There are some electrical sensors mounted on the robot other than camera. The application of these sensors is shown below.
- 3 axis accelerometer（ADXL345, Analog Devices）：Posture change detection, Getting off the track detection, Obstacle detection
- Hall effect sensor（A1324, Allegro MicroSystems）：Touching ground detection, Obstacle detection
- Servo motor（KRS-3301,Kondo Kagaku）：Posture change detection, Obstacle detection
For 3. servo motor, it may be not called as sensor. However it is comprised of motor and sensor so that I’ll add it to the list. As I posted earlier, accelerometer can be used to avoid obstacle and we can calculate velocity and displacement as well by taking cumulative sum of acceleration signal once and twice respectively. Accelerometer is cheap while it’s very useful. Here I’ll describe how to use accelerometer for posture change detection.
- We can detect posture change by taking inner product of current gravity acceleration and that of initial level and comparing with 1.0. Of course, we need to normalize two vectors as having length 1.0 before taking inner product. If current posture is close to initial level, inner product value is close to 1.0. If the robot is positioned at upside down, its value is close to -1.0.
- More gravity force is always applied to front side of the robot because the center of mass of scissors and arm is located away from that of robot platform. Therefore we need to correct posture in order to get closer to initial level of gravity acceleration vector as much as possible. When more gravity force is always applied to front side of the robot, it is applied to front legs most. Next is middle legs. And the force is applied to back legs least. Mechanical structure to measure pressure from the ground is comprised of hall effect sensor, diameter 3mm x hight 3mm cylinder Ferrite magnet, steel spring and support. Moreover, since the position of scissors and arm is off to the right, the force is applied to front-right leg most and it’s least at back-left leg diagonal to front-right leg.
- Simultaneously gap from desired angle of servo motor is largest at front legs. Next is middle legs. And gap is smallest at back legs. Moreover, since the position of scissors and arm is off to the right, gap is largest at front-right leg and it’s smallest at back-right leg diagonal to front-right leg.
- As I described, there is a relationship between signals from accelerometer, hall effect sensor and servo motor. So, we need to decide how to correct current posture using these 3 sensors comprehensively. It’s possible for us to deside whether posture change actually occurs or not accurately from multiple point of views.
- We can calculate desired servo angle by inverse kinematics so as to coincide foot tip with pre-defined touch point on the ground.
- I’ll implement the above procedure in the case robot doesn’t start walking. And then I’ll try to correct posture in real time.