Taking a better way off course.

I recently found out that my QEI (Quadrature Encoder Interface) codes were not correct. How can your brain help you stop from being hit by a street lamp if you are seeing it at the wrong place from its original place? Savvy? So we followed an advice from our mentor and sketched a module tree of our system and picked out the lower most branch of the module and start perfecting each module in the reverse hierarchy of their appearance. The scribbled rough diagram looks like this, I will try to fair it out ASAP and have my readers a look to it

I tried to vary frequency of QEI and looked if there is any redundancy in the readings and the fluctuations in the readings. While redundancy was not an issue, at 40Hz of QEI the readings fluctuated a lot. As the frequency was decreased the readings started to smoothen. This was observed and plotted on MATLAB and attached to the experiment's excel sheet which can be found here.

The next phase of the RPM calculator module will be proceeded with, tomorrow. Till then, ciao.

- Aniket

Three wheel driving - without control system

From my previous post, you can find a pdf regarding a "n wheeled driving equations". From the paper I was able to formulate equations for a three wheel drive and it followed as below,

v1 = -vx/2 + vy*sqrt(3)/2 + R*w
v2 = -vx/2 - vy*sqrt(3)/2 + R*w
v3 = vx + R*w

where,
v1 , v2, v3 are RPMs of the three wheels
vx is velocity of robot in x direction, vy is velocity of robot in y direction, w is angular velocity, R is radius of wheel

So I coded an interface for PS2 and motor control on ATMega128 controller and started testing a no control system run of the system. The code can be found here.
Considerations of no angular velocity was taken and hence w = 0. vx and vy were equalled 100cm/s initially but the response at this RPM was sluggish and often the mechanical errors dominated a lot leading to stoppage of wheels due to gear friction itself. Later the vx and vy were updated to 200cm/s and it got rid of gear friction and mechanical imperfections. Several runs were made on robot and the PWM on each wheel was "biased" according to the compensation required. For example, if the robot moved more to the left then the left wheel PWM was decreased. After testing several sets, today the driving concluded with the below results of no control system three wheel driving.

Obviously, the response is really bad. But it could be finer with some more tuning. This clearly declares the need of a control system.

Mechanical difficulties faced :

• The L mountings of the motor has very less ground clearance and hence touches unlevelled ground.
• Positioning wheels Allen bolts might have threads worn out.
• Gear friction is considerable.
• Some eccentricity on gear mountings.

Getting started with Three wheel omnidrive driving

So I will put up from where I left last time. I was working on four wheel omnidrive robot till my teammates worked on fabricating the three wheel chassis. Finally it was completed last week and we have started shifting the driving from four wheel to three wheel.

Probably, shifting the electronics and control system might only round up to changing the equations that correlate the individual RPMs and linear and angular velocity of robot. I found a really good paper work on it which can be found here.

Today we completed rewiring the triwheel chassis and managed to test all motors. Successfully tuned two of the motors but couldn't tune the third due to mechanical malfunctioning observed in it. The following are the videos of the P-regulator responses by each wheel, Used Tiva TM4C123 Launchpad. Due to lack of resources, we could accomplish only this much today.

The next steps will probably be to arrange the resources as soon as possible and make the changes in the equation and start trying out a no heading feedback system ASAP to compare triwheel and four wheel and solve actuator problems.