Motion control for the University of Liverpool’s ‘bedside’ diagnostic instrument for sepsis

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 £2billion. The project was funded by the National Institute for Health Research (NIHR) under the  Invention for Innovation (i4i) programme (II-LS-1010-10045) which provides guided progression of innovative medical product prototypes via commercial development; aiming for eventual use in the clinical environment for the benefit of patients in the NHS.

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 a very economic as well as effective solution.

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 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 exclusive UK distributor for Applied Motion Products (AMP), its ST series intelligent microstepping drives proved to adequately provide the instrument control requirements. 

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^TM’ 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 with the results displayed on screen. It would be possible to output this information to a PC for further use.

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.

Graham Wingate, Mclennan’s Sales Director summarises, “Research based applications like the sepsis instrument may not yield generous income – we rely on our volume OEMs and end-user business for that - but they do give us a great deal of satisfaction and experience; and leave the door open for future business as university led projects progress through potential technology transfer initiatives.”