Ideally, these flexible joints would behave as perfect spherical joints, that is to say no bending and torsional stiffness, infinite shear and axial stiffness, unlimited bending and torsional stroke, no friction, and no backlash.
Deviations from these ideal properties will impact the dynamics of the Nano-Hexapod and could limit the attainable performance.
During the detailed design phase, specifications in terms of stiffness and stroke were determined and are summarized in Table \ref{tab:test_joints_specs}.
After optimization using a finite element model, the geometry shown in Figure \ref{fig:test_joints_schematic} has been obtained and the corresponding flexible joint characteristics are summarized in Table \ref{tab:test_joints_specs}.
This flexible joint is a monolithic piece of stainless steel\footnote{The alloy used is called \emph{F16PH}, also refereed as ``1.4542''} manufactured using wire electrical discharge machining.
Sixteen flexible joints have been ordered (shown in Figure \ref{fig:test_joints_received}) such that some selection can be made for the twelve that will be used on the nano-hexapod.
First, the flexible joints are visually inspected, and the minimum gaps (responsible for most of the joint compliance) are measured (Section \ref{sec:test_joints_flex_dim_meas}).
The development of this test bench is presented in Section \ref{sec:test_joints_test_bench_desc}, including a noise budget and some requirements in terms of instrumentation.
Two dimensions are critical for the bending stiffness of the flexible joints.
These dimensions can be measured using a profilometer.
The dimensions of the flexible joint in the Y-Z plane will contribute to the X-bending stiffness, whereas the dimensions in the X-Z plane will contribute to the Y-bending stiffness.
\caption{\label{fig:test_joints_profilometer}Setup to measure the dimension of the flexible beam corresponding to the X-bending stiffness. The flexible joint is fixed to the profilometer (\subref{fig:test_joints_profilometer_setup}) and a image is obtained with which the gap can be estimated (\subref{fig:test_joints_profilometer_image})}
A histogram of the measured beam thicknesses is shown in Figure \ref{fig:test_joints_size_hist}.
The measured thickness is less than the specified value of \(250\,\mu m\), but this optical method may not be very accurate because the estimated gap can depend on the lighting of the part and of its proper alignment.
Using this profilometer allowed to detect flexible joints with manufacturing defects such as non-symmetrical shapes (see Figure \ref{fig:test_joints_bad_shape}) or flexible joints with machining chips stuck in the gap (see Figure \ref{fig:test_joints_bad_chips}).
To estimate the bending stiffness, the basic idea is to apply a torque \(T_{x}\) to the flexible joints and to measure its angular deflection \(\theta_{x}\).
To apply torque \(T_{y}\) between the two mobile parts of the flexible joint, a known ``linear'' force \(F_{x}\) can be applied instead at a certain distance \(h\) with respect to the rotation point.
In this case, the equivalent applied torque can be estimated from equation \eqref{eq:test_joints_force_torque_distance}.
Note that the application point of the force should be sufficiently far from the rotation axis such that the resulting bending motion is much larger than the displacement due to shear.
Such effects are studied in Section \ref{ssec:test_joints_error_budget}.
Similarly, instead of directly measuring the bending motion \(\theta_y\) of the flexible joint, its linear motion \(d_x\) at a certain distance \(h\) from the rotation points is measured.
\caption{\label{fig:test_joints_bench_working_principle}Working principle of the test bench used to estimate the bending stiffness \(k_{R_y}\) of the flexible joints by measuring \(F_x\), \(d_x\) and \(h\)}
The bending stiffness is foreseen to be \(k_{R_y}\approx k_{R_x}\approx5\,\frac{Nm}{rad}\) and its stroke \(\theta_{y,\text{max}}\approx\theta_{x,\text{max}}\approx25\,mrad\).
The height between the flexible point (center of the joint) and the point where external forces are applied is \(h =22.5\,mm\) (see Figure \ref{fig:test_joints_bench_working_principle}).
The bending \(\theta_y\) of the flexible joint due to the force \(F_x\) is given by equation \eqref{eq:test_joints_deflection_force}.
To test the full range of the flexible joint, the means of applying a force (explained in the next section) should allow a motion of at least \(0.5\,mm\).
