Opto Electronic Interfaces MD-220
in Field Boxes 1 and 2
|
A detailed description of the dual opto-electronic interface MD-220 and its working principle is given in its users manual. There are, however, some special aspects to be addressed with respect to the present application.
To be used in this project the MD-220s will be modified. The angled screw terminal will be replaced by a pluggable version and there will be an external connection for the reset switch. The operation of the optocoupler outputs will also be changed. There will be an OR-ed output instead of the original AND-ed output and the logic of both this output and the error output will be inverted from normally open to normally closed. Outputs of this kind can easily be OR-ed again by simply being connected in series. This is how the OR functions shown as logical gates in Drawing 7 and Drawing 9 will be established.
The way the interfaces will be mounted inside the field boxes has not been finally defined yet. The issues left to be considered include vibration resistance but also accessibility of the optical and electrical connectors.
The MD-220 has a dynamic range of about 24dB and can work at a sensitivity of 0.2% of light change. That means that the transmittance of a connected sensor strip is allowed to vary by a factor of 200 and even at the lowest light level a variation of 1/500 is safely recognized. This in turn means that the required resolution of sensor transmittance corresponds to 1/100000 of the maximum allowed light level. I should be clear that it is not possible to meet these requirements with a fixed amount of light fed into the sensor and the returned light measured with a fixed sensitivity. If then the maximum light would for instance give a signal of 10V this would lead to a trigger threshold of 0.1mV in the worst case, and this in turn would mean that the noise level would have to be still well below 0.1mV – a requirement hard to meet.
With the MD-220 the problem is solved first by enabling the light fed into the sensor by a factor of 20 and second by increasing the detection sensitivity also by a factor of about 20 while subtracting excess photo current when the input signal becomes clipped. This reduces the required signal resolution by a factor of 400, and this can be handled.
However – there are now two quantities adjusted in order to adapt to the given sensor transmittance: the light power fed into the sensor and the amount of photo current subtracted in order to get the current light level into the measuring range. These quantities may not be altered when the interface is in the triggered state, i.e. when a decrease of the detected light below the trigger threshold has been detected. This would result in drifting of components, especially the optical transmitter, and this in turn would alter the voltage signal the original sensor transmittance would be translated into. A correction is not possible because due to the sensor activation this original transmittance is not present atthe moment. Proper detection of the unloading of the sensor would thus severely be put at risk.
As a consequence sensor failure can not be detected at once. If a sensor strip breaks this results in the received light decreasing and forces the interface into the triggered state where it is no longer allowed to alter its adjustment. This, however, would be necessary to find out that the light has indeed dropped to zero in order to tell that the sensor is failing. The maximum allowed trigger time (currently 30s) must first elapse before the interface can attempt to adapt to the new sensor transmittance which may then found to be insufficient.
Therefore it must be noted that a sensor failure will always first be signaled as a sensor activation. In no case it will remain unnoticed.
|
|
|
|
|
