servo and sensor control on small mobile platforms - Engineering ...

J. Blanch, S. Tosunoglu, ASME Southeastern Region XI Technical Journal, Volume 2, Number 1, April 2003; also presented atthe ASME Southeastern Region XI Technical Conference, Miami, Florida, April 4-5, 2003.detailed review of the main components such as the<strong>servo</strong>s <strong>and</strong> <strong>sensor</strong>s <strong>and</strong> how they are <strong>control</strong>led tomake this an effective <strong>and</strong> mechanically stableplatform. Finally, some conclusions will be drawn asto the quality <strong>and</strong> precision of a platform built withoff the shelf components meant for hobbyists.PLATFORMThe platform used in this case is HANCOR(H<strong>and</strong>held-Controlled Rover), a small rover createdat Florida International University to test the viabilityof using h<strong>and</strong>held devices as platform <strong>control</strong>lers.The platform is based on a Pontech SV203 <strong>servo</strong><strong>control</strong>ler board that uses Sharp GP2D120 infraredranger <strong>sensor</strong>s to detect obstacles <strong>and</strong> Futaba-type<strong>servo</strong>motors (<strong>servo</strong>s used in radio-<strong>control</strong>led modelairplanes, cars, etc.) to drive the wheels. An onboardPalm III h<strong>and</strong>held <strong>control</strong>s the platform [4,5,6].SERVOSSmall budget platforms like the HANCOR oftenreplace expensive <strong>servo</strong>s <strong>and</strong> position encoders withoff-the shelf radio <strong>control</strong>led <strong>servo</strong>motors. RC<strong>servo</strong>s are cheap, easy to <strong>control</strong>, come in a convenientform factor <strong>and</strong> are available in different sizes,speeds <strong>and</strong> power ratings. Servomotors are generallyemployed for position <strong>control</strong>; they use a potentiometeras feedback to determine their position. The<strong>servo</strong> compares its current position to an input PWMsignal, <strong>and</strong> then moves until its position matches theinput signal. The “width” of the signal pulse may lastanywhere from 0.6 milliseconds (minimum position),to 2.4 ms (maximum position). The Pulse is repeatedevery 14 to 20 milliseconds (figure 2) [9]. Regular<strong>servo</strong>motors are used for the positioning of the range<strong>sensor</strong>s, but <strong>control</strong>ling the driving <strong>servo</strong>s requiresome modifications.Figure 2. Futaba-type <strong>servo</strong> timing diagram.Figure 1. HANCOR platform (h<strong>and</strong>heldconnected but not mounted).The <strong>servo</strong> <strong>control</strong>ler board is based on aPIC16C73 microchip; it accepts serial data from ahost computer (replaced by the Palm III h<strong>and</strong>held inthis case) <strong>and</strong> outputs a PWM (Pulse WidthModulated) signal to <strong>control</strong> up to eight RC <strong>servo</strong>motors[10]. Two <strong>servo</strong>s, modified for continuousrotation, provide power for the driving wheels.Besides <strong>servo</strong>s, the board can <strong>control</strong> other digitaldevices that require an on/off signal to be activated.Up to 5 <strong>sensor</strong>s can be connected to the A/D inputheader, which reads analog voltages between 0 <strong>and</strong> 5Volts. The <strong>sensor</strong> package consists of two infraredrange <strong>sensor</strong>s <strong>and</strong> one digital compass. The digitalcompass has a resolution of +/- 22 o (it detects N, NE,E, SE, etc.), so it cannot be used as a reliable <strong>sensor</strong>input, but can later be used as aid during mapmatching [7]. The IR <strong>sensor</strong>s are mounted on twosmall <strong>servo</strong>s so that they can pan to any direction in a135 o forward-looking field of viewFutaba-type Servomotors were not developed forcontinuous rotation. In order to utilize <strong>servo</strong>s insituations that require continuous rotation they haveto be modified. Servos were not designed forvelocity <strong>control</strong> either. The velocity of a <strong>servo</strong>depends on how far it is from the desired position.The <strong>servo</strong> will spin at top speed until it gets close tothe desired value, then it will quickly slow down toavoid over-shooting the target position. Usually,continuous rotation <strong>and</strong> velocity <strong>control</strong> go h<strong>and</strong> inh<strong>and</strong>. Servos that have been “hacked” for continuousrotation have the feedback potentiometer de-coupledfrom the output gear <strong>and</strong> set to a constant value (forexample 90 o ). The velocity is <strong>control</strong>led by givingthe <strong>servo</strong> a target position (0 to 180 o ), but since thefeedback has been fixed at 90 o , the <strong>servo</strong> will spinforever towards the target position. The further thetarget position from the fixed position (90 o ), thefaster the <strong>servo</strong> will spin.The Pontech SV203 <strong>control</strong>s its <strong>servo</strong>s via an 8-bit signal (PWM). An 8-bit signal can take 2 8 values(from 0 to 255). If we define the 0 signal as 0 o <strong>and</strong>