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Servo drive controls diagnostic instrument

Servo drive controls diagnostic instrument

Scientists and researchers at the University of Liverpool have been working to develop a portable prototype diagnostic instrument that will perform a straightforward, quick, and robust test for detecting sepsis in patients from a small sample of blood. Sepsis is a bacterial infection of the blood, causing potentially fatal whole-body inflammation that injures tissue and organs.

The condition may be overcome if treated early with antibiotics and intravenous fluids, but current tests can take up to two days to identify the condition, by which time a treatment regime can be ineffective. Scientists at the University have a successful method of detecting the infection at its early stages and are translating this test into a point-of-care device for improved ease and speed of diagnosis.  

It is envisaged that rapid diagnosis can be a reality in the future to combat the condition which annually affects 18 million people worldwide and reportedly costs the NHS over £2 billion. The sepsis instrument and test, which has been protected via several patent applications, is a fairly simple application that could be described as a textbook example where motion control is used as an enabling technology to facilitate amazing science.

With the small blood sample loaded into a specially developed multi layered disc which is about the size of a CD, the single axis machine combines high speed controlled 'spinning' of the disc to centrifugally separate the blood plasma and then performs precision indexing to rotate it through one or two revolutions to various positions where chambers containing biomarked and unmarked 'control' plasma samples are presented for microfluidic examination. Simply put, the difference between the sample and control determines the result. The disposable discs would be manufactured relatively inexpensively making the whole test very economic.

Initially, a very brief specification was delivered to engineers at Mclennan and as with many research projects there was sparse information about load characteristics and even some secrecy about the application as well as budget restrictions. Dr Dominic Banks, the research fellow working on the project, based at the University of Liverpool's Institute of Infection and Global Health recalls: "We had a basic outline of the functions we needed, and more than anything required the flexibility in the motion controller to develop the project as we progressed along the development path."

To meet the budget and prove the process for the first prototype machine Mclennan supplied an Applied Motion Products (AMP) ST series open loop stepper motor drive. From the control requirements given at the time, it was clear that an intelligent drive with I/O and maths capability could possibly take on most of the machine functions. As the ST series stepper drive essentially uses the same programming and has similar functionality to AMP's servo range, it was an easy transition when the instrument was further developed and servo motor control with high resolution position feedback was specified to meet more exacting positional requirements as well as provide the compactness and low audible noise levels that were needed for the envisioned point-of-care bedside version.

Dr Banks had an eye on the servo route from the outset. "We were most interested to learn that the AMP stepper and servo versions were scalable - this was a deciding factor to initially go with Mclennan so as to make the transition more seamless when funding was increased."

AMP SV7 series intelligent drives combine many progressive features for digital servo control including advanced current control and Ethernet communications that proved useful for programming and developing the application. Programming was taken care with AMP's Q Programmer language - a multi-tasking BASIC-like language with stored program execution, maths functions and conditional processing. A simple LCD display and pushbutton panel connected to the drive is used to initiate and run the SV7 program that includes data registers along with I/O for various functions including interaction with the microfluidic examination process developed by the University. Sample processing is completed automatically.

In the latest prototype version these same components are included in a miniaturised model developed by the Liverpool team that demonstrates how the machine may look in full production. "Mclennan has worked very closely with us from the outset and provided vital information and support for the instruments motion control functionality which will prove invaluable going forward", says Banks.

The project if continued would of course be cost reduced, and for Mclennan it would be unlikely that the AMP SV7 intelligent drive, servomotor and encoder would be selected but through the capability to design and supply customised mechatronic solutions the Hampshire based motion specialist may well play a role in the sepsis instrument's future.

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