A.

Environmental requirements

The environmental requirements make the design and the validation of the BSA quite challenging.

B.

Critical performances requirements

The BSM is required to steer the incoming laser beam through a range of ± 3 mrad, optical range, for both axes with a control bandwidth up to 10 Hz. Due to the 45° incidence beam on Ry axis, this performance requires a mechanical rotation of +/-2.12 mrad on the Rx defined axis and +/-1.5 mrad on the Ry defined axis at the BSM level. The critical requirements are the accuracy of the movement, the stability (short term, mid term and long term) and the repeatability.

The Tab. 2. lists the time scale and associated instability contributors while Tab. 3. gives the main performance requirements that are compliant for any contributor and for any optical angle.

Tab. 2.

Potential contributors to the stability

Tab. 3.

BSA performance requirements

A.

BSMFE mechanical design

The BSMFE is a 2-axis Tip-Tilt Mechanism (TTM), mounted on a bracket, supporting the Front end Electronic boards. Based on a two stiff push pull pairs of APA60SM® actuators [3], it produces more than 3 mrad, optical angle, with a bandwidth up to 10Hz with mechanical resonant frequencies above 1.4 kHz thus avoiding the use of a launch locking mechanism and limiting the micro-vibration susceptibility. The position of the piezo actuators was selected to minimize the volume of the BSMFE and to allow a different stroke between the two rotation axes. Such an asymmetric architecture presented on Fig. 4. is proposed to ensure similar range of applied voltage, and corresponding to the same range of optical angle. This choice enables to maximize the applied voltages on the piezo ceramics; it improves the overall sensitivity of the mechanism and then improves the accuracy of the angular movement.

Fig. 3.

BSA components overview

Fig. 4.

BSA mechanism architecture description

Once the piezoactuators sized, several simulations were performed to validate simultaneously angular range, stress resulting from integration, thermal environment, vibration/shocks environments and micro sliding at screwed links in regards of the thermal and vibration/shocks environments, see Fig. 5. All the stress contributions were summed and complied with the ECSS safety margins for both non-operational and operational modes.

Fig. 5.

FEM simulations result: Calculated maximum displacements during RX actuation

B.

Stability and accuracy smart design

This mechanism is controlled by strain gauges position sensors which support the entire stability and accuracy of the mechanism in closed loop, see Fig. 6. The contributors to these performance budgets are long term temperature variation, ageing, and repeatability and thermal noise for short term. BSA does not only require high level of accuracy and stability but also high reliability position sensors for each axis [4]. Consequently, to improve the accuracy and the stability of the strain gauge measurements, several precautions were brought. A full bridge configuration was selected to optimize the sensitivity while limiting the thermal impact and non-linearity errors on its performance, see Fig. 7. The initial sensitivity is given in (1).

Fig. 6.

Strain gauges location

Fig. 7.

Selected Full bridge configuration and its application on the push pull piezomechanism

With R₁ = R₃ = R₀ – ΔR ; R₂ = R₄ = R₀+ ΔR; and GF_x = Gauge constant factor

The full bridge configuration is theoretically insensitive to temperature effect due to the well symmetric bridge. From the previous formulae, we have introduced a thermal coefficient with an apparent strain (3).

If four strain gauges are applied to the specimen and connected into a full bridge, the thermal component in the total strain has the same sign for all strain gauges; since they are all subjected to the same change in temperature. Based on (2) and (3), we establish the expression (4):

This theory is valid as long as no thermal gradient appears between the bridge components. The degree of the compensation depends on the uniformity of the temperature at the strain gauge level. Balancing the thermal conductive paths at the bracket level is mandatory.

So, the design was optimized for this purpose, particularly choosing aluminum base plate for the mechanism. The temperature gradient in each push pull axis was studied according to each operational thermal perturbation contributor. Fig. 8. gives the result of thermal transient analysis in RY and RX push pull according to electronic start and parasite laser flux loading (corresponding to the non-reflected residual laser flux, i.e. transmitted through the mirror into the mechanism), see Fig. 8. Results are compliant with stability requirements. The thermal gradient in push pull is less than 3mK submitted to parasite laser flux perturbation. Moreover, in order to minimize the bridges sensitivity to external drifts (thermal effect, thermocouple effect…) the bridge was balanced as much as possible (i.e. to null the residual offset). An adequate pairing of the ceramics + strain gauge couple has been performed and allowed to reduce the initial offset by 99%. Additional actions were taken to further improve bridge stability: Reduction of the power dissipation, improvement of the strain gauges alignment on the ceramic, balance the lead wires.

Fig. 8.

Transient thermal analyses results in push pull RX and RY

Fig. 9.

BSA overview

C.

