An explanation of the industrial application in which spray jets are inspected with optical sensors.

A spray jet control or the inspection of the spray jet is a method to detect and measure the correct functioning and the even distribution of a spraying system. The spray quantity and the dimensions of the spray cone In the use of spraying systems have to comply with the area of application in which these have to be used. These dimensions are also dependent on a number of other factors: 

  • the used medium (primer, water, alcohol, etc.) 
  • the exit hole of the spray nozzle;
  • the overpressure;
  • and the spray quantity per spray.

Especially with spraying glue or other adhesives it is possible for the exit hole of the spraying nozzle to get clogged. Which in turn can lead to a change in the spray quantity and spraying pattern in terms of spraying direction as well as the opening angle of the nozzle. A system for spray jet inspection immediately informs the user and the controller about anomalies like this.

What does a spray jet look like?

A spray beam, a ‘loose formation’ of small droplets (with dimensions that vary between a few or hundreds of micrometers – dependent on the medium) that form at the spray nozzle by the vaporisation of the spray medium. These droplets leave the nozzle of spraying head at a certain speed and decelerate by air friction.

The spray jet is shaped by the opening angle of the spray cone and the spray quantity (droplets per time unit). With the help of a spray jet inspection this information can be mapped.

How to perform a spray jet inspection?

An inspection is performed using a laser beam that passes at least partially through the spray cone. This makes it possible, for example, to find out what the spray density is per unit of time.

Gradually through the spray cone, the laser beam is deflected by the droplets. This deflection is caused by the reflectivity of the droplets or by the focusing of the laser beam, because droplets act as microlenses if they are optically transparent. In addition, part of the light is also absorbed by the droplets. Ultimately, less (laser) light passes through the spray jet.

There are four different methods of controlling a spray jet or spray cone. Within optical sensors, these are also called through-beam methods, because the objects that have to be measured or detected pass through the emitted light beam(s). The interruption of the light helps the lasers determine the shape, presence, and length of passing objects — in this case, the spray jet and the droplets that it consists of.

Methods for spray jet control

Below there are 4 different ways of performing a spray jet inspection with the L-LAS lasers from Sensor Instruments

1. Through-beam method with one light beam

In this case a laser beam is, preferably with a slit-shaped opening, led through the spray nozzle centrally. The signal decline, compared with the absence of the spray beam, is used as a measurement of the spray volume. This method is predominantly used if there is only a need to know the spray volume or the presence of a spray beam.

2. Through-beam method with two light beams

This method is, in addition to volume control, also possible to use in symmetry control. A lateral shift of the spray cone can be detected by this method. The method with two beams is mainly used when a simpler and more cost efficient way of monitoring the symmetry of the spray cone is necessary.

3. Through-beam method with three light beams

Even the smallest deviations in symmetry and quantity can be detected with this method. Two evaluation-modes are possible here: ABSOLUTE evaluation-mode and RELATIVE evaluation-mode. In both modes the relation between the two outer beams and the relation of the middle beam with the two outer beams is evaluated. The absolute evaluation mode works with raw values and the relative mode works with values which are formed during the spraying. This method is preferable for very exact applications, such as different color patterns on bicycle frames, which should not overlap.

4. Through-beam method with a light grid

With this method a continuous light is shined on the spraying beam. The light band is wider than the diameter of The spray cone. This means that the spray beam is detected completely. On the other side of the spray beam there is a receiver that is designed as a line sensor with more than 1000 individual miniature detectors that are placed in a row. This makes a continuous monitoring of the profile of the spray beam possible. The difference in percentage between the two line signals that are registered in advance and during spraying, are used to determine the beam profile.

Inspection of spray jets in ATEX environments

The inspection of the spray jets is also possible in ATEX environments. The spray jet is then in an environment with a risk of explosion due to the presence of flammable gases, substances, vapors or mist.

With the use of a contrast sensor, like the SPECTRO-1-POFonly the optical part of the sensor is placed into the ATEX zone. An important note in such an area of application is that the optical power does not exceed the maximum allowed intensity for areas with an explosion risk. This is because of the fact that lasers or other optical beams with sufficient optical power can be an ignition source when projected onto explosive surfaces, gases, dust and chemicals. Our contrast sensors are far below this optical power range and thus are a safe and suitable solution for use in ATEX environments.

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