RioBotz Comb t Tutorial Summarized – DC Motors

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					                                              system for their arenas and             the robot from the building, then
                     LINKS                    buildings. They should not allow        remove the batteries from the robot.
        Sewer Snake and Death and Taxes       people to enter the arena until the          The builder would smother the
                  fight videos                smoke has been cleared. Due to the      batteries with the provided dry sand.
            nature of this battery chemistry and
            IUBBOPBbJpk&sns=em                the volume of smoke produced, this      The Future
           Another camera view of the         could take a long time. This is
                 fight and fire               definitely something to consider and          It should be noted that the use
            in some event venues, it may be         of other cells like the Lithium-Ion
                 amvqgyqtr74                  difficult to achieve.                   A123 brand have much less of a
                                                                                      chance of fire or this smoke hazard.
         RC Group’s complete guide to
                Li-poly batteries             At Combots, We had                      Of course, they are bulkier and a          a Simple Battery                        little heavier than many of the Li-
             thread.php?t=209187                                                      poly packs. They may prove to be a
                                              Fire Plan                               better idea but not quite yet ... trial
                                                   During the safety meeting, the     and terror testing is still the nature
                                              builders were told that they were       of the game.
       Fastrax LiPo Charge Bag. This bag is   responsible for removing the robot            As usual, combat robotics
         made from Kevlar and Nomex           from the arena and the building         consistently pushes available        when it was judged safe to do so.       technology to its limits and often
      product.php?productId=0000004476        There was a clear path maintained       way beyond; this is the nature of
                  Ansul Lith-X                from the arena to the outside doors     the sport. Hopefully, with a better
           of the building. The builders were      understanding of this impressive
            search_combo.aspx?txt             shown the path to the exit. The         and powerful battery technology,
              SearchPhrase=lith-x             builders were instructed to wear        better building practices, and
                                              leather welding gloves if they had to   improved event safety measures
     smoke contains several toxins. It is     handle the hot robot. Steel carts       we can continue to provide a great
     recommended that protective              were provided either by the builders    show and high standards of
     clothing and a full-face SCBA            or the event to place robots on, so     performance in this dangerous and
     breathing system be used by              they could be quickly wheeled           yet safe sport. SV
     anyone fighting this type of fire.       outside.
     The smoke is one of the more                  When possible, the event crew
                                                                                           Photo by Jon Swenson.
     difficult issues to control and it’s a   would put out the open fire with
                                                                                          See more of Jon’s work at
     good idea for event organizers to        CO2.                            
     provide a high flow ventilation               The builder would then remove

       RioBotz Comb t Tutorial
       Summarized – DC Motors
             ● Original Text by Professor Marco Antonio Meggiolaro; Summarized by Kevin M. Berry
          Editor’s note: Professor Marco      Chapter 5 — a major treatise on bot     opportunity to update some
     Antonio Meggiolaro, of the               motors. Marco’s book is available       information that is now obsolete in
     Pontifical Catholic University of Rio    free for download at                    the published version.
     de Janeiro, Brazil, has translated his
     popular book, the RioBotz Combot         html, and for hard copy purchase        Brushed DC Motor
     Tutorial, into English. In previous      (at no profit to Marco) on Amazon,      Overview
     editions of the Combat Zone, SERVO       published by CreateSpace. All
     has summarized many portions of          information here is provided                Motors are probably the
     the tutorial. In this article, we        courtesy of Professor Meggiolaro        combat robot’s most important
     present a much simplified version of     and RioBotz. In reviewing this          component. They can be powered
     the “Brushed DC Motors” section of       article, Prof. Marco took the           electrically, pneumatically,

