Manufacturers - 2

[Previous Page] [Table of Contents] [Next Page]

---

reprogrammed by electronically erasing the memory on the chip. ROM is normally used to control processes where the options of the customer or operator do not need to be changed.

Random Access Memory (RAM) using battery backed volatile memory components is another type of memory component. This memory requires a power supply but lends itself to modification and reprogramming. Advanced microprocessor or computer systems would normally use a combination of ROM and RAM to program control of retort functions.

A more advanced system would use a programmable logic controller (PLC) which would allow the operator or firm to alter the control limits of the controller. This type of controller would use algorithms (a programmed procedure for solving a problem) to control the loop. Algorithms are written to provide the microprocessor with a logical sequence of events for solving a problem.

Control of multiple parameters such as temperature, pressure, rotation, etc may be controlled by installation of several loop controllers controlled by one PLC, microprocessor or computer.

With the advancement of technology, defining the differences between microprocessors, minicomputers and mainframe computers has become more difficult. Size is usually the distinguishing factor. The size of the computer needed for control depends upon the number of loops to be controlled and whether the system is set up as an independent (each retort control system is self standing), centralized (one computer controls all retorts), or a distributed system (two or more computers talking to each other).

In the independent system each retort would be controlled by its own PLC or microprocessor. If one of the retort control systems failed the remainder of the systems would continue to operate.

In a centralized system all data would be collected and analyzed by a central computer. This provides for quick capture of all processing information and for control from a central location. Failure of this control system would mean that all processing systems would be down.

In the distributed system a PLC or microprocessor can be used for independent control of each retort system. The retort microprocessor is then used to supply information to a systems computer which captures all retort data for storage and printing. The systems computer in turn is used to store process programs and to program the logic controls of the microprocessor.

Computer controlled retort systems may be marketed with one or more peripherals added to the basic PLC or microprocessor (Attachment 3). Peripherals may include: keyboard or key pads for programming and entering information, printers for printing stored and captured in-process information, personnel computers for programming the PLC, CRT (touch screens) for monitoring the process and entering information, and modems which allow for connections to remote computers for programming or trouble shooting.

During inspections of lacf manufacturers using computers to control thermal processes or to control other factors critical to the process the investigator must determine the functions of the computer system.

In many cases lacf manufacturing firms will not have on hand detailed information covering the development and validation of the software and microprocessors used on their retort systems. Many firms buy the microprocessors as black boxes from the equipment vendor. The investigator must then determine the functions of the control system in as much detail as possible. It is important to remember that computer controlled retort systems must function to provide for control of the thermal process, the same as any other type of control system. The mercury-in-glass thermometer is still the reference thermometer as per the lacf regulations. The operator is required to read and record mercury-in-glass thermometer readings the same as for any other control system (these may be entered into a computer record). Each retort system is still required to be equipped with a temperature recorder as per the lacf regulations. This recorder must provide a continuous record of the thermal process; must meet the specifications of the lacf regulations; and must not record a temperature higher than the MIG thermometer reading. It is important to determine that the system is controlling the retort to meet the requirements of the firms filed scheduled processes for time and temperature; that critical factors such as rpm's in agitating retorts and come up time in water immersion retorts are being controlled; and that vent times and temperatures for steam retorts are being adequately controlled. If the firm has a schematic drawing of the control system this should be obtained or the investigator may prepare a simplified schematic drawing, which will be helpful in explaining the system operations. If the firm does not have detailed information on the microprocessor control system the investigator should obtain any limited information that is available. At a minimum this would include the vendor and model numbers of the computer control system and the functions performed by the computer control system. Observation of the system as it operates can be used to determine if critical factors such as RPM, vent times, temperatures, pressures, and thermal process times are being adequately

[Previous Page] [Table of Contents] [Next Page]

Return to: Page Top | Inspection Start