Hello,
Can I use a LIPO Battery 14,8 V 4000 mAh 3SP1 for Dagu 6WD (6 motors at 7,2 V)
using a Dual G2 18v18 for raspberry pi, servomotor?
or I strictly need to use a 7,2 V LIPO batterry? what xxxx mAh si recommandable in this case?
The LIPO battery goes directly to the Dual G2 for raspberry pi servo, but i couldn’t find an indication what is the voltage output for the dual motors…
I’m afraid not to burn the Dagu motors…
Thanks a for a quick reply,
Hello, alexm.
Your battery voltage is fine to use with the Dual G2 High-Power Motor Driver 18v18 for Raspberry Pi. As far as the motors, it is generally fine to run brushed DC motors at voltages slightly higher than what they are rated for; doing so results in higher speeds and torque at the cost of some lifetime. However, at twice the intended voltage, those motors will likely get damaged very quickly, so to help avoid this, you can limit the duty cycle from the G2 High-Power Motor Driver to around 50% (effectively bringing the voltage to the motor down to its rated voltage).
Brandon
First, I want to thank you a lot, for your help.
My next questions are :
how “to limit the duty cycle” (what does it means?)
You mean to limit the MAX_SPEED that can be passed to the motors, where 50% means MAX_SPEED / 2 ? (MAX_SPEED is conventionally set at 480 in your G2 python library. By the way do you have a C library too, for dual G2 motors for raspberry?)
Is a direct link between the voltage that is passed to the motors and the speed?
Because it seems that the servo passed the full voltage at maximum speed , isn’t it?
Or is there another way to do this?
And if I limit somehow the “duty cycle” to exactly 50% (based on your answer to 1.), I will not lose the “slightly higher” better option, that you have indicated to me?
Also “slightly higher” than nominal voltage of the motors means what ? when we choose a LIPO battery? Because if we are using LIPO batteries they go like this: 3,7V 7,4V 11,1V 14,8V etc …
regarding the amperage:
What is the nominal needed current intensity for the six motors at maximum speed?
max. 6.6 A per motor x 6 motors ?..or 6.6 A per motor x 3 motors being a dual G2 servo…this computation is not clear enough for me, because if I have only 18Vx18A. as a G2 dual servo, are the dual G2 18x18 good enough (anyway, now I have it), or would it have been better to buy 18x24 for this configuration? Is 18x24 good enough, too?
I mean. How long the G2 servo 18x18 can maintain the max. 6.6 A per motor x 6 motors ?. (…or 6.6 A per motor x 3 motors?)
What about the needed mAh of the LIPO battery in relation with the above computation ?
what about a recommendation for the LIPO batteries mAh for this configuration Dagu 6WD + dual G2 for rpi → is good to have 2200 mAh x 30C or 4000 mAh x 25C, etc
(will be a real help if your site will include in the future some clear recommendations for LIPO batteries for different combinations of servos + motors. at least some recommended combinations)
Thanks again for your answers…it will help me to better understand the right configuration.
1.Yes, the G2 High-Power Motor Driver controls the speed of the motor by outputting a PWM with the specified duty cycle. At full speed (i.e. 100% duty cycle) the output is essentially the full VIN voltage. By limiting the duty cycle to 50%, the motor will effectively run as if it were receiving 50% of the VIN voltage. We do not have a C library for the Raspberry Pi. As you mentioned, the Python library accepts speed values between -480 and 480, so to limit the duty cycle to 50%, you would make sure your program only sent speeds of -240 to 240.
2 & 3. When I said “slightly higher”, that was in relation to the motor’s rated voltage (not your battery’s nominal voltage). Those motors are intended for a maximum nominal operating voltage of 7.2V, so I would recommend keeping your effective operating voltage around 7.5V to stay within that. A 4S LiPo can be between 12.8V and 16.8V depending on its charge, so around 50% duty cycle is probably fine. You can certainly increase the duty cycle to something like 60% to get higher speeds and more torque, but as I mentioned in my previous post, doing so will likely cause your motor’s use-life to decrease faster.
4.We typically recommend choosing a motor driver that can handle the combined stall current of your motors on each channel continuously. For example, the Dagu Wild Thumper motors have a 6.6A stall (at 7.2V) and with the 6WD version, you would have 3 motors per channel. So, ideally, you would have a motor driver that can handle around 19.8A continuously. However, because of the geometry of the Wild Thumper, it is very unlikely that all 3 motors on one side would be stalled at the same time and the G2 High-Power Motor Driver 18v18 can handle brief spikes much greater than 18A, so it should be fine. Please note that stall current increases proportionally with operating voltage, so you should consider that if you try using higher duty cycles.
5 & 6. A battery’s capacity (i.e. the mAh rating) is essentially how much energy the battery can store. The main difference between using a battery with a higher capacity would be that it can go longer between charges (there are also different discharge rates, but I expect both of the LiPo batteries you mentioned to be fine in that regard). You can find a more detailed explanation of this in our Understanding battery capacity: Ah is not A blog post.
By the way, you’ve mentioned “servo” a few times; please note that a servo is different from just a brushed DC motor, which is what the Dagu Wild Thumper uses.
Brandon
Hello Brandon,
Thanks a lot for your help and especially for your very detailed answers, finally I understood all the details…
Only the Bests for You,
alexm
P.S. I have used “servo” (servomotors), not for the motors of Dagu 6WD, but for the G2 High-Power Motor Driver. Is this the correct terminology?
I am glad those answers helped! “Servo” is often used to refer to an RC servo, but more broadly refers to a control system with feedback, as described in this Wikipedia entry for Servomechanism. For the G2 High-Power Motor Driver, you might shorten it to just “motor driver” or “driver”.
Brandon
Hi Brandon,
Can you help me again, this time with the wiring of the dual G2 motor driver…
What AWG (or mm2) 2 pin red-black wires do I need for the dual dual G2 motor driver to link them to Dagu 6WD motors and to the LIPO battery? Do the wires need to fulfill other requirements (like copper? A? V?)
Can you give me a link for such jumpers?
Thanks again for your help,
marius a.
In general, sizing your wire will depend on a few factors, such as the length of the wire in your system and the voltage and current that it will be handling. You can find various charts on the Internet about appropriately sizing your wires, such as this one on TI Tech DIcoded or this one from PowerStream. We carry stranded wire in a few different wire gauge and color options.
Brandon