30   SERVO 01.2011
                                           FIGURE 2

                               FIGURE 1

hydraulically, or using fuels such as     the most used, not only in the             • τ — Applied torque at a given
gasoline. One of the most used            drive system but also to power                moment.
types is the brushed direct current       weapons. They have fixed                   • ω — Angular speed of the rotor.
(DC) electrical motor because it can      permanent magnets attached to              • Poutput — Mechanical power
reach high torques, it is easily          their body (as shown in Figure 1)             output.
powered by batteries, its speed           which forms the stator (the part that      • η — Efficiency = Poutput/Pinput which
control is relatively simple, and its     does not rotate), and a rotor that            results in a number between 0
spinning direction is easily reversed.    has several windings (Figure 2).              and 1. In an ideal motor (which
They are also a good choice               These windings generate a magnetic            doesn’t exist in practice), there
because they’re not as expensive as       field that — together with the field          would be no electrical resistance
they used to be. There are other          of the PM — generates torque in the           and no mechanical friction losses;
types of electrical motors, but not all   rotor. To obtain an approximately             in that case, η = 1 (100%
of them are used in combat. For           constant torque output, the winding           efficiency).
instance, stepper motors have — in        contacts should be continually
general — a relatively low torque         commutated which is done through                 The curves in Figure 4 show
compared to their own weight. The         the commuter on the rotor and the          the drawn current I (Iinput), angular
speed of AC motors is more difficult      stator brushes (pictured in Figure 3).     speed ω, output power Poutput, and
to control when powered by                Electrically, a DC motor can be            efficiency η as a function of the
batteries which can only provide          modeled as a resistance, an                torque τ applied to the motor by a
direct current.                           inductance, and a power source,            load such as a wheel or spinning
     The three main types of brushed      connected in series. The behavior as       weapon.
DC motors are the permanent               a power source is due to the counter             (Editor’s note: A section
magnet (PM), shunt (parallel), and        electromotive force which is directly      showing the use of many interesting
series. The series type motors are        proportional to the motor speed.           but slightly scary equations to derive
the ones used as starter motors;          The choice of the best brushed DC          the curve below and its results were
they have high initial torque and         motor depends on several                   omitted from this summary.)
high maximum speed. If there is no        parameters, modeled next.
load on their shaft, starter motors            To discover the behavior of     FIGURE 3
would accelerate more and more            a brushed DC motor
until they would self-destruct which      (permanent magnet or shunt
is why they’re dangerous. In a few        type), it is necessary to know
competitions, they can be forbidden       some parameters:
for that reason. They are rarely used
in the robot’s drivetrain because it is   • Ino_load — Electric current
not easy to reverse their movement;         drawn by the motor to spin
however, they are a good choice for         without any load on its shaft.
powerful weapons that spin in only        • Iinput — Electric current that
one direction.                              the motor is drawing.
     The PM DC motors and the             • Istall — Electric current drawn
shunt type have similar behaviors,          by the motor when so much
which are quite different from the          load is applied it can’t turn
starter motors. The PM ones are             at all.

                                                                                                           SERVO 01.2011       31
      FIGURE 4                                                   FIGURE 5

         The plot in Figure 4 shows that:    Magmotor S28-150 connected to           item). Their actual values in a
                                             one NiCd 24V battery pack. The          battery/controller/
     • The maximum speed ω no_load           guide’s “teaching point” — aside        wire/motor system would not be as
       happens when the motor shaft is       from providing sample calculations —    good. (Note: Kt and Kv — not used
       free of external loads, with τ = 0.   is twofold.                             elsewhere in this summary — are the
     • The maximum current Istall                 First, the calculated maximum      motor torque and motor speed
       happens when the motor is             input power — 5.2 HP (at stall), —      constants.)
       stalled, with speed ω = 0, so at      does not mean you’d actually get             The Bosch GPA and GPB
       Istall the motor is generating the    that much horsepower. All this          shown in the table have been
       maximum possible torque.              power is wasted when the motor is       extensively used in Brazil to drive
     • The maximum value of the              stalled; converted into heat by the     middleweights. However, they
       mechanical power Poutput happens      system resistance. The maximum          have a low ratio between
       when ω is approximately equal to      mechanical power is actually            maximum power and their own
       half of ω no_load.                    calculated to be 1.25 HP. Notice that   weight. In addition, the GPA
     • The highest efficiency happens in     the manufacturer says that the          generates a lot of noise which can
       general between 80% and 90% of        maximum power is 3 HP for that          reduce the range of 75 MHz radio
       ω no_load.                            motor which you would only get if       control systems. This problem can
                                             the battery and electronic system       be minimized using capacitors
     Example: Magmotor                       resistances were zero, leaving only     between the motor brushes, or
     S28-150                                 the motor resistance Rmotor (not a      switching to (for instance) 2.4 GHz
                                             real world case.)                       radio systems.
         The guide calculates the values          The second point is that (as it         The DeWalt 18V motor with
     (shown in Figure 5) for the popular     can be seen in the graph) the           gearbox is a good choice for the
                                                            maximum mechanical       drive system; we’ve used it in our
                                               FIGURE 6     power happens at         middleweight Ciclone. It has an
                                                            speeds that are not      excellent power-to-weight ratio. Its
                                                            necessarily efficient.   main disadvantages are that it is not
                                                                                     easy to mount to the robot
                                                         Typical                     structure, the gearbox casing is
                                                         Brushed                     made out of plastic, and its resulting
                                                                                     length including gearbox ends up
                                                         DC Motors                   very high to fit inside compact
                                                              The Magmotor           robots. Note that some older
                                                         example can be              discontinued DeWalt cordless drills
                                                         repeated for several        had other disadvantages, using
                                                         other motors. Table 1       Mabuchi motors instead of the
                                                         shows a few of the          higher quality DeWalt ones, and
                                                         most used motors in         using a few plastic gears among the
                                                         combat robots and           metal ones in their gearbox.
                                                         their main parameters.           The NPC T64 already includes a
                                                         Several parameters are      gearbox with typically a 20:1
                                                         based only on motor         reduction. The data in the table
                                                         specs (as a standalone      already include the power loss and

