Since we now have a pretty good idea of the amount of current our motors will need (if not...then check out the Motors and Batteries pages), we need some way to be able to control the speed of the motors. In the early days of combat robotics, many builders controlled the motors in their bots using combinations of relays, solenoids, switches and servomotors to control the power going to the motors. The use of servo motors with electrical switches is call “Servo Switching”, this allowed builders to use high current motors but these systems can be un-reliable, complex and only allowed motors to be run in full forward, stop or full reverse. Other builders used electronic speed controllers from radio-controlled cars. This provided the ability to be able to vary the speed of motors but lacked current capacity. Now most combat robots use Electronic Speed Controllers made specifically for the rigors of combat robotics, however some builders still use “servo switching” to control motors that need only be turned off and on.
Electronic speed controllers for combat robots use a method of speed control called Pulse Width Modulation. Instead of varying the voltage to control the speed of the motor, short pulses of electricity are sent to the motor to control how fast it runs. Switching the power off and on thousands of times per second allows for smooth precise control of the robot and has given robot builders Electronic Speed Controllers that can handle the high current needed to build powerful combat robots.
Most electronic speed controllers are very easy to hook up in your combat robot.
Innovation First (IFI) (Victor and Thor speed controllers) are very simple to wire up. These are a single channel speed controller. You would need one for each motor that you plan to use. You can use one of these to control multiple motors such as you might have them on one side of a 4 wheel skid steer bot, if they are connected in series and the total current required by the motors does not exceed the capacity of the speed controller. Because these speed controllers come from Innovation first and were originally designed for use with Innovation First control systems. To use these with conventional Radio Control systems you will need the Pulse Width Modulation Booster cable usually supplied with the controller. To use this controller the motors simply wire up to one side, your batteries connect to the other side, and the PWM cable plugs into the controller and the other end plugs into the receiver for your radio equipment. If you select the controller with the optional cooling fan, you will need to solder on a couple of small connectors and hook the fan up to the battery side of the controller. In fact if you do buy a speed controller without a fan….add one. An inexpensive cooling fan will reduce the possibility of overheating and damaging the controller.
You can then test the system with your radio transmitter. If the motor is running backwards when you press the joystick forward, simply reverse the wires from the motor. You will need to be careful about hooking up the batteries to the speed controller as hooking up the batteries backwards can destroy the speed controller. You can get the IFI Victor speed controllers in different models with different current ratings. Even though these speed controllers have proven to be very rugged it is always a good idea to shock mount electronics. Even better is to place electronic components into an aluminum or polycarbonate case to provide additional protection. The advantage of these speed controllers is the advantage of set up, the disadvantage is that if you want to run very high currents you may need to go another manufacturer of electronic speed controllers.
Vantec produces a line of speed controller for robotic combat that were originally built for other applications. Almost as easy to set up and install at the IFI models of speed controllers but the high current capacity models are dual channel. This means one unit will handle the motors for both sides of your combat robot. It may make for a simpler installation, but if the unit becomes damaged you’ll need to replace the whole unit and not just one speed controller.
Another line of speed controllers is available from Robot Power. Robot power developed a line of speed controllers specifically for robotic combat.
Robot Power speed controllers were developed as part of the Open Source Motor Control (OSMC) project. If you’re interested in electronics, all the information about the speed controllers is available. The cost compared to the current carrying capacity makes these the least expensive per amp of what is available. These speed controllers come as component systems so you may find that you will need to spend more time figuring out exactly what you need and spend more time setting things up and installing the components. Besides cost, these speed controllers also have additional features such as adjustable motor breaking, a flip control that reverses your controls if your bot is inverted, and an auxiliary control that can be used to control a weapons system. These are features that you usually won’t find on the other speed controllers, but you have to spend some time working your way through the owners manual to fine tune the set-up. The components come as bare boards that plug into each other so this means that you will need to build some sort of case to protect them, but on the other hand you can set out the components to fit your bot.
