FIR Center Report. Development of Feedback Control Scheme for the Stabilization of Gyrotron Output Power

Transcription

1 FIR Center Report FIR FU-120 November 2012 Development of Feedback Control Scheme for the Stabilization of Gyrotron Output Power Oleksiy Kuleshov, Nitin Kumar and Toshitaka Idehara Research Center for Development of Far-Infrared Region University of Fukui Bunkyo 3-9-1, Fukui-shi , Japan

2 Development of Feedback Control Scheme for the Stabilization of Gyrotron Output Power Oleksiy Kuleshov 1,2, Nitin Kumar 1 and Toshitaka Idehara 1 1 Research Center for Development of Far-Infrared Region, University of Fukui (FIR FU) Bunkyo 3-9-1, Fukui-shi , Japan 2 The Institute of Radio Physics and Electronics of National Academy of Sciences of Ukraine (IRE NASU) 12 ac. Proskura st., Kharkov 61085, Ukraine Abstract One of the main features of any electron oscillator is the stability of its parameters during the operation. In this work the gyrotron output power stabilization using proportional-integralderivative (PID) loop as feedback control system in cathode filament scheme has been analyzed and developed. Preliminary test results of the PID controller system are presented briefly.

3 1. Introduction Gyrotrons are capable of producing high-power coherent electromagnetic radiation in the millimeter wavelength ranges. The important and promising direction of gyrotron development nowadays is investigations in the sub-terahertz (sub-thz) and terahertz (THz) frequency bands. Development of compact, simple and reliable sources of coherent sub-thz and THz radiation is important for numerous applications, which include plasma physics, spectroscopy, detection and imaging of explosives and weapons, new medical technology, etc. Gyrotrons are much more compact than powerful free electron lasers because they do not require large accelerators or high-voltage modulators. From the other hand the gyrotrons can produce essentially higher output power comparing with solid state devices. One of the main features of any electron oscillator is the stability of its parameters during the operation. In this work the gyrotron output power stabilization using proportional-integralderivative (PID) loop as feedback control in cathode filament scheme has been analyzed, developed and tested. PID controller is widely used in many industrial applications, but in the case of electron device an application of PID loop has some features that are describe further. 2. Feedback control theory for vacuum electron device application. Feedback is a mechanism for regulating a physical system so that it maintains a certain state. Feedback works by measuring the current state of a physical system, determining how far the current state is from the desired state, and then automatically applying a control signal to bring the system closer to the desired state. This process is repeated iteratively to bring the system to the desired state and keep it there. Feedback can be used very effectively to stabilize the state of a system, while also improving its performance. Common application of feedback in electronics is in precision, fast- response power supplies. Constant current and constant voltage power supplies which have a high degree of stability use feedback to regulate their current or their voltage, by measuring the current and voltage across a precision shunt resistors and then using feedback to automatically correct for any deviations from the desired output. Feedback also allows the power supply to adjust its voltage or current very quickly and controllably in response to a change in load. In the case of a stabilization of a vacuum electron device (VED) output power it is required to consider at least two variables, specifically beam accelerating voltage and beam current, as it is evident from the formula: P= ηv I ac b

4 where η is VED efficiency, V ac is accelerating voltage and I b is beam current. Assume first that the stable high-voltage power supply unit is applied and it is required to provide the feedback control scheme to stabilize the beam current. This can be done in the thermionic electron gun via adjustment of filament current I fil. Consider VED characterized by a single variable I b. Under normal conditions VED has a steady state value of I b = I 0 which may vary and drift somewhat over time due to the variation of many variables such as VED tube gases pressure, reflected power from the load, cavity temperature etc. which we cannot measure or are unaware of. We possess a mechanism for measuring VED electron beam current I b as well as a control input I fil with which we can use to modify the beam current I b. Our objective is to set or lock the value of beam current to a desired value I b = I 0 and keep it there without letting it drift or vary over time, regardless of variations in the environmental variables mentioned above. Variables modify I b VED: I b I 0 Measurement of I b Calculate error e= I b I 0 Control I fil modifies I b Calculate I fil = I fil (e) Fig. 1. Scheme of VED with feedback loop in filament circuit. This feedback model can be adapted to include several state variables and several feedback variables. The most popular type of feedback stabilization control, I fil (e), is Proportional-Integral- Derivative (PID) gain feedback. PID is very effective and easy to implement in many control problems, but in some applications it does not in general provide optimal control. The fundamental difficulty with PID control is that it is a feedback system, with constant parameters, and no direct knowledge of the process, and thus overall performance is reactive and a compromise while PID control is the best controller with no model of the process, better performance can be obtained by incorporating a model of the process. The most significant improvement is to incorporate feed-forward control with knowledge about the system, and using the PID only to control error. Alternatively, PIDs can be modified in

