More simulations: We modified our earlier version of the motor simulation and replaced the current-controlled H-bridge witha simpler amplifier model that assumed rejection of back-emf effcts. This basically meant that we could get rid of one of the feedback loops and represent our model as such:
We found the transfer function of our system to be:
The corresponding Bode plot of this plant is:
After creating a Bode plot of our system, we injected sinusoids of various frequency into our system and recorded the response displacement. Note that the sinusoidal displacement of the motor (red) has a phase shift when compared to the input signal (blue). If one cycle = 2*pi, we found that the phase shift was approximately 1.51 radians, or 86.4 degrees. The frequency of our input signal was 0.2 Hz, so this phase shift corresponds with our Bode plot.
Virtual Haptic evironments:
This was one of the coolest parts of the project wherein we were supposed to replicate the feel of mechanical elements on our i touch motor.
Virtual Spring: In this exercise our aim was to introduce programmable changes to make the i touch motor feel like a spring. We used the position sensor to implement this environment. Based on the position readings from the hall effect sensor, a translation formula is applied to make the range of readings symmetrical on both sides of the center position of the armature. The program is tweaked to make the pwm value higher as the armature moves out on either side of the center position. This makes the user feel that the armature has stronger tendency to bounce back as he/she is moving it out further and hence it acts as a virtual spring!
We had initial hiccups in this exercise wherein we were able to replicate the spring feel on one side but not the other.. It was happening because of a negative pwm feedback on one side which when corrected behaved exactly as anticipated.
Virtual Wall
This was relatively simple environment to simulate once we got the virtual spring working. One position on either side of the center is chosen arbitrarily as the "wall" and the program is modified to make the motor go really stiff at those positions. We have a very high pwm value at this junction in the code which makes the armature vibrate insanely if one tried to force it beyond that point which makes the Virtual wall!!
Virtual Damper and Virtual spring-mass
A derivative control is applied in the code to get the virtual damper simulation. The concept is based on the theory that the armature should make the user feel variable resistances depending upon the applied speed on the armature. We collect the position of hall sensor every 50 ms and then calculate the velocity from that array of readings. This velocity reading is then used to control the virtual environment of the damper using the principle: the higher the velocity, higher is the impedance experienced by the user. We did implement the program for virtual damper but are not completely sure if we could replicate the feel. It was kind of difficult to sense damping if there was any and we were not able to judge objectively.
Due to time constraints, we were not able to simulate the virtual spring-mass system.
The most rewarding environment, we felt was the virtual spring. The force did increase in a linear fashion in the program and we could feel that increment in force as the armature is pushed on either side. The easiest to simulate was a virtual wall which was kind of fun too. You felt like you have just been electrocuted because the fingers start vibrating due to the offered resistance by the motor! The most difficult to feel was the virtual damper as damping was too subtle to feel. It was not a challenge from programming point of view but it was certainly hard to feel the damping. Maybe that is because of the low resolution of the hall effect sensor..
Nice visualization of input and sensor responses!
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