Add notes on actuator stroke and force

index.org

@@ -2042,7 +2042,9 @@ In this section are gathered all the specifications related to the nano-hexapod.
 The wanted dimension of the nano-hexapod are shown in Figure [[fig:nano_hexapod_size]]:
 - Diameter of the bottom platform: 300mm
 - Diameter of the top platform: 200mm
-- Maximum Height: 90mm
+- *Maximum Height: 90mm*
+
+The limiting height might be quite problematic for the integration of the flexible joints, the actuators and sensors.
 
 #+name: fig:nano_hexapod_size
 #+caption: First implementation of the nano-hexapod / metrology reflector and sample interface
@@ -2063,27 +2065,49 @@ The required angular stroke has not been estimated in this study.
 It is however simple to do so as the angular motion of each joint can easily be measured in the multi-body model used to perform the simulations.
 Typical angular stroke for such flexible joints is expected.
 
-*** Actuators
+*** Strut Stiffness
 :PROPERTIES:
 :UNNUMBERED: t
 :END:
 
-The actuation part is probably the most important part of the Stewart platform.
-
-- Stiffness:
 - Resonances should stay between 5Hz and 50Hz for payload masses up to 50kg
 - This corresponds to strut stiffnesses of $k \approx 10^5 - 10^6\,[N/m]$
-- Force:
+
-  - Weight: $60\,kg \rightarrow 600\,N \rightarrow 60\,N$ on each actuator
+*** Actuator Force
-  - Dynamic: few Newtons
+:PROPERTIES:
-- Estimation of the required stroke:
+:UNNUMBERED: t
-  - From simulation (i.e. for disturbance rejection alone), $\pm 5 \mu m$
+:END:
-  - However, the required stroke probably depends on two other factors:
+
-    - Static positioning errors of the stages
+Based on simulations:
-    - Maximum tracking errors of the stages (mainly translation stage and tilt stage).
+- Continuous Force: $\pm 5\,[N]$ (due to centrifugal forces)
-      This is probably more difficult to obtain.
+- "Variable" Force: $\pm 1\,[N]$
+
+If static deflection is to be compensated by the actuator, $\approx 100\,[N]$ of continuous force is required for each actuator.
+
+
+*** Actuator Stroke
+:PROPERTIES:
+:UNNUMBERED: t
+:END:
+
+Based on simulations, the required actuator stroke seems to be $\pm 5\,[\mu m]$.
+
+This however does not take into account two error types that will have to be compensated by the nano-hexapod:
+- the static positioning errors of all the micro-station's stages.
+  These errors have been measured by Hans-Peter, and are in the order of tens of $\mu m$ and tens of $\mu rad$.
+- the tracking errors of the translation stage and tilt stage.
+  This is probably more difficult to estimate.
   However, by limiting the acceleration of these stages, we may limit the dynamic tracking errors to acceptable levels
-  - If the chosen technology allows $\pm 50 \mu m$ that would be safer
+
+
+
+Some security margin should be taken as if the nano-hexapod has not enough stroke to compensated the above errors, the system will not be able to compensate all the vibrations.
+
+Thus, an actuator stroke of $\pm 50 \mu m$ would be quite safe.
+
+
+Note that a piezoelectric stack have a maximum strain of $0.1\%$.
+A piezo stack with a stroke of $\pm 50\,[\mu m]$ will have a length size of $\approx 100\,[mm]$ making it difficult to integrate in the nano-hexapod.
 
 *** Sensors
 :PROPERTIES: