QA / QC — Accuraspray G3

A real-time evaluation of thermal spray processes in-situ, the PC based controller is the device that digitizes the video and detector signals, performs all calculations and delivers all the results through the system’s user interface.

		In operation, the sensor head is aimed at the spray plume from a distance of 200 mm and the following eight parameters are measured: Average particle temperature Average particle velocity Position of the spray plume Width of the spray plume Maximum luminosity of the spray plume Overall intensity of the spray plume Substrate temperature (optional) Internal sensor head temperature
Figure 1: Accuraspray Sensor Head and Controller Showing the Two Main Components of the Instrument: the PC Based Controller and Sensor Head (10 m Cable is not Shown)

Values of all these parameters are continuously recorded, displayed and compared, on screen, against user adjustableacceptance intervals. An alarm can be triggered through digital I/O ports according to a user adjustable alarm protocol. A standard 10/100 Ethernet port allows the instrument to be polled and controlled by advanced spray consoles such as Sulzer Metco’s Multicoat using a standard TCP/IP protocol.

The system is also equipped with a feedback algorithm that will suggest, for plasma spray, the proper changes in arc current and primary gas flow to bring temperature and velocity back within the acceptance interval.

Principle of Operation

Simple, time-shift cross-correlation yields a very precise measurement of the time delay from which the velocity can be calculated since the gap between the measuring points is a precisely known constant. In addition, the detectors are filtered at two different colors allowing the mean particle temperature to be measured using the very well known twin wavelength pyrometry principle (assumes that the emissivity of the particles is the same for the two wavelengths).

One advantage of this method is that the value of the cross-correlation (between 0 and 1) gives an indication of the validity of the temperature measurement. A strong correlation between the two signals (above 0.6, for example) means that the two detectors are seeing the same particle population which is essential for the twin wavelength pyrometry to work properly.

Figure 2: Particle Temperature and Velocity Measurement Principle Accuraspray features a dual fiber optical device that “sees” the flow of particles at two different points along the spray stream. The signal from the down-stream detector is very similar to the first one but delayed in time because it comes from the same particles detected a few millimeters apart.

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Sensor Head

Figure 3: Inside the Sensor Head Plume position, width and intensity profile are measured from the digitized live video plume image recorded by a CCDcamera integrated into the sensor head.

Head Dimensions and Placement

The total weight of the head is 1 Kg. As shown in Figure 4, the head’s working distance is 200 mm from the plume centerline. In the “x” direction, it can be aimed at any point along the spray flow. Typically, the measuring point will be adjusted to the actual standoff when producing the coating. If monitoring is required while spraying, it is recommended to aim at least 10 mm away from the substrate because of flow perturbation phenomena as the spray plume hits the substrate.

Vertically, the head is placed so that the particle sensor aims at the center of the plume. This can easily be adjusted using the live video image of the instrument’s main screen.

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Figure 4: Head Dimensions with Mounting Holes and Measuring Point Along with the Mounting Bolt Pattern and its Location Relative to the Spray Plume.

System Architecture

The head microprocessor controls the gain of the detector amplifiers and reads the head temperature thermistor. It also memorizes the head’s serial number and calibration data. The PC and both processors are linked through standard RS232 interfaces.

The PC is equipped with two specialized boards mounted on its PCI expansion slots. The frame-grabber board digitizes the video image from the camera. The digitizing board acquires the amplified detector signals.

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Figure 5: System's Data Flow and Control Architecture As illustrated, the system has two microprocessors in addition to the PC. The main one reads the substrate pyrometer and directly controls the camera’s shutter speed as well as the digital I/O’s.

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