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

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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.Table 1. Data extracted from range of interest.Left Rightbit rps rps90 0.880282 -0.8787394 0.857633 -0.85106104 0.802568 -0.78125112 0.673401 -0.64516120 0.423729 -0.36058140 -0.4 0.454545146 -0.59032 0.648508152 -0.72464 0.761035164 -0.82781 0.859107168 -0.8489 0.874126Two observations that persuade against searchingfor more accurate approximations are: 1, theexperimental measurements are subject to error;depending on the accuracy of the reading or even onthe battery charge, values will vary slightly from onerun to the next. And 2, bit comm<strong>and</strong>s are integers;therefore there will be gaps in the velocity curves forvalues that could never be reached because theycorrespond to a bit value between two integers. Onthe other h<strong>and</strong>, the closeness of the splineapproximation to the actual data showed thatselecting a few finite points can offer sufficient<strong>control</strong> over the velocity. Not using an equation (orseveral) to find the bit value corresponding to adesired velocity, results in a crude <strong>control</strong> over speedvalues; but, by finding the bit values that correspondto specific velocities (0.8, 0.6, 0.4, 0.2, 0.0, -0.2, -0.4,-0.6 <strong>and</strong> –0.8 rps, per se), the velocity of the platformcan be properly <strong>control</strong>led for its full range of values.Figure 6. Distance <strong>and</strong> angle calculation.SENSORSThe main <strong>sensor</strong> used by the HANCOR is theSharp GP2D12 analog infrared ranger [10]. Sharp IRrangers use triangulation <strong>and</strong> a small linear CCDarray to compute the distance or presence of objectsin the field of view (Figure 6), which results ingreater reliability <strong>and</strong> accuracy than many IR <strong>sensor</strong>sthat use time-of-flight techniques. Also, these newrangers offer much better immunity to ambientlighting conditions <strong>and</strong> to the color of the reflectedsurface than other IR <strong>sensor</strong>s [11]. These <strong>sensor</strong>shave a minimum range of 10 cm (~ 4in) <strong>and</strong> amaximum range of 80 cm. The beam is very tight,just about 3 cm wide <strong>and</strong> less than 3 cm in height at40 cm. Such characteristics make the <strong>sensor</strong>s quitesuitable for unidirectional measurements, but not sogreat for general obstacle detection. Someadvantages over ultrasound ranger <strong>sensor</strong>s,traditionally used for obstacle detection, are: IR<strong>sensor</strong>s do not suffer from ghost images; furthermore,the angle at which they face an obstacle can be ashigh as 60 o without affecting distance reading [11].IR <strong>sensor</strong>s also have much lower power requirementcompared to the battery-hungry ultrasound <strong>sensor</strong>s.Finally, price becomes an issue when ultrasound<strong>sensor</strong>s are over five times more expensive than IR<strong>sensor</strong>s. Of course, sornars will always be greatdetection <strong>sensor</strong>s thanks to their range (2 to 120 cm)<strong>and</strong> their wider detection area.Table 2: IR range values for given distancesDistance (cm)10Sensor1239Sensor223512 210 20414 185 17716 169 16118 152 1412022139129132120242612011311310528 108 9930 101 9132349587877836 81 7938 79 7140 74 6742 69 6344 65 5946 61 5548 57 5250 54 48525450484341The analog output of the infrared does not changelinearly with the distance being measured; table 2shows the readings taken from the <strong>servo</strong>s from 10 to
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, 2003.50 cm. To find a numerical approximation that canyield a governing equation, the average of <strong>sensor</strong> 1<strong>and</strong> <strong>sensor</strong> 2 was found <strong>and</strong> then analyzed.value (bit)250200150100500IR readingsHoerl Pow Avg10 18 26 34 42 50 58Distance (cm)Figure 7. IR distance approximations.The average values can be approximated by thefollowing power equation:V(d) = 1917.709*d^(-0.8941)which is very similar to that of [11]V(d) = 2141.72055*d^(-1.078867)A better approximation can be found using aHoerl Model, which is a modified power equation:V(d) = 997.82*0.985^d*d^(-0.563), but it is notreally necessary to perform the extra power functionfor a marginal increase in accuracy.To turn IR range <strong>sensor</strong>s into effective obstacledetectors, the <strong>servo</strong>s must reposition the <strong>sensor</strong>s toget several readings of the terrain ahead of the robot.If the <strong>sensor</strong>s have to be reoriented, timing becomesan issue. Several factors have to be taken intoaccount to determine the frequency at which readingscan be taken. First of all, the refresh rate of the IR<strong>sensor</strong>s determines the maximum frequency at whichreadings can be taken. The velocity of the <strong>servo</strong>sdictate how long it will take to reach the desiredorientation. The response of the <strong>control</strong>ler boardindicates how long it takes for the board to poll theAD header for a new reading <strong>and</strong> return it to theh<strong>and</strong>held. The h<strong>and</strong>held connects to the <strong>control</strong>lerboard at a certain speed. Finally, the Palm has toperform many computations, <strong>and</strong> can only read fromthe serial port every so often.The IR <strong>sensor</strong>s continuously takes distancereadings every 38 ms, for a total of about 25 readingsevery second (25Hz)[10]. The Hitec ht-85 <strong>servo</strong>shave a nominal operating speed of 0.11sec/60° at4.8V. Even though the speed of the <strong>servo</strong> isproportional to the displacement covered, thenominal speed can be used to approximate the time ittakes the <strong>servo</strong> to move a certain arc. At 9600 baud,which is the st<strong>and</strong>ard connection speed for the<strong>control</strong>ler board, it takes 4ms to transmit a simpleoutput comm<strong>and</strong> of 4 bytes. For more complexcomm<strong>and</strong>s it takes the Palm 25ms to transmit thecomm<strong>and</strong> <strong>and</strong> receive the result as ASCII data. Afterthe data is received, it must then be converted to aninteger for it to be useful. In practice, is rare to get<strong>sensor</strong> readings faster than 50ms. It is possible toconnect the <strong>control</strong>ler board to transmit at up to38400 baud, which should reduce considerably thecommunication delays, but since the <strong>servo</strong> responseis the main cause for delays in the architecture, it isnot necessary.-30 o-8 o 8 o 30 oFigure 8. Obstacle detection.The GP2D12 takes continuous readings, but thePalm takes discrete readings individually. In order toeffectively cover the front area of the rover, each<strong>sensor</strong> takes three readings as seen in figure 8. Giventhe Hitec’s nominal velocity, the different times ittakes a <strong>servo</strong> to go from one position to the next are:Position 1 to Position 2: 46 o ~ 0.084sec; Position 2 toPosition 3: 24 o ~ 0.044sec; Position 3 to Position 1:22 o ~ 0.04sec. Since the IR ranger takes 38ms torefresh, the robot should wait that much longer beforereading from the <strong>sensor</strong>. Therefore, the time delaybefore a reliable reading can be taken is at least:To position 1: 0.040 + 0.038 = 0.079secTo position 2: 0.084 + 0.038 = 0.122secTo position 3: 0.044 + 0.038 = 0.082secPosition 1Position 30 o 0 o-54 o 54 o24 o 22 oPosition 246 oUnfortunatelly, PalmOS does not have a wait( )or delay( ) function, to wait long enough beforetaking the reading. Fortunately, Palm does have a
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