5 more minor ways, such as by changing the parameters (either gain scheduling in different use cases or adaptively modifying them based on performance), improving measurement (higher sampling rate, precision, and accuracy, and low-pass filtering if necessary), or cascading multiple PID controllers. In the case as it is considered above the beam accelerating voltage control is made by high-voltage power supply and the stabilization of beam current is proposed to achieve using the feedback control in the filament scheme. Thus, the expression for I fil (e) depends only on the error signal e= I b I 0 and is given by () ( τ) τ () fil p i d 0 t d I = Ket + K e d + K et dt where K p, K i and K d, are respectively the proportional, integral, and derivative gains, t is present time and τ is a variable of integration taking value from initial till present time. 3. Feedback control with the help of NI LabVIEW. At present the feedback control can be easily realized using NI LabVIEW program connected to high-voltage power supply and filament power supply. In the case of Spellman high-voltage power supply and filament power supply the feedback control scheme requires buspowered multifunction data acquisition (DAQ) device for USB connection to PC as it is shown in fig. 1. Filament Master Slave NI USB DAQ Fig. 1. Spellman HV SR20N12 with feedback control. The block diagram of PID loop and the scheme of such PID algorithm in NI LabVIEW program is shown in fig. 2.

6 a) b) Fig. 2. Block diagrams of PID loop (a) and PID algorithm in NI LabVIEW program (b). The measured value of beam current is the process value (PV) as indicated in the algorithm scheme. The desired beam current is called the setpoint (SP). SP is defined and fixed in the NI LabVIEW file interface window as it is shown in fig. 3. As it shown in fig. 3 the file interface window is divided into four parts. Upper left column shows measured value of four parameters: tube gases pressure, beam current, cathode voltage and heater (filament) current. Below it is the icon for the set value of heater (filament) current control and the switch for slow start mode. The slow start mode allows one to increase the heater current from zero to heater current set value during the fixed time, which is defined in the file block-scheme window as it is

7 shown in Appendix I. Also this slow start mode allows one to decrease the heater current from set point to zero with the same rate as in the case of current increase that is necessary at the final stage of gyrotron operation. After the program start when the heater current reaches the set value it is possible to turn on all power supply units for gyrotron operation. Fig. 3. Interactive window of NI LabVIEW file for feedback control using PID loop. The graph, which is placed in the main part of the screen in the file interface window, shows the real-time values of the measured parameters. And the PID control scheme data are placed at the bottom of the screen. Before switching on the PID control scheme it is necessary to set all the required data such as beam current set value, proportional, integral and derivate coefficients, and the range of the heater (filament) current defined by upper and lower limits. The heater current range limits are necessary not only for cathode heater protection. It should be noticed that a high proportional gain results in a large change in the output for a given change in the error. If the proportional gain is too high, the system can become unstable and in this case the upper and lower heater current limits will be the key instrument to reduce the instability effect.

