Achieving accurate dimensional measurement of diesel engine injectors

In the automotive market, dimensional accuracy of vehicle components has a great importance as an indication of the product quality. Depending on their position, components with dimensional problems may cause serious quality issues that can directly or indirectly create threats for users of the vehicle, such as rolling. Many problems--such as airbags with bad timing or brakes with long stopping distance--are examples of the effects caused by dimensional problems of the components.

Like other parts, dimensional accuracy is important for diesel engine injectors. Any kind of dimensional problems occurring during the production of an injector may cause parameters such as engine performance or fuel consumption to be outside the desired tolerances. If a vehicle purchased with a trust in its brand does not reflect the expected performance due to such a problem, this will have a direct impact on the trust of the customer in regard to that brand. Due to the fact that the automotive industry is incredibly competitive compared to other industries, such errors causing customer losses are intolerable.

Injectors pass through several shaping operations until the burr removal station, which is the last phase of their production before packaging. Each of those shaping operations has the risk of causing various dimensional problems on the parts. Consequently, the burr removal station is the most suitable point to execute dimensional controls before sending injectors to the customer.

Traditionally the injectors are manually inspected by the operators. When we think about thousand of products per day, it can be easily recognized that manual inspection, which is performed one by one, is not a cost-effective method. We should also consider mistakes caused by the human factor.

An industrial vision system developed by Vistek A.Ş. for Bosch Sanayi ve Ticaret A.Ş. solves these problems by automating dimensional measurements of diesel engine injectors with 50-µm precision. The burr removal station, built by Roboset Otomasyon Elektrik ve Makina Mühendisliği Sa. ve Tic Ltd., was the most suitable point to install the vision system and execute dimensional controls before sending injectors to the customer. Since the dimensional control operations are performed concurrently with the burr removal process, the system doesn't need additional time in the production cycle.

The main function of the system is to provide dimensional values of the injectors via processing their images. There are nine types of measurements for each injector. The measurements can be divided into two groups:

• height measurements
  
• diameter measurements

In order to achieve the desired precision, the injectors are logically divided into eight subsections. The system captures one image for each section and then performs image processing separately. The image capture is performed using high-resolution IDS (Obersulm, Germany; www.ids-imaging.de) cameras.

Image capture from different sections of the injectors is enabled by a vertically moving platform on which the camera is mounted. The movement of the platform is realized via the communication between vision system and programmable logic controller (PLC). When a dimensional measurement will be performed for an injector, the PLC first resets the position of the platform. When the vision software--developed using MVTec's's (Munich, Germany; www.mvtec.com)Halcon interactive development environment (IDE) HDevelop--sends the signal indicating that it is ready for processing, the PLC sets the platform to the altitude where the first image will be taken and informs the vision software.

At this point, the PLC is waiting for the measurement value from the vision side. With the new position information, the vision software captures and processes the image, and at the end returns the value to the PLC side. The PLC side compares the return value of the vision software and the original value of the inspected section. If a deviation from tolerances is detected, the system warns the operator via various interfaces.

After each return value of the vision software, the PLC sets the camera platform position to the next inspection section and again waits for the return value. This loop continues until all measurements on an injector are completed. It ends with resetting of the platform to the zero point. Measurement values found by the vision system are also displayed by the vision software's graphical user interface.

The interface is designed to be able to show nine measurements with their images and values on the same screen. With each measurement, injector display screens and value fields are filled with suitable images and values. In this way, when all measurements are completed for an injector, the interface contains all images and values related to this injector. Blue and green colors, respectively, signify diameter and height measurements. Values out of tolerance are displayed in red, so when the operator observes the graphical user interface, he/she can easily understand the region, the value, and the measurement type of the error.

As the system was put into operation, it provided significant labor savings for Bosch Sanayi ve Tic. A.Ş. The measurements--previously performed manually for each part--are now automated and the process has become both faster and more accurate. Moreover, since the measurements do not cause an additional load to the cycle time, the system has performed flawlessly in the adaptation to the production process.

-- Posted by Vision Systems Design, www.vision-systems.com