Speed (manual setting, or volts x 10)
3.2 DESIGN AND DEVELOPMENT OF A PC-BASED FERMENTOR DATA ACQUISITION AND CONTROL SYSTEM
3.2.1 PC-based data acquisition
This section describes the justification for a standalone PC-based data acquisition and control system for the fermentor described in Section 3.1, and its design and development.
Data from fermentations needs to be acquired from two sources, the gas analyzer, and the analog sensors. Gas analyses are made by a mass spectrometer (VG model MM80, VG Gas Analysis Ltd., Cheshire, UK). The results of analyses are stored on a dedicated IBM-compatible PC. The data can be picked up asynchronously via an RS-232 serial communications port. Analog fermentation measurements are digitised and stored on an 8-loop controller (Model 6358, TCS Ltd., Worthing, UK) and 32- channel signal processor (Model 6432, TCS Ltd., Worthing, UK). These units are on a multi-drop bus (RS422) and hence can both be addressed via the same serial transmissions cable.
At the start of this project, data acquisition from the TCS units attached to all UCL fermentors was managed by a multi-user software package (BlO-i, BCS Ltd., Maidenhead, UK) running on a micro-computer (PDF 11, Digital Equipment Corp., USA). A single monitoring rate could be specified, which then applied to all fermentors equally. Clearly, it would be desirable to vary the monitoring rate according to the duration of the fermentation, and the type of data being acquired. The software had a low reliability, and frequently ’crashed’, causing data losses during all of the first six fermentations run relating to this project. In addition, there was no facility for closed-loop control of fermentation variables, nor for the on-line calculation of derived variables. The setting of open-loop control strategies was laborious, and could only be effected via expensive TCS hardware. The set-up could not be modified without the assitance of the system manager, who often required the assistance of BCS Ltd. Data acquired from fermentations was not stored as ASCII
characters, but in a relational database format. The information was difficult to retrieve, and sometimes required screen dumping and capture by a spreadsheet package. There were typically many errors in retrieved data (Figure 12), as well as lines of text that had to be edited out. This software was therefore deemed not suitable for a project dedicated to improved fermentation monitoring and control.
In specifying an alternative data acquisition system, it was felt that the most important requirement was for a system independent of the existing software and hardware (Thornhill and Royce, 1989). This would allow the monitoring and control team to test strategies without adversely affecting the rest of the Department’s research effort. The most cost effective way of achieving such independence was considered to be by PC-based data acquisition. The PC chosen was an IBM compatible PC with an AT-bus, already existing in the Department (Model Vaxmate, Digital Equipment Corp., California, USA), as most of the Department’s PC-based data analysis is on IBM compatibles. It was also felt that, as IBM compatibles represent 80% of the market in PCs, the choice of data acquisition software and hardware would be wider, its quality greater, and its price less than for other PCs. The choice of an IBM-compatible machine with an AT rather than MicroChannel bus halves the cost of add-on boards.
In terms of software for data acquisition, the requirements were for a system capable of data storage in ASCII or Lotus-123 format, and for closed-loop control capability. The available systems fell into two groups, namely those designed for laboratory environments, and those intended for industrial production. The industrial systems were much more expensive than the laboratory systems, as can be seen in Table 8 , and so were not considered. Data acquisition toolkits were not considered, as quick implementation that avoided any "custom" software elements was desired. Any of the remaining systems (eg. Notebook, Measure, LabWindows) would have been suitable. Notebook was chosen as it is produced by a manufacturer specializing in software, rather than as an off-shoot to a hardware company s’ activity, which would