8 4. Test of the feedback control Scheme for gyrotron output power stabilization. At the first stage the scheme, which allows to measure four parameters such as the tube gasses pressure, cathode voltage, beam current and heater current, was designed and tested. This test allows us to check the beam current value realization depending on both heater current and cathode voltage. At the next stage we started the scheme of heater current control with the help of NI LabVIEW program. To this end we provided the additional block scheme, which allows us to increase the heater current from zero to defined value during determined time interval. In our case the speed of heater current increase was 1 A per 10 minutes. The magnetron-injection gun operates with the heater current around 2.25 A. During the test it was noticed that such control scheme requires protection in the case of NI LabVIEW program halt. At the initial stage such protection can be done by the control of both heater current and cathode voltage simultaneously. In such manner the NI LabVIEW program halt will result in decrease to zero both heater current and cathode voltage, but such conditions are also very dangerous for cathode heater. Therefore the protection scheme should provide very fast decrease of cathode voltage to zero with the lowspeed decrease of the heater current. Such scheme is planned to be developed in nearest future. At the last stage of our present research the PID controller scheme was used to stabilize the beam current. At this stage we provide the PID controller scheme to control the heater current with the help of NI LabVIEW program. Since the stabilization of cathode voltage was done by internal scheme of Spellman SR device the application of beam current stabilization will result in gyrotron output power stabilization. As the result of the investigation of PID controller scheme application for gyrotron output power stabilization, several advantages of such gyrotron operation should be noticed: 1. Control of heater current with a good precision. 2. Optimal start of gyrotron operation. 3. Easy in use measuring system for all necessary parameters. 4. Real compact power supply system controlled by PC. Optimal start of gyrotron operation also means the optimal time of beam current increase until the defined value with further its stabilization as it is shown in fig. 4. This is done by determine of heater current range, i.e. heater current both upper and lower limits. In fig. 4 it is evident from blue curve that switching on the PID scheme allows one to get much faster the necessary beam current value (red curve) with the small heater current range. It is also evident that without PID this current value can be reached only after more than 10 minutes.

9 Fig. 4. NI LabVIEW file for feedback control using PID loop. Also the disadvantages of such PID control scheme are following: 1. Inertion of filament system leads to the effective control only in the case of low-frequency changes and no effect in the case of fast beam current changes. 2. Requirements of additional protection scheme caused by the PC features such as program halt. The promising way to decrease the first point of disadvantages is to provide two PID schemes for beam current stabilization: PID control in both heater current scheme and cathode voltage scheme. This statement, of course, requires further investigation since the main goal of the activity is not to provide only beam current stabilization but the gyrotron output power stabilization. In this connection the changes of cathode voltage also influence on gyrotron output power as well as beam current changes and application of PID scheme in cathode voltage circuit requires additional investigation. Also it should be noticed that the control of other power supplies simultaneously can improve proposed scheme. It means that we should provide additional schemes in LabVIEW execution file to control anode voltage and electric currents of both solenoid and additional magnetic coils.

10 5. Conclusions The scheme for gyrotron power stabilization based on the algorithm of proportionalintegral-derivative controller was developed, designed and tested. The scheme was realized using NI LabVIEW program and Spellman high-voltage power supplies. Test results showed effective stabilization of the gyrotron output power using feedback control system in cathode filament scheme in the case of not too fast perturbations. In the case of fast beam current changes it is planned to apply two PID schemes for beam current stabilization: PID control in both heater current scheme and cathode voltage scheme. Also the development of protection scheme for the cases of both executive file halt and breakdowns is planned in nearest future.

11 Appendix I The block scheme of NI LabVIEW executive file with slow start mode that allows one to increase the heater current from zero to heater current set value during the fixed time, is shown in fig. 5. Also this file block-scheme consists of measuring part and PID control part. It is shown in fig. 6 that the heater control part of the executive file block-scheme can be operated in soft start mode, stable mode and low-speed decreasing mode. Such modes are required for both long lifetime and stable operation of the gyrotron cathode. The block-scheme of the slow-start mode, which is shown in fig. 6.a, can be easily adjusted for any cathode filament system by changing the timer settings. The block-scheme of the mode with the heater current preset value that is required in the case of stable gyrotron operation is shown in fig. 6.b. And the block-scheme of the mode with the slow-decrease of the heater current is shown in fig. 6.c. The PID controller algorithm should be switched off during the gyrotron operation start and finish as it is shown in the block-scheme in fig. 7. The block-scheme of NI LabVIEW file after switching on the PID control is shown in fig. 8.

12 Fig. 5. Block-scheme of the LabVIEW file in soft start mode.

13 a) b) c) Fig. 6. Heater control block-scheme: a) soft start mode, b) stable and c) decreasing mode.

14 Fig. 7. The block-scheme of NI LabVIEW file with switched of PID control.

15 Fig. 8. The block-scheme of NI LabVIEW file after switching on the PID control.