Camera systems used in industry for automatic inspection and quality assurance must focus in a matter of seconds: A maximum of 100 ms is usually available for this. Although the travel distance of 0.5 to 2 mm is not particularly large - it is often only a matter of compensating for the thickness variation of a substrate or capturing narrow stacks of images at different depths of field - this short distance can quickly prove fatal for standard focus adjusters based on linear guides. This is because the tribological system of mechanical roller guides fails prematurely with millions of small movements, as the lubricant is displaced at the contact point and cannot be distributed over the entire raceway as is usually the case. This results in cold welding, which blocks the adjuster. With movements in the micrometer and sub-micrometer range, stick-slip effects also have a negative impact, i.e. the transition from static friction to rolling friction in the guideway.

This is where Steinmeyer Mechatronik comes into play with a focus adjuster consisting of a spring parallelogram, an eccentric drive with stepper motor and an inductive linear measuring system. At first glance, this combination may seem rather strange to mechanical engineers: leaf springs as a guide element? Eccentric drives and accuracy? However, such reservations are unfounded.

Leaf springs originally come from metrology, where they have been used for over a century, and the high-tech industry also makes use of the principle. In general mechanical engineering, however, solid-state joints are rarely used. In the form of a spring parallelogram, this is a sensible solution, especially for extremely short travel distances: with small longitudinal movements, the leaf springs deform elastically so that there is no risk of wear or fatigue fracture. The upper platform performs the desired linear movement. Vertically, the shortening of the leaf springs during deflection results in a parasitic movement that is predictable, systematic and can therefore be compensated for.

If this is not acceptable, the leaf spring can be designed as a rotationally symmetrical diaphragm spring. In this case, the material expansion eliminates the parasitic movement. However, diaphragm springs require significantly more space compared to leaf springs, which is an exclusion criterion for many applications.

For very small travel ranges, diaphragm springs are flat sheet metal ends; for larger travel ranges, they are slotted or fitted with circumferential beads. When correctly dimensioned, solid-state joints allow permanent linear movements with small deflections and a long service life. They require no lubricant, operate friction-free - regardless of internal material friction - and do not exhibit any hysteresis or stick-slip effects.

Ancient technology, modern system

The concept of the eccentric drive is also well known - the technology was already used in ancient times for the transformation between a rotational and a linear movement. Today, eccentric drives can be found in presses and clamping devices, but also in car combustion engines in the form of the crankshaft. The use of eccentrics enables a simple, robust and inexpensive solution, but has its pitfalls in terms of linearity and control. For example, the system behaves non-linearly during a full revolution in the vicinity of the reversal points. This results in a reversal of the linear movement with the same direction of rotation of the motor.