2019/02 | Steinmeyer Mechatronik developed a motion unit for computer tomographs , that moves the 1.2-kilogram apertures in front of the X-ray source so that the X-ray beam can be adjusted to the minimum necessary radiation dose.

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Brilliantly positioned

High-precision aperture positioner for computed tomography from Steinmeyer Mechatronik

As a specialist for customized solutions, Steinmeyer Mechatronik has many years of expertise in the development of innovative and individual positioning systems. The fact that less is often more is impressively demonstrated by the aperture controller for a manufacturer of computer tomographs.

Basically, a positioning system consists of only four main components: Motor, screw drive, guide and feedback system. However, it is not quite as simple as it seems at first glance. For each component, a whole range of highly different principles and variants are available. Which of these is ultimately used depends on numerous factors. The decisive factors for the choice include the load to be moved, the required accuracy and the available installation space. Aspects such as the environmental conditions and the desired cost framework also play a role. The special art then also lies in the optimal selection and combination of the individual components. The goal here is the application-specific optimum of accuracy, speed and price.

Specialist for high-precision positioning systems

Steinmeyer Mechatronik has mastered this art perfectly. For more than 145 years, the name has been the epitome of quality and precision. The Dresden-based company has long since made a name for itself worldwide as one of the leading manufacturers of high-precision positioning solutions for optical and precision mechanical systems. In addition to the production of standard products, the company's core competence includes the realization of individual special projects. Together with the customer, the positioning experts develop innovative concepts perfectly tailored to the respective application. Whether it's a single functional sample, projects with quantities of a few dozen or series with several thousand units per year - Steinmeyer Mechatronik equips all products with the necessary controls, puts them through their paces according to customer criteria and delivers the positioning systems fully assembled.

The secret of the high level of innovation? Elger Matthes, Development and Product Management at Steinmeyer Mechatronik, puts it in a nutshell: "It is important to constantly rethink problems, to question established solutions and to think outside the box. We can only get there if we put the better solution above the habit and value simplicity more than complexity." What this looks like in practice is shown by the orifice plate actuator that Steinmeyer Mechatronik implemented for a customer in the medical technology secto

It goes round

The task was to develop a motion unit for a computer tomograph that moves the 1.2-kilogram apertures in front of the X-ray source so that the X-ray beam can be adjusted to the minimum necessary radiation dose. The adjustment unit is housed inside the ring tunnel (also called the gantry) and rotates continuously around the patient during CT scans - several times per second. As a result of the high accelerations, enormous forces many times the acceleration due to gravity act on the diaphragms and the diaphragm actuator. This can quickly push materials and construction to their limits. "We therefore had to find a solution that could withstand these rotation-induced vibrations and strong accelerations transverse to the direction of movement, while meeting the high requirements for operational and patient safety, reliability and service life in medical technology," Matthes looks back. "This demanded a particularly stable design with minimal room for error."

Ingeniously simple, simply ingenious

This was achieved through a carefully thought-out choice of components down to the last detail, as well as an amazingly simple structure. "Over the years, in a multi-stage process, we have simplified - you could also say "perfected" - the design to such an extent that it has become increasingly robust and thus more reliable," Matthes explains. True to the motto: What isn't there can't break. The aim was to keep the number of parts and surfaces to be machined to an absolute minimum without compromising on quality and performance. For example, the spindle is "gripped" by two bearings instead of a classic locating bearing/non-locating bearing arrangement. Also, from initially two guides, only one remained in the end. "This is hard development work. Solving problems, complexly, is relatively easy. The real challenge, on the other hand, lies in the simple," Matthes said. "It takes years of experience and a lot of know-how." The result is convincing: the combination of stepper motor, ground ball screw, profile rail and incremental encoder masters the demanding task in the computer tomograph with excellence and, with several thousand units in use, has proven itself in practice for many years. 

Polished precision

At the heart of the compact design with dimensions of 140 mm in length, 80 mm in width and 25 mm in height is the ground ball screw. Together with a stepper motor, it forms the drive system of the adjustment unit. "Only with a ground ball screw do we achieve the desired values of accuracy, reliability and rigidity," clarifies Elger Matthes, adding, "Although it has a proud price, it is nevertheless more favorable in total than a compromise solution consisting of several parts." The durable precision ball screws can be adjusted extremely sensitively and ensure the high positioning accuracy of the diaphragm actuator (unidirectional = ± 1.4 µm, bidirectional unidirectional = ± 2.5 µm). An advantage here is that the Saxon positioning specialists have a direct line to the leading manufacturer of high-precision ball screws in the form of Steinmeyer in Albstadt (Baden-Württemberg), which is also part of the Steinmeyer Group.

And what spoke in favor of the stepper motor? "We needed a high torque, extreme reliability and a low price - the stepper motor is simply unbeatable," Matthes explains. Stepper motors can be operated accurately even without sensors for position feedback. However, to increase operational and patient safety, the open-loop mode was dispensed with and an incremental measurement system was implemented with an additional encoder. This detects the rotary motion of the motor and thus monitors the degree of opening and the position of the diaphragms. Another plus: Thanks to the encoder, the stepper motor can be switched to servo mode and thus achieve higher speeds and accelerations.

Demanding series production for medical technology

Through constant adjustments and optimizations in the current series, the orifice plate actuator has been continuously improved over many years and adapted to changing customer requirements. For example, the speed requirements alone - and thus the measuring or scanning time - have long since doubled. In the meantime, several products have emerged from the original one, for example for higher loads or different traverse paths. This high degree of flexibility is no coincidence. From development to production and assembly to the test department, the entire process takes place under one roof. "This gives us the opportunity to respond to customer requests in an uncomplicated manner and to implement individually tailored solutions within the shortest possible time," Matthes emphasizes. "With us, customers get both the core component - namely the screw drive - and the design and series delivery from a single source. This clearly sets us apart from the competition." At the same time, maintaining high quality in the series business with absolute reliability of function is guaranteed even with thousands of units delivered.

Reliable, durable, highly precise and surprisingly simple in design: The aperture actuator for computer tomography exemplifies Steinmeyer Mechatronik's high solution competence and industry-specific capabilities in the development of medical positioning systems and demonstrates: Less can really be more. After all, simplicity generates robustness and thus reliability.

 

Author: Christoph John, Design Manager, Steinmeyer Mechatronik