Malcolm Barnett looks at a low cost microcontroller with integrated analogue to digital converters which can be used in scientific and industrial applications.

The majority of X-ray tubes in use in scientific and industrial applications use filaments that are floating at the high voltage. This is often in the range of 50kV to 160kV and may even be up to 300kV from ground.
The filament is used to control the beam current taken by the X-ray tube and hence the quantity of X-rays produced. The hotter the filament the greater the beam current the more
x-rays. The filament is similar to an electric light bulb and must be run at a very high temperature before any emission is produced.
Careful control of the power in the filament is desirable, as it is usually running at near to is maximum rating. Running too hot reduces the life of the filament and hence the X-ray tube. With tubes costing between E5000 and E30 000 it is worth taking special care of the filament.
Traditional methods of powering and controlling filaments use isolation transformers driven from a variable supply at ground potential and then rely on measuring the beam current to provide the control signal for the filament. The problem with this approach is that accurate measurements of the filament voltage and current are difficult to obtain, as these parameters can only be measured at the high voltage. Filament protection systems based on measurements made at the grounded end are often used but are only partially effective due to their lack of accuracy.
With the advent of low cost microcontrollers with integrated analogue to digital converters (ADCs) and memory it is now possible to achieve accurate control and monitoring of the floating filament parameters.
The system developed by HiTek Power for its new X-ray generator products uses two PIC microcontrollers, one at ground and the other floating at the high voltage. Communication between these is via a pair of transmit/receive fibre optics to give the necessary high voltage isolation. This serial link employs a full duplex, asynchronous, communications protocol running at over 600kbits per second. This high data rate allows the measured parameters to be used within the control loop for the filament, as this loop is comparatively slow due to the thermal time constant of the filament itself.
At the ground end, the PIC takes the output of the beam current control amplifier and uses this as the filament current demand signal via its internal ADC. This is then processed and transmitted to the floating PIC where it is sent to the digital to analogue converter (DAC) and then to the filament current source, which generates the demanded current to power the filament. The voltage and current at the filament output of the generator are measured directly and fed to the multiplexed ADC in the floating PIC, which then transmits these back to the grounded PIC. These values are made available digitally to the generator control system and can be passed to the X-ray control system. The filament current is converted back to an analogue signal via the DAC connected to the grounded PIC and is used to provide variable current limit and standby filament current functions in conjunction with the beam current control system.
This technique provides advantages over traditional methods. The ADCs and DACs are all 12 bit, so measurements and control accuracy are better than one part in 4000. Filaments can be run at the exact value required with very close current limit to protect against over dissipation and possible damage to the X-ray tube. The availability of an accurate filament monitor at ground facilitates the stabilisation of the beam current loop so that adjustments for different tube types is not normally required. The implementation of a standby filament current when the kV is off enables the tube to turn on quickly without having to wait for the filament to warm up from cold, while keeping the filament at a lower temperature than normal operation. This extends the life of the tube.
The microcontrollers also allow other features to be implemented to provide more sophisticated control and protection of the X-ray tubes and enable their full potential to be exploited.
More complex X-ray tubes with grid control can be accommodated with this system, making it ideal for the modern range of small analytical X-ray tubes as well as high power industrial X-ray tubes.
HiTek Power has implemented this type of control system into many of its new range of X-ray generator products, providing outputs from 50kV 1mA (50W) up to 90kV and 11mA (1kW), with and without grid control. Further developments will extend this range to higher power and voltages.
Large numbers of these products are already operational in industrial and analytical application as diverse as analysing the quality and thickness of a gold plating to inspecting pizzas and jars of fruit destined for the supermarket shelves.

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Malcolm Barnett is Director of Technology, HiTek Power Limited, Littlehampton, UK. www.hitekpower.com