Robotic Arm Add-on Pack for Starter Robot Kit

#1

I am considering buying the Robotic Arm Add-on Pack for the Starter Robot Kit, which comes with the DC Motor 3712V/50RPM, and the 37mm motor is rated at 12v and in the instructions it is connected to the 6AA battery pack that only puts out 9v for both the orion board and the motors on the starter kit. how will the 6AA battery power pack also power the 12v motor on the robotic arm?

#2

Most DC motors can run under the rated voltage no problem. There is usually a lower end of voltage that will be required to turn the motor since the internal friction and resistance must be overcome but you can easily vary the voltage to get different speeds out of a motor. So From the specs of this motor is shows a 90:1 reduction gearbox and a 12v no-load speed (assuming after the gearbox) of 50 rpm (revolutions per minute).
50x90= 4500 RPM @ 12 v
4500/12=375Kv (RPM/Volt)
Theoretically the motor (before the gearbox) will run at 375 RPM at 1 Volt (I’ll assume that it wont actually turn at such a low voltage but who knows)
So if you give it 9v instead of 12v you will get 9/12 of the output 50 RPM which should be 37.5 RPM
I would also assume you can safely run this down to 6v without too much trouble but only testing will tell ya for sure.

#3

true but the battery’s are running the Orion and the motors on the track vehicle too. i would up the voltage but the 6v motors would have a shorter life

#4

One other thing I should mention is that the batteries you use can have a big impact as the to actual current you can pull out of it and therefore the torque you can expect from the motor. For a crane you want slow speed (you have that) and loads of torque. To get more torque you can change the alkaline AA batteries for NiMH AA batteries. You will lose a little voltage since NiMH or NiCd batteries will come in at 1.2 volts per cell and alkalines give you 1.5. On a 6 cell pack you get only 7.2v versus 9v so you can safely add cells without much risk. A 8 cell pack will have 9.6v and a lot more current capability. Lithium Polymer cells are actually a bit better and there is already a port on the Orion just for that.

#5

I use NMHD battery’s they rated at 1.4v @2300 mAh so they last longer than alkaline battery’s that is 8.4 v I have a LIPO a battery’s it is 11.1v @5000mAh but it is 60USD and it is being used for my RC plane. and I am not going the spend that kind of money for a 120USD robot plus I would not use the onboard charger and if you discharge the LIPO battery you will ruin the battery

#6

Typical Alkaline AA batteries can have up to 2500mAh but are terrible at putting out current so for low current activities it is hard to beat regular ol alkalines. Rechargables (NiMH and NiCD have a long discharge curve where the voltage drops to 1.2 pretty darn fast and then flatten out for a while then drop off rapidly. This site has sold old data but the curves are quite telling.
Essentially for robotics which use high current draw motors everyone should switch to rechargable batteries since you will realize much more power in the motors.

#7

On a full charge my batterys go up to 1.48 v and they stay around at around 1.38 v for a long time I have checked this out on a mutimeter .and nicad batterys do not hold a charge very long i have useh both nicad and NMHD battery’s in the flash of my camera and the nicad’s take for ever to recharge the capacitor in my flash and NMHD are much faster.

#8

That is the no load voltage. NiMH will drop to 1.2 or near there after load. We are talking about inductive load like a motor. If you fly R/C the load is with the prop on in wind like that which the plane experiences meaning in wind and not stalled. The load of the motors in a crane is going to be dependent on the construction and load you put on it and that is what load you should be measuring with. Alkalines have much higher voltage sag due to their internal resistance and are therfore no good at high load requirements but you will see a voltage drop on NiMH batteries as well as all batteries when under load. I recommend getting in inline current and volt meter for both your robot motor and R/C airplane motor applications so you can size the speed control, wiring, and battery packs properly when hacking new things.

You can see in the last chart on the page I referred to earlier that the voltage starts near 1.4 but under various loads it goes from 1.3-1.0v.

#9

no that is with a load the 1.48 is no load voltage. Nicad’s are only rated at 1.2v period

#10

By the way AA Duracell Alkaline batteries only have 2000mAh not 2500 and Duracell batteries are one of the best. Not typical.

#11