Differences

This shows you the differences between two versions of the page.

--- marvin:ecp3 [2009/01/28 20:15] – deva
+++ marvin:ecp3 [2009/01/28 22:17] – deva
@@ Line 11: / Line 11: @@
 =====Project Goal=====
 Make robot able to drive a predefined pattern.
 =====Plan=====
   * Create a Motor Control class that can append extra power to the motors individually to control how the robot is behaving.
+=====The Motor in Theory=====
 === DC Motor Drives ===
 {{ :marvin:motor.png |The NXT DC Motor}}
@@ Line 63: / Line 64: @@
 If we apply an offset to both wheels we are removing control from the controller and disturbing the calculated error. Either the control loop will overcome this disturbance by some minor oscillations or it will become unstable as we are really adding to the overshoot. This does not happen when we add a small offset to one wheel and subtract from the other since this does not affect the overall error signal. The natural place to control a closed loop control is of course the reference values, which are applied for each state. The reason that we did not use this approach immediately is probably that the reference values have been left unused as we want all the states to be zero in order for the robot to remain in equilibrium. If a small offset is added to the tilt angle (psi) the robot must remain in motion to stay balanced. Although the robot is capable of moving forward and backward it showed an undesired tendency to oscillate between forward and backward commands. As the contra steering used to turn the robot did seem to stabilize the robot, it seemed important to make the robot occupied in between commands by adding a small amount of contra steering. This will keep the motors busy and reduce the slip when the motors are changing from forward to backward motion simultaneously. We used a sine function to add a small offset to one wheel and subtract from the other as the sine function overall should make the robot remain in the same position - also we can quite easily alter the amount of offset. The real benefit from this is due to high amount of slip in the motors in the instant where they are not active. Before, this slip would point in the same direction, but with the small contra steering it actually points in separate directions, hence stabilizing the robot.
 Using this approach we are now able to control the robot as expected, thus we are able to make the robot drive a predefined pattern. It is important to mention that the robot still has a tendency to oscillate, which often requires the control loop to "interrupt" the desired motion causing the robot to move unexpected. Therefore the the robot is not able to drive according to the navigation class, but it is satisfactory in order to implement a behaviour control model. The goal of this lab session was to make Marvin capable of driving a predefined pattern, but we did not specify anything about the accuracy of its position or which type of pattern. We are currently satisfied with the progress and we find the result satisfactory with respect to the next lab session in which we shall implement the behaviour based control model.
 ====Gyroscope Offset Drift Problem====
-  * Hjul amok løsningen
+Easy as it seems, we did not arrive at the goal easily. We had a very annoying problem, with the gyroscope offset drifting when the robot was not standing still balancing.\\
-  * Udret fejl jvf. vinkel
+The offset seemed to drift in the direction of the angle, and the larger the angle, the faster the drift.\\
-  * Batteri korrektion (fejl på reference til A/D konverter, drift i denne pga batteri spænding)
+We tried several solutions to the problem, in the following they will be drafted, together with the reason that they did not work:\\
-  * Lille spredning i gyro --> nulstil gyro
+  * The first approach was based on the fact that when the robot was in perfect balance (the angle velocities were below a threshold for some amount of time) we could determine the angle to be zero, thus reset it. This however would make the robot reset its zero angle at a position //not// zero (it would lean forward but stay in that angle for duration of its forward movement).
-  * Addering af konstant offset
+  * When the angle offset had reached a certain level, the robot continuously tried to straighten up, resulting in the wheels spinning up to maximum in the opposite direction. Based on this observation, we tried to detect when the wheels were driving above a threshold, and thereafter adjust the offset angle towards the thought actual zero. This did not seem to fix the problem at all, since the time we detected the large offset, was way too late.
-  * Gennemsnitsmålinger på gyro
+  * Next we tried to calculate the drift size by multiplying the angle with some constant. We tried to adjust this constant for a long time, with a lot of measurements, but all in vein. If the drift size could be calculated as a function of the angle, it was certainly not a linear function.
-  * Vægtet midling af alle gyrolæsninger --> VIRKER!!!
+  * We had read somewhere that there might be a problem with the ADC based on its reference voltage, due to drop in battery level. This lead to an attempt to calculate the drift size based on the current battery level measured against the initial battery level. This however did not seem to be the way to things either, since the battery level dropped faster than the drift size would grow.
+  * We later tried to simply add constant number to the calculated angle, and tried to adjust it manually, by modifying it, tweaking it, until we thought it as accurate as possible. This seemed to work to a certain point. The drift grew smaller, but did not seem to disappear.
+  * We for some time suspected the gyroscope reading in itself of being too inaccurate, which lead to a fix, where we made a number of readouts, averaging them out and using this as the actual reading instead. This seemed to work while the robot was in balance, but had negative effect when it was tilting fast, which made the gyroscope reading smaller than the actual value due to the averaging out, so this was obviously not a satisfying solution either.
+  * The last attempt was to simply make a running weighted average of all the gyroscope readings, based on the assumption, that the robot is tilting equally forward and backwards. This seemed to work, so that was the solution we chose.
 =====MotorControl=====