To determine the applied force, a load cell will be used in series with the mechanism that applied the force.
The measured deflection of the flexible joint will be indirectly estimated from the displacement of the force sensor itself (see Section \ref{ssec:test_joints_test_bench}).
Indirectly measuring the deflection of the flexible joint induces some errors because of the limited stiffness between the force sensor and the displacement sensor.
Such an effect will be estimated in the error budget (Section \ref{ssec:test_joints_error_budget})
\item Errors in the measured torque \(M_x, M_y\): this is mainly due to inaccuracies in the load cell and of the height estimation \(h\)
\item Errors in the measured bending motion of the flexible joints \(\theta_x, \theta_y\): errors from limited shear stiffness, from the deflection of the load cell itself, and inaccuracy of the height estimation \(h\)
The applied force \(F_x\) will induce some shear \(d_{x,s}\) which is described by \eqref{eq:test_joints_shear_displ} with \(k_s\) the shear stiffness of the flexible joint.
With an estimated shear stiffness \(k_s =13\,N/\mu m\) from the finite element model and an height \(h=25\,mm\), the estimation errors of the bending stiffness due to shear is \(\epsilon_s < 0.1\,\%\)
As explained in the previous section, because the measurement of the flexible joint deflection is indirectly performed with the encoder, errors will be made if the load cell experiences some compression.
Suppose the load cell has an internal stiffness \(k_f\), the same reasoning that was made for the effect of shear can be applied here.
The estimation error of the bending stiffness due to the limited stiffness of the load cell is then described by \eqref{eq:test_joints_error_load_cell_stiffness}.
With an estimated load cell stiffness of \(k_f \approx1\,N/\mu m\) (from the documentation), the errors due to the load cell limited stiffness is around \(\epsilon_f =1\,\%\).
As the measured displacement is foreseen to be \(0.5\,mm\), the error \(\epsilon_d\) due to the encoder non-linearity is negligible \(\epsilon_d < 0.01\,\%\).
The accuracy of the load cell is specified at \(1\,\%\) and therefore, estimation errors of the bending stiffness due to the limited load cell accuracy should be \(\epsilon_F < 1\,\%\)
The most important source of error is the estimation error of the distance between the flexible joint rotation axis and its contact with the force sensor.
An overall accuracy of \(\approx5\,\%\) can be expected with this measurement bench, which should be sufficient for an estimation of the bending stiffness of the flexible joints.
As explained in Section \ref{ssec:test_joints_meas_principle}, the flexible joint's bending stiffness is estimated by applying a known force to the flexible joint's tip and by measuring its deflection at the same point.
The force is applied using a load cell\footnote{The load cell is FC22 from TE Connectivity. The measurement range is \(50\,N\). The specified accuracy is \(1\,\%\) of the full range} such that the applied force to the flexible joint's tip is directly measured.
To control the height and direction of the applied force, a cylinder cut in half is fixed at the tip of the force sensor (pink element in Figure \ref{fig:test_joints_bench_side}) that initially had a flat surface.
Doing so, the contact between the flexible joint cylindrical tip and the force sensor is a point (intersection of two cylinders) at a precise height, and the force is applied in a known direction.
To translate the load cell at a constant height, it is fixed to a translation stage\footnote{V-408 PIMag\textsuperscript{\textregistered} linear stage is used. Crossed rollers are used to guide the motion.} which is moved by hand.
Instead of measuring the displacement directly at the tip of the flexible joint (with a probe or an interferometer for instance), the displacement of the load cell itself is measured.
To do so, an encoder\footnote{Resolute\texttrademark{} encoder with \(1\,nm\) resolution and \(\pm40\,nm\) maximum non-linearity} is used, which measures the motion of a ruler.
This ruler is fixed to the translation stage in line (i.e. at the same height) with the application point to reduce Abbe errors (see Figure \ref{fig:test_joints_bench_overview}).
The obtained CAD design of the measurement bench is shown in Figure \ref{fig:test_joints_bench_overview} while a zoom on the flexible joint with the associated important quantities is shown in Figure \ref{fig:test_joints_bench_side}.
