This article highlights three key elements of your 100% inspection system are; the encoder, the LED lighting solution and the lens.
All the components of your 100% inspection system need to work together in sync in order for your system to be successful. Three of the key components that make up your 100% inspection system are:
- LED lighting solution.
- Camera lens.
Understanding these key components is key when specifying your 100% inspection system.
Encoders are often left as an after-thought when purchasing a 100% inspection system, however it can be the single biggest point of failure for your 100% inspection system.
Encoders are the physical connection between your 100% inspection system and the product. Understanding why they are used, how they work, how to mount encoders and finally how to select an encoder are important.
The mechanical workings of an encoder are straightforward. A shaft rotates a coded disc. The disc has an array of holes that an LED will then shine through to a detector. This is then converted to an electrical signal. The frequency of this signal is a direct match to the rotating speed of the disc and this the shaft.
The three most common ways to mount encoders are:
- Belt driven - This is where an encoder is coupled to a motor or rotating shaft using a belt. There may be gearing involved, but this is also known as an indirect mounting.
- Surface / Friction mounted - These encoders simply lie on top of an idler and rotates as the idler rotates. The encoder assembly is normally spring mounted. Actual contact is made by a rubber O-ring which has far better contact properties than aluminium or steel.
- Shaft mounted - This is the most common and most reliable encoder mounting assembly. The assembly is mounted to the shaft of an idler usually through a coupling device as show below
The following are the main considerations for selection of an encoder that will work reliably:
- Heavy loads.
- Temperature extremes.
- Electrically noisy environments.
- Corrosive environments.
- Wet or dry environments.
LED lighting has emerged over the last number of years as the norm for 100% inspection. LED illumination should be the default of all modern inspection solutions, unless an application has a particular requirement for a characteristic found in other technologies.
LED lighting possess a number of key distinguishing factors that have allowed it to become the industry norm for 100% inspection systems;
- Very long life span - designed to last up to 100,000 hours (7 to 10 years).
- Reliable due to use of solid state technology and often temperature controlled.
- Low maintenance so no bulb replacement.
- Light output is very stable (No Flicker).
- Resulting in clean noise free inspection images thus better defect detection.
- Light output is very even along its length.
- Very high intensity.
- Lenses bundle and focus the light.
- Results in smaller apertures, sharper images and thus better inspection.
- Color temperature of light output is 6,700K which is equal to the color recommended for use with the line-scan cameras.
Choosing the right lens is not just about price. The lens will have knock on effects to the mechanical structure, the type of light used and the quality of the inspection. The three most important areas to look at with your lens are:
- How the aperture works.
- Depth of field.
- Focal length.
How the aperture works
When light passes through a camera's lens, it must pass through an opening called an "Aperture". The aperture, which is an opening in the lens diaphragm, is like an adjustable hole, that lets in more light the more it is open, less light the more it is closed. In essence the aperture is just like the pupil in the human eye. You can control the amount of light passing through on to the CCD in the camera, by setting the "Aperture Opening". The image below shows an example of an aperture fully opened allowing maximum light alongside an aperture that is almost closed, allowing very little light into the camera.
Depth of field
When you focus the camera lens on a subject to get a sharp image, other objects nearer to the lens, and further away from it, do not appear equally sharp. This decline in sharpness is gradual and progressive. There is, therefore, an area of apparent focus – ‘zone of sharpness’, where the blur is too small to be noticed and will therefore pass to the viewer as sharp, that exists in front of, and behind, the lens. Depth of field is the technical term used to describe this 'zone of sharpness' between nearest and furthest of a subject in focus, i.e. the distance of sharp focus in front and behind the subject on which the lens is focused.
Very simply focal length is the distance from the lens to the sensor, when focused on a subject at infinity. In other words, focal length equals image distance for a far subject. To focus on something closer than infinity, the lens is moved farther away from the lens. This is why most lenses get longer when you turn the focusing ring. The focal length of a lens also determines its cone of view. This has an effect on distortion. When doing 100% inspection, it is important to reduce the effect of distortion on objects on the outer edges of the field of view, else it can affect detection and classification.
Understanding these three key components of your 100% inspection system is key when specifying your 100% inspection system. The encoder, light and lens need to all work together in sync to ensure there is no disturbance to your inspection process.