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R/C Combat Vehicles

Speed Controls

This article was originally written by Marty The Legend Hayes for the R/C Warship Combat hobby. It has been edited and revised by Frank Pittelli for the R/C Tank Combat hobby, with the H-Bridge Speed Control section contributed by Joe Sommer.


Speed controls come in many sizes, shapes and prices. What is best for you is for you to decide. Items which should be included in your decision are such things as your skill, the size of your pocket book, availability of materials, the amount of current used by your motors and the size of the ship you are going to put it in. The choices run between the simplest toggle switch or brass contact type to the most expensive transistor electronic throttle.

In direct current (battery power), the motors turn in one direction when the polarity (+ and -) is applied to the contacts and the other direction when the polarity is reversed on the contacts. Motors are "tuned" to prefer one direction and will have a small power loss when running in the opposite direction (loss is small, and can be ignored.) The motor reacts to the voltage applied in a direct manner, as more voltage is applied the faster and more powerfully the motor turns. The two problems which will normally damage a motor are heat build up and too much current for the size of the wire which the motor is wound with. Both of these have to do with the application of too much voltage. The way to more power is a larger motor not more voltage past the limits of the motor windings. The amount of current drawn from your batteries and through your speed control is decided by the motors through their winding size and number. Cheap motors generally use more current (amperage) to produce the same power. Choose your speed control to carry the amount of current your motors require plus 50%.

How much control you need over your actual speed is again a matter of personal preference. The speeds which you use in combat is somewhat dependent on your style of combat. Many people use very little other than full speed, some use many different speeds, some run at medium speed most of the time to conserve energy and have full speed to use in emergencies or for the chase.

Simple Toggle Control

The one of the simplest speed controls is the toggle switch type. This is a simplistic design, requiring only a servo, the toggle switch (double pole, double throw, center position off) and a connecting rod. The wiring of the switch is such that the voltage is reversed as the switch is moved between it's extreme positions; the voltage is turned off in the center position. One of the problems with this design is, you only have full speed (forward/reverse) and off, no medium or slow speeds. Another problem which I have observed is the selection of the center position is sometimes difficult by radio control. All in all, this design is somewhat a bull in a china shop type of control.

Drawing of Toggle Switch Speed Control

A similar design to the toggle switch uses a small slide switch with four positions giving two forward speeds and one reverse. Problems with this were occasionally the servo would over-travel and hang the switch up at one end of it's travel which may or may not still be making contact and running the engines. The voltages for the slide switch throttle were pure (not using any resistors) but using different battery contacts to produce the different voltages required.

Another similar design used a multiple position rotary switch (Radio Shack) and operated either with resistors or different voltage pickups. The problems are similar to the previous designs in travel overruns and a limited choice of speeds. Advantages are cheap and easy construction. Some battlers use the overrun feature of the servo trim to activate other controls (pumps in his case). This is accomplished by the use of a Radio Shack on/off switch which turns on when it is pushed once and turns off when it is pushed again. The switch is located so that the control arm contacts it only when the servo control is brought to the end of it's travel and the trim is brought to maximum also.

Maryland Attack Group (MAG) Speed Control

Many of the mechanical short-comings of the toggle switch can be overcome by using two micro-switches in an arrangement known as the MAG Speed Control. (Technically speaking, the MAG speed control is the same as an H-bridge implemented using two switches. Nonetheless, it was first used in R/C combat by the Maryland Attack Group, so the name has stuck.) The two micro-switches are mounted on a servo with a "half moon" cam such that neither are activated at neutral position. The switches are wired with the plus voltage on the N.O. position (normally open) and the minus (or negative) on the N.C. position. The motors are hooked up to the common positions such that one common goes to one side of the motor and the common of the other micro switch goes to the other. In the neutral position, the negative (minus) voltage is applied to both sides of the motor and the motor will not run. Activating either switches removes the negative and applies the positive to the motor contact connected causing the motor to run in that direction.

