Parallax Stingray Manual page 24

Driving Brushed DC Motors
This section will guide you through connecting and testing the DC Brushed Gear Motors included with the
Stingray robot. Familiarizing yourself with the motors included in the Stingray robot kit will help you take
advantage of the benefits of using these motors as well as understanding the nuances.
DC motors are usually driven by an H-Bridge since such a circuit can reverse the polarity of the motor
connected to it. The DC brushed motors included in this kit are driven by the L6205 H-Bridge on the
Propeller Robot Control Board. Understanding how to control this H-Bridge is the key to controlling the
direction, speed and duration that the motors are on or off.
Parallax has released a Propeller object called, "PWM_32" which makes it easy to drive servos as well as
control motors using pulse width modulation. This object can be used with the Propeller Robot Control
Board to drive the on-board H-Bridge, which in turn drives the DC motors. This object can be downloaded
from the Stingray robot product page on our website or from the Propeller object exchange listed in the
Useful Websites section at the end of this document.
Brushed DC Spur-Gear Motors
The Stingray robot was designed to be fast when needed and as such a method of locomotion was chosen
to support this requirement. Servo motors used in many smaller robots are too slow and have too little load
capacity for the Stingray. Instead, brushed DC gear motors were selected. Specifically the motors used are
known as Spur-Gear motors. These motors provide a good value and performance in a reasonably sized
package.
The motors included in the Stingray kit have the following
specifications:
• 7.2 VDC Nominal @ ~180 mA (no load)
• 310 RPM (30:1 gear ratio on output shaft)
• 6mm diameter output shaft w/5.5mm detent for hubs with
set screws
• Output shaft length: 18 mm (0.70")
• Motor shaft extends from back of motor allowing encoder
support
• Motor shaft length: 10 mm (0.40")
• Four M3 size mounting holes
Internally the Spur-gear motor looks like the picture shown below, left. The back side of the motor is shown
in the figure on the right. Take note of the (+) sign that is used to indicate polarity. While motors don't
have a specific polarity per se, the plus signs help to match polarity of the motors on either side so you know
the direction the motor will turn when you apply power to it. When you apply power to the motor so that
the positive supply is connected to the (+) terminal and the other terminal connects to ground the internal
motor shaft turns clockwise while the output shaft spins counter-clockwise.
Copyright © Parallax Inc.
Spur-gear motor
internals
Back side of motor; note
the ⊕ symbol to help
match polarity.
Stingray Robot (#28980) v1.1 10/6/2009 Page 24 of 26