 # Controlling a 12V/3.4A DC Motor with R3-Motor Shield

Hello

I'm a computer scientist and roboticist (rather on the software side) and new to the Arduino World. In my current project I want to slowly (10rpm) rotate a large disc (D=2.5m) and our engineer says we need a torque of about 2Nm. Since I use a Leonardo to interface with two sensors, I thought about using a motor shield to talk to the motor so that I could also run a control loop on the arduino. My current plan is to use a DC motor, although I'm not sure if a step motor would be a better choice, so please tell me what you would use.

To be more specific, I want to control this motor: http://www.conrad.com/ce/en/product/198486/DOGA-DO31627612H003120-12V-DC-Motor-38RPM-2NM with this shield: http://arduino.cc/en/Main/ArduinoMotorShieldR3

My problem is this: "Max current 2A per channel or 4A max (with external power supply)"

Can I connect both outputs to the DC motor? I found some threads to this topic but only found the answer "Could be possible, depends on shield and motor" so I hope to get a definite answer. Or should I just use a step motor?

Cheers, Nikolas

The spec's for that motor don't make sense - the speed and torque come to 8W, the electrical input is 40W.

Could you maybe explain how you computed this values? (Or give me a link where I such computations are explained?

Can I connect both outputs to the DC motor?

The device allows parallel connection according to the specs: www.sparkfun.com/datasheets/Robotics/L298_H_Bridge.pdf See Figure 7.

should I just use a step motor

Stepper motors allow easy speed control. If you need a stable or precise speed the DC motor requires a feed back sensor such as a hall sensor or an encoding disk. If you only need an approximate speed adjusting the pwm duty cycle is sufficient. If the continuous current is near the device limit you may get thermal cycling. The disk will speedup and slow down as the device protects itself from overheating.

What the heck. Give it a try and see what happens :)

Thank you :)

I think I'll try it with the DC (I already have sensors that show me the effect of the motor so I probably won't need the accuracy of a step motor). Unfortunately, I haven't found a wiring example for combining the channels, so I would be happy if you could have a look:

I found an intructable on using two DC Motors: http://www.instructables.com/id/Arduino-Motor-Shield-Tutorial/step5/Two-Motors/

Is it correct to just join the two black and two red wires and connect them to my motor?

And how do I control this motor? For example, the Direction can be set using D12 and D13, is it enough to execute two digialwrite-commands without any delay in between or do I have to physically remove one connection to the Arduino (e.g. D12) and connect D12 to D13 with a wire and only write on D13 so that the command is synchronized?

And how do I control this motor? For example, the Direction can be set using D12 and D13, is it enough to execute two digialwrite-commands without any delay in between or do I have to physically remove one connection to the Arduino (e.g. D12) and connect D12 to D13 with a wire and only write on D13 so that the command is synchronized?

Yes common control as well as common output. Common direction and also only one PWM signal. Otherwise a software bug could lead to the well known "smoke test".

Ok, Thanks : )

FooTheBar: Could you maybe explain how you computed this values? (Or give me a link where I such computations are explained?

Basic mechanics:

Power = torque x angular velocity (this is just like power = force x linear velocity).

Using SI units (newton-metres, watts, radians/second) means no constant factor is involved.

So you convert rpm to rad/s (multiply by pi, divide by 30), torque units to newton-metres (people seem to use all sorts of arcane units for torque, like ounce-force-inches, kg-force-cm, its a nightmare).

Electrical power in = 12V x 3.4A = 41W, mechanical power out = 2 Nm x 3.98 rad/s = 8W, efficiency = 20%, which is suspiciously low - perhaps just a poor quality gear train?

Thank you, now it's a bit clearer. I learned all this stuff in my undergrad physics lectures too many years ago, but it rang a bell..