Drawing of Maryland Attack Group Speed Control

This throttle provides full-forward and full-reverse with no mechanical linkages and is very compact. Unlike the simple toggle throttle, however, this approach creates a dead short across the motor leads in the neutral position. In many situations, this is a desirable feature, because it causes the motor to stop spinning immediately after the servo returns to neutral. That is, the throttle acts as an electronic brake for the motor. When used in warships, this causes the prop to immediately stop spinning, instead of slowing winding down, which causes the boat to stop faster. For land vehicles, the electronic brake locks up the motor, which facilitates skid steering.

Because of the dead short across the motor, it is essential that the micro switches used are not only rated for the continuous amp draw expected, but also for the current spike generated when shorting the spinning motor. Fortunately, micro-switches that can handle 15-20 amps are readily available and are very inexpensive. It is also possible to use multiple micro-switches to handle higher loads, provided the servo cam is designed accordingly.

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Here is an example of a typical implementation of the MAG speed control, with the micro-switches mounted on either side of a servo. The switches and servo are mounted independent to a brass bracket so that either can be replaced easily in the field. This particular speed control served many years inside of a remote controlled goose decoy that saw plenty of salt water action without a failure.

Another variation on the MAG speed control provides multiple speeds by mounting multiple micro-switches on each side of the servo, positioned at different points around the circumference of the servo cam. In that way, as the servo advances in one direction it trips each switch in succession. If the first switch operates as a normal MAG speed control, then the next switch can be used to boost the voltage, say from 6 to 12 volts. Such a two-setting speed control is quite useful for operating a vehicle at a slow speed for normal traveling, while providing a fast speed for emergency or critical operations.

A different layout for the MAG speed control clearly shows the simple wiring design. The positive (red) and negative (black) wires from the battery are connected to the normally-open (NO) and normally-closed (NC) poles of each switch, and the motor control wires (brown) are connected to the common pole of each switch. In this case, all connections are made with quick connectors so that a faulty switch can be replaced without soldering.

This layout also shows how a small servo cam can be used to actuate the push-button on each switch, without the need for any rocker arm or roller. This increases the reliability of the speed control (less moving parts) and makes for a compact arrangement. Furthermore, by mounting the switches above the servo, everything is easier to reach when testing or repairing.

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The MAG speed control is a very flexible arrangement that can be configured in a number of different ways. One such variation is to use two high-amp relays as the main throttle, wired exactly like the micro-switches would be wired. Then, two low-amp micro-switches are operated by the servo to trip each of the relays. Such an arrangement allows all of the high-amp motor wires to kept separate from the radio equipment, which can help to reduce radio interference. It also allows the motor to be controlled without a radio from a test box, by using a simple toggle switch to activate each relay.
The picture shows a dual MAG speed control implemented with 20amp automotive relays that is used to control two independent tracks. The relays are installed in sockets that come with all of the wires attached (can't beat that for $1.29) The battery is connected using the red and black wires on the bottom. The common ground (black) is connected to the normally-closed (NC) pole on each relay and to one side of each relay coil. The positive wire (red) is connected to the normally-open (NO) pole on each relay and through 4 individual switches (not shown) to the other side of the relay coil via the white wires. Finally, the yellow wires are connected to the common pole on each relay and are used to drive the motors.

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Tri-Pact Speed Control

The Tri-Pact Speed Control combines two relay-based MAG Speed Controls and a simple diode circuit to provide a speed control that drives two motors using a single transmitter joystick in a natural manner. Specifically, the Tri-Pact Speed Control provides the following motor controls based on the position of the joystick:

Joystick Position Left Motor Right Motor
Center Locked Locked
Up Forward Forward
Down Reverse Reverse
Right Forward Reverse
Left Reverse Forward
Upper Right Forward Locked
Upper Left Locked Forward
Lower Right Locked Reverse
Lower Left Reverse Locked
The Tri-Pact Speed Control requires a small number of relatively inexpensive parts that are easily obtained in any auto parts or electronics store, and can be built with nothing more than a set of wire strippers and a soldering iron. For example, the following parts list could be used to acquire all of the necessary components from All Electronics (last updated 8/11/2007):
Component Part Number Quantity Each Cost
12 VDC SPDT 30 AMP AUTO POWER RELAY RLY-351 4 $2.40 $9.60
SPST N.O. SNAP-ACTION SWITCH SMS-167 4 $0.50 $2.00
3AMP/1000 PIV DIODE 1N5408 12 $0.33 $4.00
      TOTAL $23.60