Beam Steering Electronic

The BSA electronics is split in three flex-rigid PCBs:

BSFE electronics: The Front End Board (FEB) provides the most critical functions of the BSA:

Pharao proximity electronics where thermally controlled within 30mK tolerance, while the BSA proximity electronics are submitted to PLH thermal conditions with some temperature variations up to 4K on long term. Nevertheless, BSA stability performances are better because of electronics improvement and TTM optimized architecture. The PCB outlines and its routing are also particularly complex in order to be compatible with the specified mechanical and electrical interface.

Others particularities or difficulties associated to the FEB electronics are the need of interchangeability and the need of cleanliness, combined with the necessity to be immune to the close high current laser pulses of more than 100A which exist beside the BSMFE.

BSME electronics: The Main Electronic Board (MEB) provides BSA other specified functions which can be put far away from the mechanism:

The first part is the power supply of all the BSA circuits by a single fully shielded dc-dc converter

The second one are the digital functions associated to the management by a FPGA in order to:

The third one are the analog functions associated to the mechanism command and control:

The outlines of this PCB are far much classical and spacious than the others two. It is simply fold inside the BSME case.

A.

Piezoelectric components LAT

As The Piezoelectric components (or Multi-Layer actuators – MLA) are procured in advance and are standard components, the chosen philosophy is to realize on a part of the batch a Lot Acceptance Test (LAT) to accept the lot before MLA integration on the flight models. The LAT tests validate piezoelectric components procurement of BSM. The BSA LAT sequence is mostly in line with the ECSS draft generic and detailed specification for space piezoelectric components.

As some tests cannot be conducted with piezoelectric components alone, some integration sequences are necessary. Some samples are tested alone and some others were tested in APA shape, integrated in their titanium shell.

All the tests described in Tab. 4. were performed, using all the 25 samples, and the tests results demonstrated the compliance to all criteria. So, the acceptance of MLA batch used for EQM and both flight models was successful after LAT achievement. It is worth noting that this is the first time the LAT sequence has been achieved with piezoelectric ceramics equipped with strain gauges.

Tab. 4.

LAT sequence description

B.

Cleanliness management and results

LIDAR instruments in general involve high fluence; consequently, to ensure safe operation of the instrument a very careful monitoring of the cleanliness is required. Cleanliness is a generic issue for optical space instrumentation (particulate and molecular contamination requirements) but in the case of active instrumentation trace contaminants, especially as it concerns silicones and other compounds which are not easily removed by oxygen cleaning, cleanliness topic becomes a vital issue [5].

On ATLID project the severe cleanliness specification requires to take into account this matter at each step of the project, starting from the design, naturally during integration and tests, up to the storage. To respect the cleanliness requirement the following parameters were filled by both Sodern and Cedrat Technologies:

Concerning the design optimization, the following design rules have been applied on each part of BSMFE:

Operationally, the BSA was submitted to a severe cleanliness and contamination control plan during all activities of integration and tests. Two cleanliness breadboards were used, at mirror assembly level and at TTM level, representative of APA assembly and electrical connection. The cleanliness breadboards allowed validating design choices with respect to cleanliness requirements.

The cleanliness activities also consisted in repeating interventions with: Visual inspection and cleaning, thermal vacuum chamber blank test and bake out, particular and molecular contamination control, clean room and tools cleaning. Cleanliness tasks were taken into account at each step of the project, giving excellent results with a molecular contamination measurement at 2 10^-8g/cm² for FM2 and a successful particular management.

C.

System performance test results

The BSA tests campaign, including qualification and acceptance tests, gave excellent performance results, much better than required for some of them.

BSA provides a deviation range of +/-3mrad, optical angle, on each axis, with a typical power consumption of 3.6W taken on a standard 28V power bus. The mechanism response time is less than 100ms for small displacement up to 40μrad, and less than 300ms for larger displacement. The angular resolution is better than 0.4μrad, optical angle. It ensures stability performances of very high quality with long term stability inferior to 100μrad, including repeatability inferior to 30μrad.

Stability performances in different bandwidth and thermal stability are given hereafter:

The measured gap between set point and realized angle also gave excellent results,

The non-spherical Wave Front Error generated by the BSA mirror is also very good with a value less than 4nm RMS on 7,8mm beam diameter

In conclusion, main system performances are compliant with the requirements, and some like the stability values are even far better than specified.

D.

Environmental test results

A large qualification tests campaign was performed on BSA product The BSA was designed and tested according to mechanical, thermal and EMC critical environments. Tab. 5. gives worst case level of each environment requirements with respect to BSA components: BSME and BSMFE.

Tab.5.

BSA worst case environments levels

Both BSMFE and BSME were submitted successfully to shock, vibration, EMC and thermal cycle tests. Results of test campaign show that BSA is compliant to all these requirements without any performance degradation.