The manufacturing of pharmaceutical tablets is carried out using machines that appear, at first sight, to be very simple. First, a die is filled with the correct amount of powder. Next, two opposing punches compress the powder to form the tablet. Finally, the tablet is ejected from the top of the die. In a typical manufacturing machine, rotating mechanical cams control the movement of the punches.
The apparent simplicity of the manufacturing process hides extensive development that pharmaceutical companies undertake for each different formulation to ensure the tablet is properly formed, the active ingredients are not denatured, the tablet does not disintegrate between formation and ingestion, and that it dissolves or breaks up at the correct time when swallowed. To fulfill all of these requirements, the principal characteristics of the manufacturing process must be established: the speed at which the tablet should be formed, the cam profiles used to drive the punches, and the minimum distance between the punch faces (which determines the thickness of the tablet). To do all of this, pharmaceutical companies use a compaction simulator.
The Compaction Simulator
In the compaction simulator, high-performance servo-hydraulic rams, instead of mechanical cams, drive the punches. With this arrangement, engineers can quickly evaluate different punch movement profiles, speeds, and approach distances to easily adjust all of these parameters using the simulator’s operating software. To fully automate the simulator and, therefore, generate a number of tablets over a range of conditions, pneumatic and stepper-motor-driven subsystems control the operation of powder-filled hoppers, a tablet removal arm, a tablet storage magazine, and other items. The simulator must be fully instrumented so position, force, and temperature profiles may be recorded for the formation of each tablet.
The servo-hydraulic performance of the simulator is critical to the successful operation of the machine. Tablet manufacturing machines operate at a high speed. For example, a typical upper punch profile might require the punch to descend 15 mm in 15 ms, then immediately reverse back to the starting position at the same speed. This profile must be well controlled even though the punch load increases rapidly when the punch hits the powder. For this reason, engineers use very-low-friction hydraulic rams and high-performance servo valves (~1 ms time constant).
The Machine Control System
Product Technology Partners (PTP) of Cambridge, UK, developed an original control, data acquisition, and analysis software for a compaction simulator. The control and data acquisition system for the machine is complex, consisting of the following main items:
- A special-purpose, high-performance, two-axis servo-hydraulic controller for the punches.
- A special-purpose, three-axis stepper motor controller for control of the powder hopper, tablet collection arm, tablet magazine, and various pneumatic actuators.
- An industrial PC providing supervisory control, coordination, data acquisition, multiple control functions, and the human machine interface (HMI).
Previously, to achieve the required machine control, engineers heavily interlinked the control system in a rather complex arrangement. It therefore suffered from the following deficiencies in both performance and maintainability: the lack of flexibility in the servo-hydraulic controller limited performance; the data acquisition subsystem and the controllers had to be separately calibrated; the controller structure required extensive wiring and complex application software that is costly to develop and maintain; the PC was required to undertake real-time control and synchronization functions for which it is not well suited; and the control system hardware is costly.
A New Approach to Control
Having established the deficiencies of the existing control system, PTP was asked to simplify the system using different hardware. The company conducted a study to establish an alternative controller design. Starting from a blank sheet, it quickly became apparent that PTP could effectively address these identified deficiencies with a system based on a National Instruments (Austin, TX) NI PXI controller running LabVIEW RealTime graphical development software, combined with a host PC to provide the HMI. The new system offered the following benefits:
- Precise control of the punches using an NI R Series PXI module programmed with LabVIEW FPGA technology for advanced, nonlinear control algorithms.
- Hardware reuse by using the same R Series module for data acquisition control and avoiding the requirement for separate calibration.
- Containment of the complete controller within a single LabVIEW RealTime system to dramatically reduce both wiring and application software complexity.
- All real-time tasks were undertaken within a real-time environment.
- A significant cost saving was realized with the new hardware configuration versus the old one.
The I/O configuration of our PXI controller includes the NI PXI-7833R R Series module for servo-hydraulic control and fast data acquisition; the NI PXI-6123 S Series data acquisition module for additional fast data acquisition; the NI PXI-6220 M Series data acquisition module for auxiliary data acquisition; the NI PXI-7334 stepper motor controller for hopper, tablet removal, and tablet magazine control; two NI PXI-6515 digital I/O modules for pneumatic and safety systems control and monitoring; and the NI PXI-8420/8 RS232 S Series interface for communication with optional machine subsystems such as the die heater controller.
The change in controller required redevelopment of the machine operating software. However, it was calculated that the cost of redevelopment would be recovered from the reduction in hardware cost with the sale of only two machines.
Servo-Hydraulic Control Performance
Prior to implementation of the revised controller, the machine manufacturer required assurance that the R Series module, programmed with LabVIEW FPGA, was capable of at least matching the performance of the existing, special-purpose controller.
To provide this assurance, PTP constructed a prototype to control the punches of an existing machine using a PXI-7833R module. A simple controller was developed in LabVIEW based on PID control. It was combined with static compensation for the nonlinear servo-valve characteristic and gain scheduling based on the pressure difference across the valve. It took only about 2.5 man-days to develop the prototype and the trial software. This fact reflects the simplicity of the LabVIEW FPGA approach to application development. However, it must be noted that significant care had to be taken with the algorithm development because LabVIEW FPGA supports only integer and fixed-point arithmetic.
Comparison measurements were then conducted using both the experimental controller prototype and the existing controller to perform V-profiles. Profiles were executed at several different speeds to form tablets using different maximum punch loads.
From a comparison of the error span results, it was shown that, with only two days of software development effort, the performance of the LabVIEW FPGA test controller prototype exceeded that of the existing controller when under load. Under no-load conditions, the demonstrated performance was not quite as good, but this could be improved through the application of a more sophisticated control algorithm.
A complex R&D machine such as a compaction simulator places many demands on its control system, including precision servo-hydraulic control; high-speed data acquisition; and control, sequencing, and monitoring of the auxiliary subsystems. In comparison to a conventional control hardware design, which combines special-purpose controllers and a PC in a real-time role, PTP demonstrated that an integrated controller embracing PXI, LabVIEW RealTime, and LabVIEW FPGA technologies can have significant benefits.
This article was written by Dr. Martin Saxon of Product Technology Partners Ltd. For more information on the NI products described here, visit http://info.hotims.com/34457-321.