Solenoid Control of Combat Robot Drive and Weapon Motors - Ask Aaron

Solenoid control can be a reasonable option for simple on/off control of a brushed weapon motor. For weapon or drive motors that must be able to reverse direction, an electronic speed controller costs very little more than a solenoid system of comparable capacity. An ESC is more reliable, lighter, less bulky, and provides much greater control.

On / Off Weapon Control with a Solenoid

Q: I want to activate a 24v NPC motor for a weapon using a 24v solenoid and a Battleswitch. How do I use the Battle Switch to power the Solenoid? A: Follow the diagram at the right. Use the 'SC' and 'S2' connections on the BattleSwitch and connect the output from the BattleSwitch to a small 'coil' connection on the solenoid. The receiver energizes the BattleSwitch relay, the BattleSwitch sends a small current to the solenoid coil, and the solenoid switches the large current to the weapon motor.

Q: I'm using a White Rodgers 24v 124-series SPDT solenoid to activate an AmpFlow motor for a weapon. How do the limits of the R/C switch I order need to compare to the capabilities of the solenoid? For example, can I use a 10 amp Battleswitch to activate my solenoid? Does '10 amp' refer to the current needed to switch the solenoid, or do I need a 100+ amp switch for the times I expect my motor to draw that much current? A: Go take a look at the data sheet for that solenoid. The power consumption of the 24 volt coil that activates the solenoid is listed at 12 watts, and that's all the power your R/C switch has to handle: that's 0.5 amp at 24 volts. Any R/C switch with at least that rating will do fine. If you use a solid-state switch like the little PicoSwitch, you'll need to install an antiparallel flyback diode [diagram at right] because the solenoid coil is an inductive load that create a voltage spike when turned off. An electro-mechanical relay switch like the 10A BattleSwitch doesn't need the diode -- it's more robust.

Q: Hey there, I forgot to design in a brake system for my middleweight's spinning weapon. (Rookie mistake, I know!). I haven't seen too much info on motor brake systems.. Are there any techniques you can suggest? I'm going to be activating my weapon motor with a DPDT solenoid; As of now the weapon is only designed to operate in one direction, so the opposite throw of my solenoid is unused. Is there some mechanical device I can hook up to it? Perhaps something that contacts the shaft to slow it? Thanks for the help. A: The most common spinner braking method is called dynamic braking, and it uses nothing more than the components you already have. Take a look at the diagram to see how to wire the DPDT solenoid. Connecting the motor leads together turns the motor into a generator and dissipates the rotational kinetic energy of the weapon by converting it back into electricity and then into heat as it passes thru the resistance of the motor armature. Dynamic braking works best when the weapon is spinning fast. The braking effect lessens as the weapon slows, but it is a simple and effective way to reduce spin-down time. We can discuss mechanical braking systems if dynamic braking isn't enough, but try it first.

R/C Switch Interface Selection

What do you recommend? Thank you.

A: Either option is workable to activate a low-current solenoid valve.

• The Pololu R/C switch with MOSFET is inexpensive, very compact, and has a built-in flyback diode to allow direct control of a solenoid. It does require a receiver cable and bit of soldering to hook up. See the wiring diagram at right. Good choice for a small combat robot.

• The Fingertech tinyESC would provide a forward/reverse current rather than on/off if plugged into a switched receiver output. You could get around that with transmitter programming, but a simple switch would be easier.

Forward / Off / Reverse Motor Control

Q: Hi Aaron!! I want to know whether the twin stick RC system can be used in conjunction with relays to operate a robot. Thanks. [Mumbai, India]

A: Yes, but there are several drawbacks. You will require a special R/C interface between the receiver and the relay to translate the R/C signal into an on/off current to control the each relay -- you can't just plug the relays into the receiver. If this is a fairly large robot, the relays (solenoids, contactors) needed to control the high current levels the motors require are expensive, heavy, and bulky. A standard tank-steer robot will require at least four relays and interfaces to provide forward/reverse/off/left/right control. A reverseable weapon motor will require two relays. Relays do not provide speed control. A robot controled by relays will be difficult to maneuver precisely and will be frustrating to operate. Although there are commercially available dual relay boards with a built-in R/C interface that can be used to control a single low-amperage motor forward/off/reverse, a dual channel electronic speed controller is more compact, lighter, more reliable, provides better control, and costs less than two relay boards of the same capacity.

Solenoid Selection

A: To have reversing capability, you'll need two DPDT solenoids rated close to 300 amp inrush current on all contacts (see diagram in post above). Too small an amp rating and the solenoid contacts can weld themselves shut! The Normally Closed (NC) contacts on DPDT solenoids are typically rated for less current than the Normally Open (NO) contacts, so check the current ratings carefully. A good high-power DPDT solenoid isn't cheap, and you need two of them plus two R/C switch interfaces to communicate with the receiver. You aren't going to save much (if any) money over a suitable speed controller.

A: We recommended the WR-586 for good reasons. The TW-C1 contactor has a couple of drawbacks:

• Fragility: the TW-C1 contactor body is made of a brittle material and does not withstand shock well. These contactors have been reported to fracture when the robot is hit hard -- even if shock mounted. The big White-Rodgers solenoids have a metal body and are much more shock resistant.

• Uncertain capacity: contactors almost always have lower current capacity on the Normally Open (NO) contacts than on the Normally Closed (NC) contacts. The specs for the TW-C1 contactor don't mention the capacity of the NO contacts. If you're using the contactor for single-direction on/off control this isn't an issue, but since you're using two of the contactors and all of the contacts for forward/off/reverse control, all contacts should be rated for the surge current capacity required to control the selected weapon motor.

A: You can read the engineering specs for the 124 series solenoid as well as I can. At 24 volts, the rated inrush current for the NC contacts is 100 amps, and your selected motor can pull more than 250 amps at startup. That solenoid may survive long enough for your purpose, but I don't recommend stressing a component that far beyond its rating. How many $85 solenoids are you willing to burn up before you decide the $110 solenoid is a better value?

Solenoid control of a motor is a reasonable option for single direction weapons. However, for a reverseable weapon an electronic speed controller is typically more reliable, more compact, lighter, and provides better control for about the same cost. If you are on a very tight budget, you may be better off to redesign for single direction Weapon operation, or perhaps select a smaller weapon motor.

A: The solenoid you found gives no specifications beyond '100 amp' -- there's no way for me to comment on its suitability to control an Ampflow E30-150. The peak 120 amp consumption of the E30-150 motor assumes that it is starting from a dead stop. If the motor/weapon is spinning in one direction and is being reversed without first bringing it to a stop, the potential peak current can be much greater and can continue for longer period of time. Use combat-proven components you know you can rely on.

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