32   SERVO 01.2011
weight increase due to the gearbox               instead of powered by its nominal        have overvolted it to 96V, but
which explains the relatively low                12V. If used with solenoids to           current limiting is highly
power-to-weight ratio. But, even                 power weapons, make sure that            recommended.
disregarding that, the performance               they can take the high currents                A few DC motors allow the
of this motor is still not too high.             involved. This motor is difficult to     permanent magnets fixed in their
The reason many builders use it is               find even in the US.                     body to be mounted with an
due to its convenience, it is easily                   The Etek motor is really           angular offset with respect to their
mounted to the robot, and it is one              impressive. It may deliver up to 15      brush housings (typically about
of the few high power DC motors                  HP (1 HP = 746W), and it can deliver     10 to 20 degrees, it depends on
that comes with a built-in gearbox.              high torque and high speed at the        the motor) which allows you to
Care should be taken with the NPC                same time. It is a little too heavy for  adjust their phase timing. If the
T64 gears (shown in Figure 6, with               a middleweight. We ended up using        motor is used in the robot drive
red grease). They might break under              it in our spinner Ciclone but we had     system, it should have neutral
severe impacts if used to power                  to power it at only 24V because the      timing. In other words, it should
weapons. As recommended by the                   additional battery packs that would      spin with the same speed in both
manufacturer, only use them as drive             be needed to get to 48V would            directions, helping a tank steering
motors.                                          make the robot go over its 120 lb        robot to move straight. If it is used
     An excellent motor for driving              weight limit. The super-heavyweight      to power a weapon that only spins
middleweights is the Magmotor S28-               shell spinner Super Megabyte only        in one direction, you can advance
150 (a.k.a., Ampflow A28-150). It is             needs one of these motors                the timing to typically get a few
used in our robots Titan and Touro.              (powered at 48V) to spin up its          hundred extra RPM. (On the other
A good weapon motor for a                        heavy shell. A few daring builders       hand, in the other direction the
middleweight would be the
Magmotor S28-400 (a.k.a.,
Ampflow A28-400) with
                                      TABLE 1
higher torque and power,
which we use to power
Touro’s drum. Using a             Name                   Bosch GPA        Bosch GPB          D-Pack             DeWalt 18V
single S28-150 to power           Voltage (V)                 24               12        12 (nominal)                24
the weapon of a
                                  Poutput_max (W)           1,175             282            3,561                  946
middleweight is not a good
idea. There’s a good chance Weight (lb)                      8.4               3.3             7.7                   1.0
that it will overheat.            Power/Weight               140               85             462                   946
     Because of that, to spin I /I                            23               25              63                   128
                                   stall no_load
the bar of our
                                  Kt (N×m/A)                0.061            0.042           0.020                0.0085
middleweight Titan, we use
two Magmotor S28-150s             Kv (RPM/V)                 167              229             485                  1,100
mechanically connected in         Rmotor (W)                0.13             0.121          0.00969                0.072
parallel by acting on the         Ino_load (A)               8.0               3.9            19.6                   2.6
same gear of the weapon
     The D-Pack motor is a
good candidate to replace
the Magmotors, besides
being much cheaper.               Name                      Etek      Magmotor S28-150 Magmotor S28-400 NPC T64 (w/gearbox)
However, its electrical           Voltage (V)                 48               24              24                    24
resistance is so low that it      Poutput_max (W)          11,185            2,183           3,367                  834
almost shorts the batteries
                                  Weight (lb)               20.7               3.8             6.9                  13.0
and electronics. Because of
that, its current must be         Power/Weight               540              574             488                    64
limited if used with speed        Istall/Ino_load            526              110             127                    27
controllers. Otherwise,           Kt (N×m/A)                0.13            0.03757         0.0464                  0.86
there’s a good chance of
                                  Kv (RPM/V)                  72              254             206                    10
damaging the electronics,
especially since this motor is Rmotor (W)                   0.016            0.064           0.042                  0.16
usually overvolted to 24V         Ino_load (A)               5.7               3.4             4.5                   5.5

