Upload
preston-dalton
View
220
Download
2
Tags:
Embed Size (px)
Citation preview
Time Varying Structural Behaviour of the PETS “On-Off” mechanism piston movement impact
Riku Raatikainen
18.7.2011
Introduction Piston kinematics
Force impulse Model Description
Results
Conclusion
INDEX
INTRODUCTION
Aim:
1. Study the shock wave caused by the PETS On-Off mechanism 2. Calculate the PETS response (displacement) to the corresponding load
Overview:
The On-Off mechanism is a component for reflecting or transmitting the high RF power within PETS
Mechanism is driven by a linear actuator, which creates the movement of the piston between the two operation modes
The actuation time for the mechanism is ≤ 18 ms in which time the piston (150 g) moves inside the choke chamber for 8 mm
The mechanism is brazed to the PETS → Forces created within the mechanism have direct impact to the PETS causing time varying distortions, which could have an influence to the sustain alignment (PETS 20 µm)̴
ON OFF
Main components of the mechanism Illustration of the piston movement
As the piston is being moved, the structure exposes to impulse forces, which last very short period of time but causes forces of a high magnitude
The impact time is the driving parameter to the results. The smaller impact time, the larger magnitude of impulse force is generated
The piston movement kinematics presented below shows how the velocity of the piston is first increased and then suddenly decreased
PISTON KINEMATICS
Velocity of the moving piston as a function of timeDisplacement of the moving piston as a function of time
acceleration
deceleration
FORCE IMPULSE
Δtimpact
Relation of the piston movement and the force impulse. The acceleration/force value are proportional to the impact time.
Ideal impulse excites all modes of the structure → the response of the structure should contain all mode frequencies
However we cannot create an ideal impulse force numerically → force is applied over a discrete amount of time
For comparing the effect of the impact time to the PETS response, three different cases were studied:
Case Impact time (ms)
1 2
2 1
3 0.5
High deceleration during impact
MODEL DESCRIPTION
The FEA modelling was done to the level of the PETS, which is supported by using a fixed support in the middle and a sliding supports at both ends. Vacuum and waveguide connections were omitted to make the computation lighter.
The impulse force was applied on the On-Off mechanism As a first step towards the transient simulation, a modal analysis was carried out for the PETS. As a result the following natural frequencies were obtained:
F(t)
Mode Frequency (Hz)
1 540
2 550
3 770
4 810
5 1210
6 1260
Next, the response of the PETS was calculated when an impulse force shown earlier is applied to the structure (with a structural relative damping ratio of 3 %)
RESULTS
Maximum displacement of the PETS (compact coupler) as function of time in three different cases Response in the frequency domain is also presented
Case 1: Impact time of 2 ms (impulse magnitude 66 N)̴
Case 2: Impact time of 1 ms(impulse magnitude 133 N)̴
Case 3: Impact time of 0.5 ms(impulse magnitude 266 N)̴ The frequency is at the range of
the 3rd eigenvalue.
According to the results, the impulse force created by the On-Off mechanism causes a response in the magnitude of few micrometers at the most . The displacement has its maximum value around the Compact Coupler and the On-Off mechanism. On the DB axis the displacement has its value in the order of 200 nm (case 3)
In reality the displacement will be even smaller since the compact coupler is connected to the waveguide as well (making the structure more rigid)
The generated shock wave in the structure decays in approximately 40 ms from the impact
The impact time has a major influence to the shock wave amplitude (e.g. with a 2 ms of impact, the piston experiences nearly 50 g acceleration)
Based on the results, the response of the structure caused by the shock wave stays well below the alignment tolerance in all simulation cases. The results obtained from the simulation could be compared later on with the actual On-Off mechanism test results.
CONCLUSION
RESULTS-EXTRA
Case 1: Impact time of 2 ms (impulse magnitude 115N)̴
Case 2: Impact time of 1 ms(impulse magnitude 230N)̴
Case 3: Impact time of 0.5 ms(impulse magnitude 460 N)̴ The frequency is at the range of
the 3rd eigenvalue.
Obtained results for the PETS (Compact Coupler) response in case of the piston having a mass of 260 g instead of 150 g DB line response still in the order of few hundred nanometers