Flow and jam control of granules with the MICROmote® amplifier S294
The predominant part of applications for optical sensors deals with object detection. The monitoring of the conveyance of granules in a transparent delivery system, such as a glass or plastic tube or duct is a special form of it but with a specific difficulty:
Regardless of whether it is grains, mechanical parts, or some other sort of particle with a granular structure, three working conditions - 1. granules in motion, 2. no granules, 3. material jam - must always be reliably distinguished.
In comparison to other applications typical for optical sensors, there are two particularities:
Due to the large number of individual particles in the material flow, there is no discrete signal with a corresponding ‘peak value'. The individual grains provide signal portions which are extremely difficult to distinguish. Rather there is a constantly varying base signal (for example, from a change in the colour of the granules) with a superimposed ‘jittery' dynamic share of erratically varying amplitudes. In addition, three different statuses must be able to be represented at the amplifier output.
This task can be solved with the V91/S294-AP amplifier with dynamic signal evaluation: with a "high-pass" function in the electronics, the rapid signal changes, such as those generated by individual grains flying through the detection area, are filtered out largely independently of their amplitude. If such rapid signal changes are detected, the associated dynamic output switches an ‘On' signal.
Parallel to the dynamic evaluation, there is also a static evaluation. It compares the received signal classically with a defined comparison value and delivers an ‘On' or ‘Off' signal, depending on whether it is above or below the value.
Thus, all three operating conditions can be depicted through the logical combination of the two outputs:
1. Object is not present = no output signal
2. Object is present and moving = dynamic output signal + static output signal
3. Object is present and not moving = static output signal
Like all MICROmote amplifiers from STM, V91/S294 can also be used with any MICROmote sensor, regardless of whether they are through beam or reflective sensors. The choice of the right sensor depends rather on the application or the granules to be sampled and the geometry of the application:
Through beam sensors do deliver the clearer signal in principle. However, they can only be used as long as the mass stream is not so dense that light can no longer penetrate and strike the receiver on the other side. They are the right solution when relatively few objects fall through the detection area. But it must also be borne in mind that accompanying material, such as dust or grit, can quickly lead to false signals.
For large mass flows, such as in the supply of granules, sensors on the outside of a transparent supply tube make much more sense. Design, positioning, and selected wavelength must be optimally selected in accordance with the monitored material and existing geometry.
