low pass characteristic of the circuit. Diagram 5.19 shows a simplified view of the process.

Interference component: Useful component:

i_r

i_r

Diagram 5.19: The low frequency interference is filtered out by the band-pass filter.

Interference Useful component:

component:

Intergference component:

Useful component:

5.3.2.3 Interference suppression using blanking

The processed signal from the receiver amplifier is the digitised by an A/D interface. The interface consists of a comparator with a decision threshold Ues (See diagram 5.20).

Interference components greater than Ues pass unimpeded to be also digitised and remain part of the total signal. A useful signal can be expected a very short time after an emitted light pulse. The blanking suppression utilises this fact.

Diagram 5.20: Weak signals are filtered out by the comparator.Interference component Useful component

The symbolic switch in Diagram 5.21 is closed for a very short time only following an emitted pulse, when an useful signal is expected. For this reason the switch is

synchronised with the emitter. A number of interference pulses are in this way eliminated during the emitter "off" time. It should be noted, that if by chance an interference pulse occurs at the emitter "ON" time then it will remain part of the total signal as before and without further measures could lead to incorrect operation of the optical switch.

Diagram 5.21: By clocking the receiver interference pulses outside

5.3.2.4 Interference suppression by digital filtering

In order to suppress the disturbance a statistical evaluation of the frequency is required.

The decisive assumption here is that the previous signal processing has eliminated interference signals to the level that the frequency of these signals is small compared to the useful signals.

A simple and very effective method consists of connecting the data stream to a digital up/down counter, which is synchronised with the emitter pulse generator. If immediately after an emitter pulse the receiver data bit logic "1", then the counter increments, the opposite occurs if a logic "0" is present and the counter counts down.

When the counter reaches the maximum or minimum value it is reset. The output Q of these Flip-Flops represents the switch output of the optical switch (Diagram 5.22).

Diagram 5.22: Interference suppression using Digital Filtering

Object No Object Object

Counter

Counter State

Flip-Flop

Interference component

A delay time ts is associated with this technique between the occurrence (Object/No Object) and the reaction of the output Flip-flop. When no interference is present this time is given by:

t_s = T·(2ⁿ - 1),

here n is the "filter depth", this means the maximum counter steps and T the emitter pulse repetition rate.

When interference is present the time t_s is increased by amount depending on the

frequency of the interference. It can be seen that as n increases the noise immunity of the system increases, since more interference pulses can be tolerated before incorrect

switching occurs. On the other hand with increase in n the delay time t

s is greater and the maximum switching frequency is reduced.

1 f_s =

—-2·ts

From the above relationships we can obtain the product 1 -Switching Frequency * Noise Immunity T

The product of switching frequency and noise immunity should be large. The emitter cycle time T must therefore be as small as possible; to achieve this it is indispensable that emitter pulse time t

i is as short as possible, because of the pulse loading of the emitter diode. For this reason in high performance optical sensors the relatively slow phototransistor is not used as the receiver (detector), instead a fast PIN diode together with a receiver amplifier is used, which has a higher frequency limit.

Some sensor types have the facility to select the "filter depth" depending on the application. A selector switch in the terminal compartment of these sensors enables selection of the switching frequency between 200Hz and 1.5kHz. Internally the "filter depth" is changed correspondingly. At 200Hz the counter reaches it's highest value after 15 pulses, with 1.5kHz the highest value is reached after 3 counter

5.3.3 Function Reserve

5.3.3.1 Static Functions Reserve

For the input amplitude an upper good region (OG) is defined, in which information

"sufficient reflection" is available, also a lower good region (UG) is defined related to the information "no reflection"(Diagram 5.23).

Between these two regions is the region of the Function Reserve (FB). The switching thresholds also lie in this region.

When the amplitude lies in this region incorrect switching may occur due to external factors (temperature, dirt, reflections, etc.). In this situation a special output FRA indicates that the received signal lies in this unfavourable region FB; this enables suitable

precautions to be taken in time.

