User manual

RP6 ROBOT SYSTEM - 2. The RP6 in detail
obstacles, ramps and uneven floors. On such surfaces, the encoders are extremely
helpful, as they allow optimal speed regulation under all load conditions, completely
independent of surface condition, motor load and Battery voltage.
At a rate of 50 segments per second we have a speed of 1.25 cm/s assuming a value
of 0.25mm per segment. This speed is the minimal speed, which can be controlled
reliably (at least with the standard software implementation). The exact value may
vary for individual robots. A rate of 1200 segments/second corresponds to the maxim-
um possible 30 cm/s (at 0.25mm resolution, whereas 0.23 corresponds to 27.6 cm/s).
Maximum speed depends on battery charging status and 30cm/s will are not possible
for too long with usual Batteries. Because of this, the function library forces a limit of
1000 segments/second to be able to maintain a constant maximum speed for longer
battery discharge periods. Additionally, the life time of gears and motors will be pro-
longed when using lower speeds most of the time!
Whenever the robot has counted 4000 segments, it will have covered a distance of
approximately one meter. As already explained, this specification is valid for exactly
0.25mm resolution - without proper calibration you will notice more or less severe
deviations. If you do not care for precise distance calculations, you just do not need to
calibrate the encoders and simply assume a value of 0.25mm or better 0.24mm!
Good navigation systems usually do not rely completely on encoders for distance and
angle controls, but use external fixed markers such as infrared beacons and a preci-
sion electronic compass. So it is usually a good idea to use external systems to correct
odometry deviations as often as possible.
2.4. Drive System
The RP6 drive system consists of two DC motors with attached gearing systems for
powering the caterpillar wheels (see preceding figure). The motors can consume a
fairly high amount of power and a microcontroller can not directly serve such high cur-
rents.
Thus we need powerful motor drivers. We
use two so called H-Bridges for the RP6 Mo-
tors. The diagram on the left shows the ba-
sic principle. There you can also see why it
is called like this: The Switches and the Mo-
tor form the letter “H” together.
Now let us assume all switches are open. If
we close switches S1 and S4 (red) a voltage
will be applied to the motor and it will start
turning, say to the right. If we now open S1
and S4 again and subsequently close S2 and
S3 (green), we reverse the applied voltage
and the motor will start turning in the op-
posite direction (to the left). Of course we
will have to take care not to close S1 and S2
or S3 and S4 simultaneously. Each of these combinations would result in a short cir-
cuit and might destroy the activated switches.
Of course the RP6-design will not be using mechanical switches but so-called
MOSFETs, which are conductive if a suitable voltage is applied to the gate connection.
MOSFETs can switch very fast at a rate of several Kilohertz is possible.
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