\caption{\label{fig:test_joints_bench}CAD view of the test bench developed to measure the bending stiffness of the flexible joints. Different parts are shown in (\subref{fig:test_joints_bench_overview}) while a zoom on the flexible joint is shown in (\subref{fig:test_joints_bench_side})}
In order to estimate the measured errors of the load cell ``FC2231'', it is compared against another load cell\footnote{XFL212R-50N from TE Connectivity. The measurement range is \(50\,N\). The specified accuracy is \(1\,\%\) of the full range}.
\caption{\label{fig:test_joints_force_sensor_calib}Estimation of the load cell accuracy by comparing the measured force of two load cells. A picture of the measurement bench is shown in (\subref{fig:test_joints_force_sensor_calib_picture}). Comparison of the two measured forces and estimated non-linearity are shown in (\subref{fig:test_joints_force_sensor_calib_fit})}
To do so, a stiff element (much stiffer than the estimated \(k_F \approx1\,N/\mu m\)) is mounted in front of the force sensor, as shown in Figure \ref{fig:test_joints_meas_force_sensor_stiffness_picture}.
Then, the force sensor is pushed against this stiff element while the force sensor and the encoder displacement are measured.
The measured displacement as a function of the measured force is shown in Figure \ref{fig:test_joints_force_sensor_stiffness_fit}.
The load cell stiffness can then be estimated by computing a linear fit and is found to be \(k_F \approx0.68\,N/\mu m\).
\caption{\label{fig:test_joints_meas_force_sensor_stiffness}Estimation of the load cell stiffness. The measurement setup is shown in (\subref{fig:test_joints_meas_force_sensor_stiffness_picture}). The measurement results are shown in (\subref{fig:test_joints_force_sensor_stiffness_fit}).}
The actual stiffness is now estimated by manually moving the translation stage from a start position where the force sensor is not yet in contact with the flexible joint to a position where the flexible joint is on its mechanical stop.
Three regions can be observed: first, the force sensor tip is not in contact with the flexible joint and the measured force is zero; then, the flexible joint deforms linearly; and finally, the flexible joint comes in contact with the mechanical stop.
The angular motion \(\theta_{y}\) computed from the displacement \(d_x\) is displayed as function of the measured torque \(T_{y}\) in Figure \ref{fig:test_joints_meas_F_d_lin_fit}.
The bending stiffness of the flexible joint can be estimated by computing the slope of the curve in the linear regime (red dashed line) and is found to be \(k_{R_y}=4.4\,Nm/\text{rad}\).
The bending stroke can also be estimated as shown in Figure \ref{fig:test_joints_meas_F_d_lin_fit} and is found to be \(\theta_{y,\text{max}}=20.9\,\text{mrad}\).
\caption{\label{fig:test_joints_meas_example}Results obtained on the first flexible joint. The measured force and displacement are shown in (\subref{fig:test_joints_meas_bend_time}). The estimated angular displacement \(\theta_x\) as a function of the estimated applied torque \(T_{x}\) is shown in (\subref{fig:test_joints_meas_F_d_lin_fit}). The bending stiffness \(k_{R_x}\) of the flexible joint can be estimated by computing a best linear fit (red dashed line).}
The same measurement was performed for all the 16 flexible joints, both in the \(x\) and \(y\) directions.
The measured angular motion as a function of the applied torque is shown in Figure \ref{fig:test_joints_meas_bending_all_raw_data} for the 16 flexible joints.
\caption{\label{fig:test_joints_meas_bending_results}Result of measured \(k_{R_x}\) and \(k_{R_y}\) stiffnesses for the 16 flexible joints. Raw data are shown in (\subref{fig:test_joints_meas_bending_all_raw_data}). A histogram of the measured stiffnesses is shown in (\subref{fig:test_joints_bend_stiff_hist})}
The measured bending stiffness values exhibited good agreement with the predictions from the finite element model (\(k_{R_x}= k_{R_y}=5\,Nm/\text{rad}\)).
These measurements are helpful for refining the model of the flexible joints, thereby enhancing the overall accuracy of the nano-hexapod model.
Furthermore, the data obtained from these measurements have provided the necessary information to select the most suitable flexible joints for the nano-hexapod, ensuring optimal performance.
