Machine Vision Reveals Defective Magnets and Qualifies Parts for AssemblingA machine vision system, NEOVISION, has been developed for controlling the dimensional quality of flat NEO- type permanent magnets. The partners of this European HPCN TTN project are CCD Photonics Ltd from Espoo and Neorem Magnets Oy from Ulvila, Finland.
Both Rare Earth (RE) and
NEO (Nd-Fe-B) -type permanent magnets are manufactured using a complex
pressing/sintering/grinding powder metallurgy process. The grinding tools
are used to meet the tight dimensional tolerances required by stepper motor
manufacturers and other end users. The magnet material is as hard as glass
and it is susceptible to chipping and cracking during grinding. Pressing/sintering
process variables drift slowly and their statistical changes have an effect
on the shape of magnet parts.
Fig. 1. NEOVISION operates
using a reflection principle, which reveals also partial edge chipping.
A selection of actual magnet
shapes is shown in Fig. 1. Should a certain magnet product be rectangular,
a slight variation in the process may mean that the shape slowly drifts
to a banana, barrel or tapered form. The appearance of internal cracks,
as in Fig. 2, or edge chipping is also possible. The internal crack sometimes
gives a broken part to visual inspection, which is a much better situation
than having a bad part inside a final product.
Fig. 2. Internal crack formations in disc magnets.
NEOVISION System Checks Magnets on Conveyor Belt
Each magnet piece should be checked before shipping them to an end user; oversized, undersized and also deformed parts as well as parts suffering from cracks or insufficient clean up must be rejected. This work has been done using a mechanical calliper with human eyes. A typical tolerance of length is from +0.1 mm to -0.6 mm for a 25 mm part, whereas the acceptable variation of width is from +0.01 mm to -0.20 mm for a nominal measure of 5.0 mm.
The NEOVISION camera system
is based on electronic imaging using a linear type CCD (Charge Coupled
Device) sensor having 5000+ light sensitive elements. The principle of
imaging is illustrated in Fig. 3. A fluorescent tube lamp illuminates magnet
parts moving on a conveyor belt and the specular reflection from the flat
surface is focussed onto the light sensor. The system recognises only the
rays reflected from machined horizontal surfaces, which means that the
appearance of cracks or insufficient clean up, as in Fig. 3, is reliably
revealed. The total observation width of NEOVISION can be chosen according
to the size of magnet parts in production; a part measuring 50 mm across
the conveyor belt can be inspected with the nominal resolution of 0.01
Fig. 3. A thin magnet showing insufficient clean up enters the control
Experience at Neorem Magnets
The NEOVISION system has
been installed for industrial use at Neorem Magnets Oy in Ulvila, Finland.
Dr. Mauri Veistinen says that machine vision can be utilised at different
stages of the powder metallurgical production;
location of green compacts on sintering trays, feeding of sintered parts
into grinding machines, in visual inspection of machined parts as well
as in packing of finished parts. "We have also applied vision technology
very successfully for automatic feeding of arc segments into a through
feed grinding machine. I believe that our system is the first of its kind
in magnet business."
Fig. 4. NEOVISION system in full operation at Neorem Magnets.
NEOVISION will give significant
cost savings to Neorem's production and also
a more consistent and standardised quality of finished products, which
also gives a clear cost saving to the final customer. The data obtained
from the computerised visual inspection system also gives an important
feedback concerning the statistical variation of the manufacturing process.
Says Dr. Veistinen: "We have our own cookbook for certain peculiarities
of the process!" - A typical variation of product width is shown in Fig.
5. Only six parts have been rejected due to width violation in a batch
of 250 magnet parts.
Fig. 5. Typical variation of magnet width at Neorem Magnets.
NEOVISION Qualifies Lock Parts for Assembling
The imaging system developed by CCD Photonics Ltd has been used in many applications outside magnet industry. Joensuu Factory Automation Oy (JFA) have installed NEOVISION for controlling operation of a vibrating feeder in the assembling line at ABLOY Oy, Joensuu, Finland. Kauko Paljakka (JFA) says that their assembling robot picks the lock cylinders from the moving belt assuming that the open end of a lock cylinder always comes first. NEOVISION is used to reject parts entering in wrong position, and they have a new chance as they come to the conveyor belt again. The variety of lock cylinders is vast as can be seen from the collection of Fig. 6. A lock cylinder can move in any tilted position on the belt and NEOVISION algorithms have been oriented to match any possible appearance.
CCD Photonics Ltd is a spin-off
company from Technical Research Centre of Finland (VTT), founded in 1990
by Dr. Kimmo Simomaa, one of the founding members of the Vision Club of
Finland. -CCD Photonics Ltd is AAA- rated by Dun & Bradstreet Finland
Fig. 6. Small selection of lock cylinders.