## 2 Prototype of Lightweight Manipulator for Ornithopters
### Description of the Lightweight Manipulator Prototype  
Underactuated mechanical systems with a passive joint at its base, like the one of the discussion above, are very unstable and very difficult to control. For that reason, the motors have very strict  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & **Number** & **Friction on the** & **Swing-Up** & **Stabilization around** & **Trajectory** & **Experimental** \\  & **of Links** & **Passive Joint** & **Approach** & **Equilibrium Point** & **Tracking** & **Results** \\ \hline
[13] & 2 & & ✓ & & & & \\
[14] & \(n\) & & ✓ & & & & \\
[15] & \(n\) & & ✓ & ✓ & & & \\
[16] & \(n\) & & & ✓ & & & \\
[17] & 3 & & & ✓ & & & \\
[18] & \(n\) & ✓ & & ✓ & & & \\
[19] & 3 & ✓ & ✓ & ✓ & & ✓ \\ Our work & \(n>2\) & ✓ & & ✓ &requirements, specifically, they need to be: (1) very precise; (2) high torque; (3) high speed, in order to compensate for the inaccuracies of the system. Additionally, if they have to be mounted in ornithopters, a fourth requirement of the size and the weight is necessary. Very lightweight and precise motors that provide high torque can be obtained using high-ratio reduction gears at the price of reducing the speed of the system. On the contrary, motors that moves at high speeds providing high torques have to be very powerful, thus increasing their weight and the weight of the required batteries. Therefore, all of these requirements can not be achieved simultaneously. Thus, the prototype developed can be seen in Figure 3, representing the links, joints and electronic components (see the similarity with Figure 2). The ankle joint is passive, it can rotate freely around an aluminum shaft, emulating the claw of the bird grabbing a branch. The knee and hip joints are actuated by two customized servo actuators shown in Figure 4.  
The lightweight design approach considered in this work is mainly associated to the choice of the actuators and the features of the frame structure that supports the manipulator. Since the control performance of the smart servo actuators like Dynamixel or Herkulex typically employed in aerial manipulation is constrained by the serial communications (with update rates below 100 Hz) and the embedded position controller, it is necessary to develop customized actuators that provide sufficient torque/speed with higher control rates while reducing their weight. In order to achieve the adequate balance between weight and control performance, two groups of actuators are considered: medium weight Maxon-Harmonic Drive (knee joint), and low weight Pololu micro servos (hip joint). Polylactic acid (PLA) and polymer parts were used to assembly the motor with the gearbox and the support  
Figure 4: Customized servo actuators: (**a**) knee joint; (**b**) hip joint. Figure 3: Lightweight manipulator prototype: (**a**) 3D model; (**b**) experimental prototype. frames, whereas the frame structure of the manipulator is manufactured with flat and L-shaped aluminum frames and thin tubes. In this sense, it is desirable that the structure is lightweight while providing a certain rigidity and robustness, also facilitating the assembly of the manipulator. Note that the weight can be optimized replacing the aluminum parts by carbon fiber, or using nylon screws instead of steel ones, although this is out of the scope of this paper. Table 2 indicates the mass density of these materials to evidence the relative weight of each component. The knee actuator (70 g weight) consist of a Harmonic Drive CSF-5 gearhead combined with a Maxon EC20 flat motor, assembled through a 3D printed plastic frame. This actuator provides zero backlash and high torque performance to satisfy the control requirements. The Maxon motor is controlled in torque by an ESCON 36/3 controller interfaced in PWM mode at 500 Hz. This motor set works with a power supply of 12 volts. The hip actuator is a customized micro servo (25 g weight) based on the Pololu micro metal gear motors and it works with a power supply of 7.5 volts.