0 . 0 5 0 6 7 . 0 1 0 8 6 . 7 0 0 5 0 0 . 0 0 0 0 . 0 0 0 7 . 7 3 0 0 . 0 6 3 9 7 . 0 2 0 8 6 . 7 0 0 5 0 0 . 0 0 0 - 9 9 9 9 . 0 0 0 7 . 7 3 0 0 . 0 8 0 3 7 . 0 1 0 8 6 . 6 0 0 - 9 9 9 9 . 0 0 0 0 . 0 0 0 7 . 7 3 0 0 . 1 1 3 6 7 . 0 1 0 8 6 . 5 0 0 5 0 0 . 0 0 0 0 . 0 0 0 7 . 7 4 0 0 . 1 3 0 0 - 9 9 9 9 . 0 0 0 8 6 . 5 0 0 5 0 0 . 0 0 0 0 . 0 0 0 7 . 7 3 0 0 . 1 4 6 9 7 . 0 1 0 8 6 . 4 0 0 5 0 0 . 0 0 0 0 . 0 0 0 7 . 7 3 0 0 . 1 6 3 3 7 . 0 1 0 8 6 . 4 0 0 5 0 0 . 0 0 0 0 . 0 0 0 - 9 9 9 9 . 0 0 0 0 . 1 8 0 0 7 . 0 1 0 1 0 0 . 8 0 0 5 0 0 . 0 0 0 1 0 . 2 0 0 7 . 7 4 0 0 . 1 9 6 7 7 . 0 1 0 1 0 8 . 3 0 0 5 0 0 . 0 0 0 - 9 9 9 9 . 0 0 0 7 . 7 2 0 0 . 2 1 3 3 7 . 0 2 0 1 0 1 . 4 0 0 5 0 0 . 0 0 0 5 . 2 0 0 7 . 7 3 0 0 . 2 3 0 0 7 . 0 2 0 1 0 0 . 3 0 0 - 9 9 9 9 . 0 0 0 5 . 0 0 0 7 . 8 7 0 0 . 2 4 6 7 7 . 0 3 0 1 0 0 . 1 0 0 5 0 0 . 0 0 0 5 . 0 0 0 7 . 8 8 0 0 . 2 8 0 0 7 . 0 2 0 1 0 0 . 0 0 0 5 0 0 . 0 0 0 5 . 0 0 0 6 . 9 4 0 0 . 2 9 6 7 - 9 9 9 9 . 0 0 0 1 0 0 . 0 0 0 5 0 0 . 0 0 0 5 . 0 0 0 6 . 9 0 0 0 . 3 1 3 3 7 . 0 3 0 1 0 0 . 0 0 0 5 0 0 . 0 0 0 5 . 0 0 0 - 9 9 9 9 . 0 0 0 0 . 3 3 0 0 7 . 0 3 0 1 00 . 100 5 0 0 . 0 0 0 5 . 0 0 0 8 . 0 2 0 0 . 3 4 6 7 7 . 0 3 0 1 0 0 . 1 0 0 5 0 0 . 0 0 0 5 . 0 0 0 8 . 1 3 0 0 . 3 6 3 3 7 . 0 3 0 1 0 0 . 1 0 0 5 0 0 . 0 0 0 - 9 9 9 9 . 0 0 0 8 . 1 6 0 0 . 3 8 0 0 7 . 0 3 0 1 0 0 . 1 0 0 5 0 0 . 0 0 0 5 . 0 0 0 8 . 2 2 0 0 . 3 9 6 7 7 . 0 2 0 1 0 0 . 1 0 0 - 9 9 9 9 . 0 0 0 5 . 0 0 0 7 . 0 3 0 0 . 4 3 0 0 7 . 0 3 0 100 . 100 4 8 6 . 0 0 0 5 . 0 0 0 6 . 9 9 0 0 . 4 4 6 7 - 9 9 9 9 . 0 0 0 9 9 . 1 0 0 1 2 5 . 0 0 0 5 . 0 0 0 7 . 0 1 0 0 . 4 6 3 3 7 J33C 9 8 . 5 0 0 1 2 5 . 0 0 0 4 . 900 7 . 0 1 0 0 . 4 8 0 3 7 . 0 3 0 9 7 . 9 0 0 1 2 5 . 0 0 0 5 . 0 0 0 - 9 9 9 9 . 0 0 0 0 . 4 9 6 7 7 . 0 3 0 9 7 . 5 0 0 1 2 4 . 0 0 0 5 . 0 0 0 7 . 0 1 0
F igure 12: A sam ple o f data retrieved from Bio-i in 1989 by screen dum ping, after rem oval o f lines o f text. The -9999.000 entries indicate failed data acquisition.
PACKAGE NAME AND SUPPLIER COMMENTS COST (1989 prices) LT/Control, Laboratory Technologies, Wilmington, USA Industrial data acquisition; £2750 CAMM Technology, Worthing, UK Industrial data acquisition; £3250
Paragon 500, In tec Controls Corp., Chichester, UK Industrial data acquisition; £3500 Tactician, TCS Ltd, Worthing, UK Industrial data acquisition; LT/Notebook, Laboratory Technologies, Wilmington, USA
Supports many different suppliers hardware platforms;
£ 895
Asyst, Keithley Instruments, Reading, UK
Drives Metrabyte hardware only;
£1670 Lab WINDOWS, National
Instruments, Austin, USA
Emphasizes interfacing over data analysis;
£ 349 Measure, National
Instruments, Austin, USA
Program calls not supported; recent change of ownership;
£ 395
Table 8 : Commercial software packages considered for use for the monitoring and control of the 42L fermentor
increase the availability of supported hardware. It was also well reviewed in the literature (eg. Tinham, 1989).
A block diagram of the data acquisition system that was set up is shown in Figure 13. Labtech Notebook is able to acquire data serially via an RS-232 port. To communicate with the TCS units, it was necessary to install a 422/232 converter, already existing in the Department. Notebook does not support process control via a serial port, but only by analog output. For control of flowrates (of acid/base and substrate) this was not a disadvantage, as automatic control of the lOlU/R pumps is in any case by an analog signal. A 6 -channel analog output board was specified (Model DDA06, Metrabyte Corp, USA, supplied by Keithley Instruments, Reading, UK). A four serial-port board (Model Async 4-II, Accent Computers Ltd., Burgess Hill, UK) was also specified, to add to the single RS-232 port that is standard on the Vaxmate PC.
3.2.2 Data Analysis
In this work. Notebook was directed to store data in files in ASCII format. The files were copied onto 5M” floppy disks. Off-line data analysis was performed on a PC (Model 310, Dell Ltd., USA). The data analysis packages used were Lotus 123 (Version 3.0, Lotus Development Corp., Cambridge, USA) and Freelance Plus (Release 3.0, Lotus Development Corp., Cambridge, USA). Where sophisticated mathematical analyses were required, a library of mathematical routines (Numerical Recipes C library. Numerical Recipes Software, Cambridge, USA) was used. These routines were applied using the Quick C and C5.1 compilers (Microsoft Corp., California, USA).
RS422 RS232 ] C0M1