Embedded Controller CPU inside Evaporative Light Scattering Detector
Low-cost PC-based controller runs Chromatography instrument

Chromatography is a separation method used in the analysis of chemical substances including food, plastics and many commercial drugs. It relies on a two-stage process: an initial partitioning phase to separate the components in a mixture, followed by quantitative detection of these components by measurement of one or more physical properties. Many types of detector are available, including ultra-violet absorption, refractive index, and viscometry. The choice of detection method is dictated by the substance under analysis - ideally there should be a large concentration-dependent change in the chosen parameter. Problems can arise in the analysis of unknown substances and with ‘difficult’ samples which do not give useable responses with conventional detectors. Enter the new PL-ELS 1000 detector manufactured by Polymer Laboratories (PL) who are based in Church Stretton, Shropshire, UK. Polymer Laboratories have been involved in the chromatography market since 1976. The company was founded to develop techniques and instrumentation for the characterisation of polymer systems, and to develop high technology polymer products for use in chromatography, as well as diagnostic and pharmaceutical applications.

A new and dynamic part of their product range includes the next generation of detector – the economically priced PL-ELS 1000. This machine uses a technology called Evaporative Light Scattering Detection (ELSD) which has been developed to new heights by Polymer Labs.

The PL-ELS 1000 is the size of a PC mini tower case and offers a highly reproducable, high sensitivity method of analysis via simple interfaces; either front panel or PC-based. The method of operation is simple. The medium (eluent) which carries the chemical sample is fed into the side of the unit and then into an innovative high-efficiency nebuliser device where a small quantity of compressed nitrogen is used to produce a finely divided droplet spray. This passes through a pre-heated chamber where the eluent droplets are evaporated completely. Waste eluent and exhaust gases are expelled from the detector.

Solutes less volatile than the eluent create a particle stream which intersects a collimated light beam. The scattered light is detected by a low-noise, high-sensitivity photodetector. The output of the photodetector is proportional to the amount of solute present over a wide concentration range.

At the centre of the PL-ELS 1000 is a combination of computer based control equipment. A single board I/O sub-system, designed by PL, takes care of physical connection to all devices in the instrument - such as display, gas flow controller, temperature monitors and heaters. A secondary peripheral interface controller (PIC) is used for monitoring the numerous safety systems and interfacing to the infra-red I/O circuitry required to communicate with the detachable remote handset.

The CPU core of the instrument is an embedded controller board supplied by Arcom Control Systems. A low-cost single board micro-controller which features a 25MHz Intel 80188EB processor, on-board digital I/O, a PC/104 expansion interface, and two serial ports. Flash and SRAM memory devices are also on-board.

This single board controller is used for the main CPU activity of the machine. It runs all the software required to control the instrument, to interface with users via serial and IR links and also to drive the two front panel alpha-numeric displays which report both exhaust temperature and output signal voltage.

The Arcom controller board has 24 lines of digital I/O. These channels are used for the high-level control of devices around the unit, including the displays and the photodetector module. Also controlled from the digital I/O ports are a set of outputs on a standard ‘D’ type connector on the rear panel which are used for safety control of external equipment, typically pumps which are pumping solvents into the PL-ELS 1000.

An on-board PC/104 interface is passed to PL’s custom I/O board via a ribbon-cable, which gives the full features of a standard PCbus architecture. This bus is used to interface with expansion I/O, non-volatile memory, multi-channel ADC and DAC devices and to communicate with the PIC.

Of the two serial ports available, a single serial channel is routed from the 80188EB board to the back panel of the unit to provide a standard RS-232 port used for interfacing the PL-ELS 1000 to innovative Windows-based control and analysis software.

When choosing a base for the control CPU, Paul Claes, Instrument Development Engineer with Polymer Laboratories, had already decided to opt for single board technology. Advantages include size and packaging, which would fit with the instrument’s space availability and, more importantly, EMC requirements. Also using an assured ISO9001-certified supplier such as Arcom with a commitment to long-term 5 year availability was very important. After initial discussions with Arcom, Paul chose Arcom’s TARGET188EB. Deliberately designed around the embedded micro-controller Arcom were able to offer PL a development environment which would suit Paul’s familiarity with ‘C’ programming, in addition to a C-based remote debugging tool. The combination of all of these offered a cost-effective PC-based solution, reducing development time, and in turn, brought his product to market within PL’s time-scales.

Using Borland ‘C’ programming tools running on a desktop PC, PL’s R&D team was able to develop all the required embedded software. Arcom’s supplied library functions proved to be very useful, and fragments of existing ‘C’ code could be incorporated. With Arcom’s proprietary SourceVIEW product, live source code was downloaded (via a serial comms link) to the target development system and run. Using Borland’s remote debugger tool, any problems or tweaks with code were ironed out prior to assembling the final executable code which is stored in Flash EPROM on the TARGET188EB. Any development difficulties with code and hardware implementation were teased out with Arcom’s team of technical support engineers based in Cambridge.

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