How to control the Batbot?

 The main goals of the batbot's control are enabling batbot of accurate forward and turning flight capacity and achieving efficient flight. To achieve these goals, we construct the system of robot (Figure 1).

Figure 1. Whole system diagram of Batbot

 As you can see, the system is consists of outer loop and inner loop. Inner loop is called 'morphing-wing modulation' and outer loop is called 'attitude regulation'. From now on, we will see each loop more specific.

 1) Inner loop(morphing-wing modulation)

Inner loop is closed loop with sensor. Therefore, the most important thing that you have to consider is this loop is feedback control. In addition, the interesting thing is that the actuator is SMA(Shape Memory Alloy) and the sensor is SMA. The control goal is to efficiently drive SMA actuators aimed at the fastest modulation of the wing shape (contraction and extension). From Figure 1, inner loop consists of Motors, Morphing control, SMA actuations, Wrist mapping, SMA Resistance measurement, and Elbow motion estimation.

     a) Motors

      Motors actuate the q1 and q2 motion. q1 motion is about flapping the wing to go upward and downward and q2 motion is about the rotation wings forward and backward. q1 and q2 motion allow the Batbot to go pitch motion.

     b) Morphing control

      From the feedback control, the input of morphing control is the position error of elbow joint motion and the output is the control signal Uheating that derives SMA actuator.

     c) SMA actuation

      Using the SMA actuator, it is possible to control the q3 motion. Then, What is SMA actuator? A Shape-memory alloy is an alloy that remembers its original shape and that when deformed returns to its pre-deformed shape when heated. This material is a light weight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, andmotor-based systems. The advantage of SMA actuator is that it's weight is very light, it is cheap, and there's no noise. In the case of Batbot, SMA actuator can make q2 joint moves freely. By calculating the change of electric resistance of SMA actuator, SMA actuator acts like sensor itself. Therefore, it is very convenient to use SMA actuator for Batbot.

     e) Wrist mapping

    Because q4, q5 and q6 motion is under-actuated, joint torque T4,T5, and T6 are generated by the T3.

     f)Elbow motion estimation

     Resistance change of SMA is lineally with respect to the strain rate of the SMA wire. Moreover, elbow motion can be estimated as a function of electrical resistance measurements : q3= f(Rsma)

 2) open Loop(attitude regulation)

    a) Wing trajectories 

      From getting the position data from the remote controller, the position data can go in closed loop system. Then, it activates Motors and Morphing control.

    b) Attitude control

      By using the remote control, the angle of roll, yaw and pitch are controlled. Inputs are the desired joint trajectories and attitude references. Outputs are the signals that derive the inner loop.

  

 3) What is Shape Memory Alloy?

 SMA actuator is most popular actuator for very small and tiny robots. A Shape-memory alloy is an alloy that remembers its original shape and that when deformed returns to its pre-deformed shape when heated. This material is a light weight, solid-state alternative to conventional actuators such as hydraulic, pneumatic, andmotor-based systems. For Batbot the SMA actuators are attached the upper side and bottom side of each wings.  When the wings goes up, the SMA actuator of upper side is heated and contracted. Therefore, the wings go up and the up-stroke motion is activated.

References:

1. J.D.Colorado, 2012, BaTboT: a biologically inspired flapping and morphing bat robot actuated by SMA-based artificial muscles, Universidad Politecnica de Madrid

2.  Colorado J, Barrrientos A, Rossi C, 2012, Biomechanics design of smart wings in a Bat robot : morphing wings SMA control