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