The static reserve indicator FRA is unsuitable for dynamic applications, where there is continuous switching of the switched output, since with each switching operation the signal passes twice through the function reserve region and the FRA indicator operates even though the optimal conditions exist. Here the dynamic function reserve indicator is used.

Diagram 5.23: Static Function Reserve

Function

5.3.3.2 Dynamic Function Reserve

Following each switching operation the signal amplitude is checked to see if it lies within or without the function reserve region.

The interference output is set when the receiver level, before a switching operation, has not left the function reserve range.

The report is independent of time, that is it is independent of the speed of the object and the frequency of the switching of the functions reserve indicator due to dirt or

maladjustment.

The interference indication remains on until the correct switching conditions are again provided. A switching operation can take place even if the ideal level has not been reached (Diagram 5.24).

Diagram 5.24: Dynamic Functions Reserve Switch "OFF" point

Switch "ON" point

DUAL-LED

green-supply voltage red-Functions Reserve

Switched output (N.O.) LED yellow

Function reserve Output

Funktions-reserve

Background reflection

Receiver Level

Switching Hysteresis

Dirt Poor reflecting object

5.3.4 Protection against mutual interaction

A number of optical sensors may be found in the same effective optical field.

From a sensor, here called the interference, pulses are emitted and received and processed by a second sensor, the operation of which is then disturbed. The emitter pulse repetition time of the interference is T₁ and of the disturbed switch is T₂.

Under the ideal conditions that T

1 and T

2, are at all times the same size, if in addition a time shift t0 is present there would be no mutual interference, since the received pulse of the interference arrives in the blanking space of the other sensor.

These ideal conditions are highly improbable. In reality the times T

1 and T

2 will always be different from one another. This produces a form of beat frequency, with periodic

repeating time zones, in which the received interference pulse can pass the interference blanking unhindered.

A remedy is to produce different but fixed times for T

1 and T

2, as is possible with some sensors switching from frequency 1 to frequency 2 with the selection switch in the terminal compartment.

T

2 = T

1 + ∆T

v

The number S of the interference pulses which occur can be calculated:

t_i S =

—-∆T

v

By suitable selection of ∆T a good compromise can be found between on the one hand the filter depth n and on the other hand the highest possible switching frequency of the optical sensor.

5.4 Types

5.4.1 Reflex photoelectric sensor with polarisation filter 5.4.1.1 Polarisation Filter

In addition to the propagation direction or characteristic the oscillating behaviour can be modified.

This is achieved using optical filters, which are only translucent to light of a definite wavelength or definite range (e.g. red filter, UV filter). The filtering out of a component of light can be achieved by reflection, absorption or deflection. A part of the energy is always lost with these methods of filtering, since only a portion of the light is allowed through.

Linear polarisation filters are of special importance to optical sensors, because they only allow light waves a particular oscillating plane to pass through (Diagram 5.25).

With polarisation filters the light is split by either reflection or refraction, during which one of the two beams or both are linearly polarised. This effect, for both reflection and

refraction, is dependent on the optical properties of the medium used.

These filters also absorb or reflect a part of the light, so that only part of the light energy available can be used. Polarisation filters are used with reflex photoelectric sensors.

Diagram 5.25: Polarisation filter

5.4.1.2 Retro Reflectors

While with the two optical surfaces, a mirror or contact surface, an angle is produced between the incident and the reflected light beam, with a retro reflector the reflected light is parallel to the incident light. This is achieved either by an arrangement of many three sided pyramids (tetrahedron) or by the use of special folia coated with hollow glass balls.

Triple reflectors consist of extremely exact cube corners (pyramids) of glass or plexiglass (Diagram 5.26). Here the specular reflection is produced by three total spatial reflections .

Diagram 5.26: Triple Retro Reflector

Retro Reflectors, because of the multiple refractions or reflections, are capable of turning polarised light into depolarised light and/or rotating the polarised plane through 90°.

