Running a 4-DC motor Circuit

dc motormotorpcb

Currently I'm doing a project where I am simply trying to get a robot to move forward without code by using say a push button or a potentiometer. I plan on moving on to micro controller later, but right now I'm trying to work with my first perfboards and soldering irons. It has 4 DC motors, each with a free current of about .4A and a stall of 5A, but I don't really know how to hookup the circuit correctly. Each motor uses about 7 volts. I was wondering
a.) Is there any specific way I should construct motor
b.) What type of battery should I use to power the circuit saftely

Best Answer

I would start off with an old drill battery pack and maybe even the drill speed control. An old 9.6V Makita or DeWalt drill would work well.

The advantage of using a decently-sized battery pack from a battery drill is that you have both the battery and charger. These battery packs are generally 1.2 - 2.0 Amp-Hours and can supply really significant amounts of current for short periods of time.

You will need 8 wires going from your controller to the robot. These should be fairly beefy - I'm going to suggest 18 AWG if the cable is long (10 feet) or 20 AWG if the cable is short (5 feet or less). 8 conductor 20 AWG rotor cable works well and is inexpensive. Black polyethylene jacket, conductors have PVC insulation. Available from lots of places.

Your control box will have 4- DPDT center-off switches and a push-button. The DPDT switches are easy - you can use mini-toggle switches or full-size switches. The full-size switches can handle more current but the mini-toggles will be okay so long as you don't move them while the motors are energized.

The push-button is more of a problem. Most of the hobby-grade buttons can't handle any significant current. However, there are lots of automotive switches that are beefy and readily available. Head over to your friendly automotive parts store and ask the counter people for a rugged momentary push-button switch.

You will wire all 4 DPDT switches to have a reverse function. Power to all of the switches comes through the push-button from the drill battery pack.

Simple schematic diagram follows:

schematic

^{simulate this circuit – Schematic created using CircuitLab}

To use this, set all of the motor switches OFF except for the motors that you want to use. Then press the button for as long as needed. Let go of the button, change the motor switches, then press the button again.

Have fun!

Come back when you are ready to add some electronics to this.

Related Solutions

Electronic – Circuit design for controlling 6 individually selectable motors

Your initial thought is not bad: one H-bridge to determine the direction, and some kind of switch for each motor.
Forget triacs. They're only for AC, and besides will have a too high voltage drop at 6V power supply.
Relays would be the best thing, also because, thanks to the insulation between control and contacts, you can place them anywhere in a circuit. You'll just have to pay attention that this is not going to cost you more than a separate H-bridge for each motor.
MOSFETs are also often used to switch motors, but (especially power) MOSFETs have an internal diode which makes them unsuitable for bidirectional use, and besides, they're not symmetrical anyway.

So the relays seem to be the remaining option, apart from a series of H-bridges. Like I said you'll have to check if they're the most cost-effective solution.

Electronic – arduino – Motor speed control using arduino and quadrature encoders

It looks to me that your control loop is essentially:

if speed is too slow:
    increase PWM duty cycle one unit
if speed is too fast:
    descreate PWM duty cycle one unit

As you have observed, this doesn't work so well. Your control loop cyclically overshoots the target speed.

The canonical solution to this sort of problem is a PID controller. The concept is essentially the same, measure a thing and compare it to a target to calculate an error. Then adjust something (in your case, PWM duty cycle) based on the error.

However, a PID controller has three error terms:

P: the current error
I: the integral of the error
D: the derivative of the error

For each of these terms, the controller has programmed some gain. It then multiplies each term by the respective gain to calculate how much the control input (PWM duty cycle) should be changed. Properly tuned, this allows you to get a feedback loop that at first ramps up the duty cycle, then as your vehicle approaches the target speed, backs off smoothly so you get a minimum of overshoot.

Best Answer

Related Solutions

Electronic – Circuit design for controlling 6 individually selectable motors

Electronic – arduino – Motor speed control using arduino and quadrature encoders

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