                                                                                                               SERVO 01.2011      33
                                                                           FIGURE 7   a featherweight can even drive a
                                                                                      lightweight. Other lightweight drive
                                                                                      options are 18V DeWalts connected
                                                                                      to custom-made gearboxes, such as
                                                                                      the DeWalt Powerdrive Kit or Team
                                                                                      Whyachi’s TWA69 gearbox with
                                                                                      Astroflight 990 Cobalt motors which
                                                                                      we use in Touro Light.
                                                                                           For middleweights, S28-150
                                                                                      Magmotors are usually a good
                                                                                      choice for the drive system,
                                                                                      connected (for instance) to Team
                                                                                      Whyachi’s famous TWM 3M
                                                                                      gearbox (pictured in Figure 8). The
                                                                                      S28-400 Magmotors are more
     motor speed would decrease.)            small gears can break due to the         appropriate for the drive system of
           To advance the timing, loosen     higher torques at 24V — we’ve            heavyweights and super
     the motor screws that hold its body,    broken quite a few 12V Pittmans          heavyweights, connected (for
     power it without loading its shaft,     after abusing them in battle at 24V.     example) to the TWM 3M gearbox
     and slightly rotate its body (where     The only way to know whether             (used in Touro Maximus) or to the
     the permanent magnets are               they’ll take the overvolting is by       stronger TWM3.
     attached to) until the measured         testing them. It’s also a good idea to        A good option for the drive
     Ino_load current is maximum, and        always have spare motors.                system of beetleweights used in
     then fasten the body back in place.          There are much better               Mini Touro is the Beetle B16
     For neutral timing, rotate the body     gearmotor options for hobbyweights       gearmotor (Figure 9), sold at The
     until Ino_load is identical when        and even heavier robots than the         Robot Marketplace (www.robot
     spinning in both directions.            ones from Pittman and Buehler,  For antweights
           Regarding hobbyweights (12        however, they usually need some          and fairyweights, the Sanyo 50
     lbs, about 5.4kg), a few inexpensive    modifications to get combat-ready.       micro geared motor (Figure 10) is
     gearmotor options for the drive         We’ve been using Integy Matrix Pro       a very popular choice.
     system are the ones from the            Lathe motors (Figure 7) for the drive         There are several other good
     manufacturers Pittman and Buehler,      system in our hobbyweights, I            brushed DC motors besides the
     which can be found in several junk      adapted to 36 mm BaneBots                ones presented above, not only for
     yards. Our hobbyweight drumbot          ( gearboxes             the drive system, but also to power
     Tourinho originally used (in 2006)      that were modified following Nick        the weapon. Brushless motors (to
     two Buehler gear motors (with           Martin’s recommendations that            be studied in a later article) have
     300 grams each, about 0.66 lb),         were described in the March ‘08          been successfully used as weapon
     and our hobbyweight wedge               edition of SERVO Magazine.               motors in several weight classes. It
     Puminha used four Pittmans (with        Recently, we’ve upgraded the             is useful to do research on which
     500 grams each, about 1.10 lb).         gearboxes to BaneBots’ P60.              motors have been successfully used
     We’ve bought used ones in Brazil for         A good combination for the          in combat. Several motors can be
     about US$10 to US$15 each (after        drive system of a featherweight is       found at The Robot Marketplace
     bargaining). Most of them have          the discontinued 42 mm BaneBots          and much more information can
     nominal voltage at 12V. However,        gearbox (or its improved version         be obtained in the RFL Forum
     we’ve used them at 24V for three        Magnum 775), connected to a 775-         (http://forums.delphiforums.
     minute matches without overheating      sized motor. For Touro Feather’s         com/therfl).
     problems. Remember that by              drivetrain, we’ve adapted 18V
     doubling the voltage, the power is      DeWalt motors to modified                Summary
     multiplied by four.                     Magnum 775 gearboxes with great
           The only problem is that the      results. This overkill combination for        I’ve managed to grossly simplify
                                                                                      this subject in this tutorial. Detailed
                                                                                      calculations are available for those
                                                                                      wanting to understand the math
                                                                                      behind these seemingly simple (but
                                                                                      actually amazingly complex) pieces
      FIGURE 8                    FIGURE 9                  FIGURE 10                 of bot technology. SV

34   SERVO 01.2011

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