Retro reflectors are used, for example, to improve the recognition of objects in traffic (reflecting signs) and are used with reflex photoelectric sensors.

5.4.1.3 Reflex Photoelectric Sensor

With reflecting objects, which are to be detected using a reflex photoelectric sensor, there is only one position in the optical path where the light from the emitter is reflected back to the receiver. Since the reflex sensor is activated by a break in the light beam in this special case it cannot distinguish between the reflecting surface and retro reflector.

To prevent this fault occurring polarised light is used. In diagram 5.27 the principle of operation is shown.

The emitter and receiver are provided with 90° displaced polarisation filters F1 and F2.

The non-polarised light leaving the emitter has only a horizontally oscillating component after passing through the linear polarisation filter F1. The receiver can only receive light polarised in the vertical plane, since the filter F2 is arranged perpendicular to filter F1.

Retro reflectors have not only the property of mainly depolarising light, but also to rotate the oscillating plane through 90°. Both effects now enable the vertical component of light to reach the receiver. In this way the reflector is recognised.

Ideal reflecting objects however rotate the polarisation plane through 180°, so that the horizontal oscillating plane is maintained. Since the receiver filter F2 blocks horizontally polarised light the light reflecting object is detected with certainty, based on the criteria

"receiver beam is interrupted".

5.4.2 Direct detection sensor with background scattering

As already mentioned optical sensors evaluate the intensity of the reflected diffused light, whatever the source of the light. In unfavourable applications it is possible that the

interference reflection from the background scene has an amplitude at the receiver similar to the object itself. In this case it is difficult, if at all possible, to detect the object.

A remedy is the so called background scattering, where a clear boundary between the sensing range and the background region is provided. There exists a series of

procedures, from which one will be described here:

The emitter and receiver optics are arranged so that their optical axes cross (diagram 5.28), because the two cones of light intersect an optical active space R is formed. It can be seen clearly, that an object can only produce a diffused reflection in this space, while the background light is effectively cut out. By turning the optical axis the useful space can be selected for a particular application as required.

Diagram 5.28: Cutting out the background light.

5.4.3 Direct detection sensor with light guides 5.4.3.1 Light guides

5.4.3.1.1 Principle of operation

Light guides are fibres of glass or plastic, which are capable of transmitting light fed into it. The light follows the shape of the light guide even when it is bent. This achieved by the use of the total reflection at the boundary surface of two media (diagram 5.29). The light is fed into a fibre of optically "dense" medium (e.g. glass or plastic), the diameter of which is selected so that at the optically rarer surrounding medium the critical angle for total internal reflection is exceeded. The light beam is reflected back from, the boundary between the optically dense fibre and the rarer surrounding medium, into the core and travels in a zig-zag course to the other end of the fibre.

5.4.3.1.2 Glass fibre light guides

Glass fibre light guides are normally combined together in a bundle of a large number of single fibres (approx. 0.05mm) within a sheath made from PVC, Silicon or stainless steel.

In this way the required flexibility is guaranteed. The individual fibres can divided in various ways between emitters and detectors (Diagram 5.30).

Depending on the application various arrangements can be chosen. Usually a half circle arrangement is used; for the detection of small objects a concentric or segment shaped arrangement is useful.

The diameter of the light beam which can be transmitted depends on the number of glass fibres available; with this the loss increases or decreases and the sensing distance at the end of the light guide which can be achieved. Tables in the catalogue give the relation-ship between the sensing range determined by the sensor, the glass fibre diameter which should be selected and the distance which can be covered with the light guide.

The diameter of the light beam which can be transmitted depends on the number of glass fibres available; with this the loss increases or decreases and the sensing distance at the end of the light guide which can be achieved. Tables in the catalogue give the relation-ship between the sensing range determined by the sensor, the glass fibre diameter which should be selected and the distance which can be covered with the light guide.

In document Departament de Medicina Facultat de Medicina UNIVERSITAT AUTÒNOMA DE BARCELONA (página 31-37)