Delft University of Technology Uses HBM Force Sensor for Scientific Research on Friction

Research results can lead to new technology for form guidance of scopes

The Biomechanical Engineering research group at the Delft University of Technology has opted for an HBM force sensor in their research on friction between steel cables and various types of rubber. At first glance, this may seem like a marginal area of research, but the findings may lead to new technologies for form guidance of flexible endoscopes (long, flexible hoses with a camera on the end), possibly resulting in considerable savings in the medical sector. Over the short term, it may be possible to go ahead and implement the technology in defense and police applications for the detection of explosives, for instance, and peering into enclosed spaces.

Force transducers from HBM

The research on friction between steel cables and rubber was part of a PhD thesis study by Arjo Loeve, who is now a post-doctoral researcher. This study was geared towards the development of new technology for form guidance of scopes, to be used for medical examinations in the human body. During insertion of flexible endoscopes, undesired bends sometimes occur in the endoscope hose, which may cause issues such as painful stretching in the patient's intestine. This may even give rise to adverse complications, such as perforation of the intestine during a colonoscopy. Some manufacturers have actually introduced technologies for form guidance onto the market, but their functioning is not yet optimal and they are extremely expensive. Most endoscopes do not use form guidance yet, and only offer limited options to fix the shape of the hose.

Close up of the clamp module. In the foreground blocks, which are covered with rubber or steel cables.

Faster and cheaper - new technology for the challenges of healthcare

The new technology, which Loeve researched, consists of an inflatable rubber hose surrounded by a ring of steel wire inside a stainless steel spring. By using a liquid to put the rubber tube under tension, the tube clamps the steel wire against the inside of the spring and makes the endoscope guidance rigid. This way, the physician can complete the work more quickly, the patient experiences less discomfort from the examination and the risk of complications is greatly reduced.

Endoscopic examinations often have to be interrupted on account of pain or complications, which means not all of the anomalies in the patient are detected, sometimes requiring additional examinations with costly and time-consuming CAT scans in order to arrive at the right diagnosis.

This new technique should enable an increase in the rate of problem-free completion of internal exams. In contrast to other possible techniques for form guidance and fixation, the technique studied at Delft does not require an expensive control unit, which keeps endoscopic systems affordable. Thanks to the new technology, operation has also been greatly simplified, so endoscopic exams do not necessarily have to be performed by expensive specialists.

Therefore, this new technology potentially offers a perfect solution to the challenges of healthcare: efficient treatment methods that reduce hospital stays and, most importantly, provide considerable savings. This new technology may create a strong impetus for the introduction of minimally invasive technologies, endoscopic surgical techniques and incision-free surgery.

First research on friction between steel cables and rubber

The study conducted by the Mechanical Engineering research group was unusual. In the study of friction, or "tribology", a great deal of fundamental research is conducted on friction between different types of materials. “But not much research has been done yet on friction between steel cables and rubber,” explains Loeve. “Rubber is soft and has a tendency to adhere to other materials at the molecular level, which causes a lot of friction.

However, the most similar study previously conducted was aimed at reducing friction for applications such as conveyor belts or prevention of wear. For applications in scopes, you actually want to maximize the friction, in order to get as much rigidity as possible in the guidance. Also, most rubber types are not very popular in the medical sector because they are not biocompatible, they sometimes release poisons or carcinogens and have a low resistance to sterilization. In the application of the new technology for scopes, the rubber does not come into contact with the human body because it is located inside the mechanism.

Close up of the clamp module. Two rubber-clad blocks between which a block, wrapped with steel wire was pulled.

Which material combination provides most friction?

The study examined three types of rubber and five different types of braided steel cables in fifteen different combinations. The objective was to determine what type of rubber and what type of cable provided the most friction. The cables were standardly braided steel cables with thicknesses from 0.18 to 0.45 millimeters that are used in everyday applications for hanging picture frames or as bicycle brake cables. 

For the test, a setup was developed by the team. It consisted of a clamping module with two rubber-clad blocks, with a third block wrapped in steel wire pulled between these two. Measurements were taken of both the clamping force of the clamping module and the tensile force of the block with the steel wire. The setup needed a suitable force sensor to measure the force exerted on the block with the steel cable. This ultimately enabled the friction to be mapped out. The force sensor had to meet specific requirements related to measurement range and accuracy.

The specifications were drawn up at what is known as the "Meetshop" (Measurement Shop) in the Faculty of Mechanical, Maritime and Materials Engineering, where students and doctoral candidates can request support with measurement applications. Because the Meetshop does a lot of business with HBM, and uses an MGCplus data acquisition system by HBM for instance, it came as no surprise that HBM was selected to supply a force transducer. The force transducer was exposed to weights of up to 180 kg. Incidentally, the force sensor was not only used for the tests with the clamping module, but also for calibration of the tensile module.

“Including development of the test setup and clamping modules, the study lasted a year and a half and has produced a treasure trove of useful data,” adds Arjo Loeve. “After publication of the study in WEAR, the scientific journal for tribology, and in IEEE Transactions on Biomedical Engineering, the basic principle of the technology attracted the interest of other organizations. The stiffening principle also appears to be applicable for the stiffening of controllable forceps during operations. The friction data from the measurements have proven their worth in many areas where steel cables come into contact with rubber parts. Even the army and the police are interested in testing the technology for stiffening hoses in the field. Some other examples of potential applications include detection of explosives and booby traps and visual inspection of enclosed spaces.

MGCplus data acquisition system
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