H-Bridge Speed Control

One of the most common circuits used for controlling the direction of a motor is known as an H-bridge (named because the circuit diagram looks like the letter 'H'). H-bridge controllers can be built using a variety of components, including mechanical Single-Pole-Single-Throw (SPST) relays, electronic transitors, power MOSFETs or Solid State Relays (SSRs). If mechanical relays are used, an H-bridge circuit provides essentially the same full-forward/full-reverse (on-off) speed control as provided by the MAG speed control. On the other hand, if the H-bridge is implemented using any of the electronic component types (transitors, MOSFETs or SSRs), then it can also be used to provide variable-speed control in both directions (see Electronic Speed Controls below for more details). This is one of the key benefits of an H-bridge speed control, but it also makes it more complex to build.

Relay-Based H-Bridge

An H-bridge using four SPST relays is shown below to demonstrate the basic operation of the circuit. When relays A and D are closed at the same time, current passes through the motor causing CW rotation. When relays B and C are closed at the same time, current passes through the motor in the opposite direction causing CCW rotation. Closing relays A and B at the same time causes a dead short across the motor to provide electrical braking. If all four relays are left open, the motor can rotate freely.

Be careful when using H-bridges. Accidentally closing relays A and C at the same time, or closing relays B and D at the same time, causes a dead short across the battery which could ruin the batteries and/or start a fire. Additional logic circuits are often used to prevent these catastrophic conditions.

Transitor-Based H-Bridge

Equivalent H-bridges using electronic transistors and power MOSFETs are shown here:
A transitor is basically an electronic switch that allows current to flow from one pole to another when a small control current is applied to another pole. A power MOSFET is a special kind of transitor that is capable of dumping large amounts of current. Two MOSFETs and a couple electronic components can be made to act exactly like a single relay, so four power MOSFETs can be used to create a speed control.

Surge suppression diodes should be connected across the transistors or MOSFETs to prevent damage caused by sudden motor stall or reversal. Amplifier chips (not shown) are often required to bias base or gate control voltages high enough to activate the transistors or MOSFETs in the upper legs.

SSR-Based H-Bridge

Solid State Relays (SSRs), as the name implies, are the solid-state equivalent to mechanical SPST relays as shown here:
They contain MOSFETs controlled by internal LEDs to provide optical isolation between inputs and battery connections. Inputs can range from 3 to 24 VDC and draw about 1 mA/V. Consequently, SSRs are particularly easy to interface to TTL digital logic (5 VDC) and microprocessors. SSRs also usually contain internal protection circuits for current limiting and surge suppression.
Two H-bridges using eight Crydom 100V, 20A SSRs are shown here.

Electronic Speed Control (ESC)

If an electronic H-bridge is used to control the direction of a motor, then it can also be used as an Electronic Speed Control (ESC) to vary the speed of that motor using an approach called Pulse Width Modulation (PWM). PWM turns the motor on and off rapidly (thousands of times per second) so that it is only powered a certain percentage of the time, called the duty cycle, as depicted here:
Motor speed is roughly proportional to the duty cycle if the modulation frequency is significantly higher than the motor's frequency response. (That is, you need to turn the motor on and off fast enough so that the motor doesn't "see" the changes, just like your eyes don't see a television picture refreshing 30 times per second.) For CW rotation, H-bridge legs A and D may be modulated simultaneously. Alternately, leg D may be turned on at 100% duty cycle while leg A is modulated.