4QD produces a line of speed controllers that offer a built
in “fail safe” circuits that will bring your bot to a stop in the event that
radio signal is lost. This is a line of speed controllers designed to handle the
high current requirements of some bots and like the IFI speed controllers come
as single channel units so that you will need to purchase one for each motor you
need to control. You will also need to purchase the interface cable to be able
to hook these units up to your radio control receiver.
Robotec is another high current capacity dual channel speed
controller. This speed controller however allows the user to hook the controller
up to a computer and change a variety of operating parameters such as current
limiting which will reduce the possibility of burning up small motors. These
speed controllers come in a fully enclosed case to prevent chunks of debris from
shorting out electronics.
Robowars IBC is a speed controller that Robowars calls an
Integrated Bot Controller. This is another bare board controller that plugs
directly into your RC receiver for easy set up. This is also a dual channel
speed controller with an auxiliary channel for controlling your weapon system.
This controller allows only 50 amps per channel, for bigger combat robots you
might find that this controller comes up a bit shy, but for smaller bots this is
a less expensive option than buying individual speed controllers for each motor.
There are many different speed controllers out there, some have high current capacity, some require a interface board or cable, some with multiple channels and functions. The important thing for the first time builder who is afraid of electronics is to find a speed controller that they are comfortable with. First time builders are usually surprised how easy the electronic components are to work with. In many cases the manufactures have the installation manual available online. You can read the manual to see if it’s within your abilities. Once you’ve read the installation instructions carefully and thoroughly and you’ll be well on your way.
Of course not all motors need to have a speed controller, we’re referring to weapon motors and motors that perform other functions such as operating a self-righting mechanism. You can in fact have a few options to control these. Servo switching is still used by some builders to control weapon motors etc. The idea here is to use the small servo motors that usually come with the radio system that you bough to turn a switch off and on and thus turn a motor, solenoid or actuator off or on. The problem is that if you want to control something that requires more than just a few amps of current you will need to add a solenoid to the system and thus the solenoid is activated by the switch and provides the necessary power to the item being controlled.
You will have to be careful to ensure that the servo motor will “fail safe”, we will talk about “fail safes” a little later on. Also, servo motors can be a bit sensitive to severe jarring and jolts, so you will need to ensure that you have adequately shock mounted the components to be sure they will not fail. Fabricating a rugged servo switch system can be more challenging that would first appear and technical inspectors at events will scrutinize your servo switch closely to make sure that it will operate safely.
An option to the servo switching system is to use a spike relay module which is an electronic version of the servo switch. Again, if you plan to turn on components that require more than a few amps, you will need to add a solenoid to the system. There are also items called R/C relay switches that in effect perform the same function. The R/C relay switches and spike relay modules can both be found in models that can handle up to 20 amps. Which is fine for controlling smaller motors but may not be enough for a large weapon motor.
So how to we switch a weapon motor off and on? Now we add a solenoid to the system. Whether we use a servo switch, R/C relay switch or control module doesn’t really matter to the solenoid. The solenoid is in effect another switch only more capable of handling the high current demands of larger weapon motors. The most commonly used solenoids are referred to as SPDT or Single Pole Double Throw. The trick being used here is that when the solenoid is activated, electricity flows from the batteries to the motor through the Solenoid. When the solenoid is deactivated, the solenoid has a second switch position that shorts out the electric motor and forces it to slow down instead of coasting freely. The obvious question here is “Why is this important?” Most events will require that if you are employing a spinning weapon on your bot, the weapon must come to a complete stop in 60 seconds after you turn it off. At some events this time has been reduced to 30 seconds, so you will need to check the technical regulations of the events you plan to attend to make sure you can pass this requirement. If you were to allow the motor to spin freely then you may have to design and fabricate some sort of mechanical break that automatically engaged when the power is turned off or if the robot looses radio contact.
