Download Modular - microwave power system based on microcontroller unit

STUDIA UNIVERSITATIS BABEŞ-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
MODULAR - MICROWAVE POWER SYSTEM, BASED ON
MICROCONTROLLER UNIT
V.Surducan, E. Surducan, C.Neamtu
National Institute of Research and Development for
Isotopic and Molecular Technology, Donath 75-103,
Cluj-Napoca, Romania
The “Modular-microwave power system” was designed for fine
adjustment and control of any microwave power generator, based on
continuous wave magnetron, with output power of max. 1000W. The
destination for such power generators covers many scientific or industrial
applications, from drying, microwave assisted digestion, polymerization,
sample extractions etc. For scientific and other sensitive applications, it’s
imperative to control precisely both the output power and the energy of
the microwave generator. The scientific key for this complex power and
energy adjustment is the use of a smart device, like a personal computer,
or a standalone microcontroller unit, to control the microwave generator’s
power supply. An user-friendly interface, having only four functional
buttons and a large LCD display with 2 rows and 16 characters/row,
assures the input parameters for the time pulse (energy) and power. A real
time clock measuring the programming time and generates the markers
for energy adjustment. Two different programming modes are
implemented: a local mode and a programed mode via an optoisolated
RS232/485 interface, using an external personal computer unit. Various
resolutions for the microwave output power may be achieved, with a
simple and stalwart procedure by switching the power supply coils of the
high voltage transformer. Sample’s temperature can be logged and
collected on the PC using some smart temperature sensors connected to
one measurement bus.
Design consideration
Our goal was to design and manufacture an universal microwave power
system, being able to generate inside the microwave coaxial chamber, a range
between low microwave power (beyond 50W for small sample volumes) to high
microwave power (900W for large sample volumes), in different effective power
steps using the same magnetron (900W/2.45GHz) and the same microwave circuit,
because the microwave sample preparation for small (less than 6ml) and very small
sample volumes are requiring an expensive and specialized microwave digestors.
This has the effect of decreasing the system cost and increase of versatility. It was
necessary to modulate the output energy in a precisely time quantum, because 50W
V.SURDUCAN E.SURDUCAN C.NEAMTU
of absorbed microwave power radiation is still too much for small sample volumes.
Thus, the probe may be iradiatted with any possible energy, by using a Pulse Width
Modulation tehnique [fig.2], with four different amplitude energy pulses. If the
pulse width is small enough and the pulse rate is extremely stable, then a precise
Microwave power versus output energy
900
microwave power [W]
800
700
600
500
full power
10% energy
50% energy
90% energy
400
300
200
100
0
1
2
3
4
power modes
fig. 1 The microwave generator output power versus 10% to 90% energy mode incident
fig. 2 Puls Width Modulation output tehnique for 10% output energy. The signal period is
kept constant while the pulse width is increased untill it equals the period time for 100%
energy mode.
MODULAR - MICROWAVE POWER SYSTEM, BASED ON MICROCONTROLLER UNIT
power is generated in the probe chamber as the fig. 1 shows. However the user
must know how much from this microwave radiation was absorbed by the probe,
and for this purpose we have designed and patented a microwave thermal
transducer [1]. Another problem in such systems may be the noise generation into
the mains, the tehnique used to supress this noise is zero crossing firing for the
power semiconductor devices involved in driving the mains load. The mains zero
crossing is synchronised with the rising edge of the power pulse width. [fig.2].
Power design and control overview
The generator [fig.3] is designed using a 900W CW magnetron [1]. The
microwave power is extracted using a TE microwave guide and a coaxial adapter
to the reaction chamber [2]. The chamber temperature is measured using a digital
temperature sensor. The power supply [3] is entirely electronically controlled, both
the anode voltage and the filament voltage can be directly controlled by the
required program sequence. The system controller [4] is built with a PIC16F628
microcontroller. It uses a smart LCD driver [4b] with HD44780 which is able to
display all the important parameters: the output power level (in relative power
steps), the output energy (in procent indications), the treatment time (in
minutes/seconds) and the chamber temperature (celsius degree). Al settings can be
set from a small four buttons functional keyboard [4a]. A functional keyboard is
sharing the button functions and displaying those on the LCD. When a button is
pressed the microcontroller is running the specific routines and change the button
function for the next push. Some buttons may have the same function in different
screens displayed, other buttons may have different functions as the software
requires. From one screen, the PC communication menu may be set. In this mode
the whole generator control is under the PC command via the RS232 interface [5].
To avoid the computer breackdown, because of possible leackage currents from the
high voltage modules, the RS232 interface is fully optoisolated.
fig. 3 Modular microwave power system, block diagram
V.SURDUCAN E.SURDUCAN C.NEAMTU
Conclusions
The modular-microwave power system can be used in any microwave
application which request variable output power and energy. With a small firmware
reconfiguration the system can be used in totally different aplications. This is
possible because of some logic/analogic reserved input/ output ports in the
microcontroller unit and interchange features of the microwave circuit. One usage
example is our versatile and cheap microwave digestion system [fig.4].
fig. 4 Microwave digestion unit using the modular microwave power system
Bibliography:
1. E.S ur d u ca n, V .S urd u ca n, Thermographic Transducer for Microwave
Power Radiation, Patent RO116506 / jan.2001