ESCs are usually implemented by a micro-processor (such as a Basic Stamp or PICAXE) that varies the duty cycle to the H-bridges based on the position of a mechanical dial (a potentiometer) or by plugging directly into a radio receiver. In fact, the micro-processor can be programmed to read two separate servo signals from the radio receiver and vary the speed and direction of two motors, providing variable speed and turning capabilities. More advanced programming can also be used to ramp-up and ramp-down the motors smoothly to reduce the dynamic loads on the system when they are suddenly started or reversed.

Low-amp ESCs are readily available from a variety of manufacturers for a variety of purposes, such as R/C race cars or ships. Unfortunately, such low-amp controllers will quickly fail when attempting to drive the high-amp motors typically required by large-scale R/C tanks. Furthermore, such controllers are typically intended for less than 12v. Thanks to the robot warriors, high-amp/high-voltage ESCs are also available from a range of commercial manufacturers, but they are also more expensive.

Of course, if you're proficient at micro-processor programming or know someone who is, then you can build your own ESC using any of the electronic H-bridge designs shown above. Be prepared, however, to spend an appropriate amount of time first bench-testing and then field-testing your design before you consider the task completed. Often times, the cost of a commercial speed control starts looking much better after you're home-grown design has failed a couple of times. (Now you know why the simple, relay-based speed controls described above are widely used in the hobby.)

The advantages of an ESC are:

  1. they provide variable speeds,
  2. they connect directly to your radio receiver, and
  3. they are easy to install.
But, when it comes to battling, there are some key disadvantages that must be considered as well, including:
  1. high-amp ESCs are relatively expensive,
  2. high-amp ESCs are complex to build,
  3. they require heat-sinks or fans, and
  4. they cannot be easily repaired.
Although many of the disadvantages can be overlooked, the last one is particularly disturbing in the R/C Tank Combat world. If your ESC fails during one battle, there is little chance that you'll be able to participate in any more battles until you plop down more money and wait for another to arrive in the mail.

Other Issues

Here are a couple of suggestions that may save you some time and effort when using speed controls:

Power Surges
One thing to consider is the use of fuses in your control system. Fuses are devices which burn out at a specified current (amperage) and open the circuit to whatever device is on the other side of the connection. Many people use a fuse for each of the motors in their vehicle. If the motor jams up or is shorted out for any reason, the fuse is the item which fails first (hopefully) saving a possible fire or enormous drain on your batteries. A single fuse on your battery pack will also work but if it burns out, then your vehicle will be left sitting on the field without any power (this is not an advantageous position from which to do combat).

In general, it is best to use a fuse that is double your expected total amp draw. That way, it won't trip during even heavy usage, but it will trip if a dead short develops that would otherwise destroy your wiring, batteries and possibly your vehicle if a fire occurs.

Radio Interference
Another thing to consider on your drive system is the use of capacitors on each motor to reduce the radio interference that is generated by them. Twisted wires also cut down on the EMI (electrical magnetic interference) as does shielded wiring (ground shield to the negative lead of the supply batteries). Also, do not run your power leads for your motors close to your antenna lead in and be cautious in running high current flow wiring anywhere near your radio receiver and servo wiring, because the current flowing through one wire can cause electrical interference on the other wire. In extreme cases of radio interference, power the motors from relays located near the motors operated by switches in your radio box thus keeping the spikes far from your receiver.

To reduce radio interference when using relays, you should solder a diode in a backwards orientation across each relay coil. This helps to dissipate the electric pulse that is generated when the coil field collapses. To see just how strong such a pulse can be, simply turn a relay on and off near a television set and you'll see a burst of static across the screen whenever the relay is turned off. With a properly installed diode, no such burst will be seen.

The Right Stuff
Speed controls should offer reliability above everything else, a sitting duck on the battle field is soon a dead turkey. The second consideration is power drain since there is only a limited amount of power available. If you can move on the battlefield longer than the next guy, who will win?

Finally, practical experience has shown that a small number of speed ranges are sufficient during a battle. So, before you buy that fancy, expensive, proportional electronic speed control consider what your state of mind will be when (a) you are chasing someone or (b) someone is chasing you; Will the ability to travel at 56.3% of your maximum speed really matter much?