After completing my first DIY Mars-style rover project, I am looking to make a bigger and better version, this time with solar recharging like the real deal. The main components I have planned so far are:
4 x CEO9376 servos for steering the front and back pairs of wheels
6 x DFR1114 metal gearmotors for driving
WS-26363 for recharging a lipo / 18650 cells with the solar panel, and it provides a 5V 3A output
SS313070001 3W panel
However, my main concern is power consumption. The current limit of the solar power manager is 3A, and looking at the stall currents of each of the motors, their sum is well over that limit. Considering their normal operating currents, it is also pretty high (about 3-4A) and that isn’t taking into account the microcontroller, or possibly adding camera vision with something like a Pi Zero instead of just a Pico.
Does anyone have any ideas how to improve this design so it can safely power everything?
So far, my solutions are: trying to manage stalling through software by soft-ramping the PWM signals, going to two power systems with two separate power managers that share the load (say, one for the left side, and another for the right side), or maybe trying to only use four drive motors instead of 6. However, my ideas either don’t instill a lot of confidence safety-wise, introduce added complexity and high costs, or design challenges with ensuring the middle, unpowered wheels in the rocker-bogey have enough mass so they don’t get stuck in an upwards position after traversing an obstacle, respectively.
Hi Euan.
I think you are going to just find or provide the power you need. You might be able to find more economical motors or something like that or go through the requirements again to filter out things or bits that really do nothing.
Very difficult to do. You have to share via a diode for each power supply and straight away you have a bit of wasted power here. The main worry is that for equal sharing the power supplies have to be EXACTLY the same voltage. If not the higher one will supply the whole load until it sags to the value of the lower one then sharing can take place. But in reality each supply at some time will have to supply the WHOLE load. This arrangement works for a HOT STANDBY arrangement where the second load picks up the supply in the event of the main supply failure or depletion.
My short answer here would be do not go down this path. Far more economical to increase the existing capacity as to share you would need 2 supplies capable of providing the whole load each anyway.
Cheers Bob
Hi Euan
Just looked a bit closer and at the motor numbers
It seems that the total motor (6) stall current would be 3-4A. and the total no load current about 0.3-0.4A with actual operating current somewhere in between.
Something does not add up.
Power manager output is 5V @ 3A = 15W
Solar panel output = 3W. Just a bit short and that is not considering conversion losses and you only get this while in the sun.
Every time you change voltage there will be a “conversion loss”.
Watts is Watts no matter what the voltage is. To estimate requirements it is easier to use the Watt term and allow for any conversions.
I highlight the controller output quoted as 3A or 15W and the solar panel 3W. Now the controller can only output what is put into it so to get 15W out you have to put 15W plus conversion loss into it which a 3W solar panel cannot do. OK so a battery could make up the shortfall but that has to be charged to replenish.
I think a rethink of all this might be in order. It would be easier from this end if we could see a sketch or something of EXACTLY what you have in mind.
Cheers Bob
Those are very good points; my thinking was to rely solely on the solar panel to recharge when the robot is stationary / turned off / in sleep mode, rather than providing continuous power.
Maybe cutting down on the power requirements could be good. i.e using only four drive motors and two servos for steering, and the four-wheel steering can be achieved with the rear wheels being on castors. Operating said motors all at 5V theoretically lowers their current consumption by 17%, so that should also contribute to savings.
I have been able to achieve the same (4 drive motors, 2 servos) using just AA batteries, so something like 18650 cells should be able to handle it.
Great points all round - and such a sweet starting point. What a dope project Euan!
In keeping to the essence of the project, and from what I’ve seen - is usually deployed in larger vehicles.
The power system would look something like:
Depending on your power setup, you might be able to insert a solar charger downstream from the BMS (to still give protection) and complement the power.
With this setup, you’ll likely need a “more dumb” solar charger that is happy to output power while the batteries are being used, or a BMS system that can take a solar input (camping setups typically have all-in-one units).
Running any more than a couple of motors from a regulated output usually hits the limits of any ratings pretty quickly.
While this is the way its “usually done” there aren’t all that many parts available for this kind of project unfortunately (especially once you get to a 2S setup (2 LiPo’s in series)).
That is very interesting, thank you so much, Liam!
Would you be able to elaborate on what a BMS would entail? I like the idea of including a main switch to connect/disconnect the battery from the motors and charger, respectively. The question then becomes bringing the 7.4V down to the 6 or 5V needed for the motors. The integrated drive on the DFR1114 was appealing as it reduced the number of required components. Do you suggest a dedicated motor controller would be better?
I definitely agree that something like a 2s cell or maybe 18650s in series would be a better choice than small lipos.
I am definitely on board for custom-making a circuit if necessary, as those are good skills to learn. Feel free to correct me if I have misinterpreted anything as well!
In short, a BMS is there to protect the battery pack. The 3 Cell 18650 BMS Module is a good example of that — its product page lists over-charge, over-discharge, short-circuit and over-current protection, with a continuous discharge range of 5–8A and 10A peak.
While it is completely up to you, I’d say a dedicated motor controller would be better since it removes any regulators from your system (if a motor driver has current limiting, you can use higher voltages).
I’d say your project is pretty close to the point where it would be a good idea to go either way.
Picking your power topology is definitely a good first step though!
Liam
Hi Liam, Euan
Please correct me if I am wrong but aren’t the motors linked glorified continuous servos. If so if you use a dedicated motor driver (brushed or brushless) you would need to replace the motors with the appropriate type. The control signal requirements of the selected motors look very much like the 360º servos.
If the motors are already purchased (6) this would be an added expense unless they can be exchanged.
Cheers Bob
I haven’t purchased anything as of yet; I am in the planning stage at this time. If I were to use dedicated motor drivers (I am thinking the makerverse 2-channel driver), I would get a version of the motor without the attachment. This is what I used previously, only that model of motor was about one-half the RPM, so I might have to do a bit of digging to find it (or I could just desolder the integrated drive).
One more question for Liam, given the output voltage of the BMS is 11.1 V, to recharge it using a switched solar charger, would I have to isolate each cell, given the solar chargers I have found have a max battery voltage of 3.7-4.2V? Or would that not be safe, charging each cell individually while they are connected to the BMS?
I wouldn’t charge each cell individually while they’re still tied together in a series pack and connected to a BMS. That gets messy fast, and unless the charger is specifically designed for a multi-cell pack with balance connections, it’s not the safe path.
charge the whole pack as a pack with a charger made for that pack voltage/chemistry
then step down pack voltage for logic, and use a motor driver that accepts the pack voltage directly
Just a heads up, this will mean the voltage will drop as you use the batteries (a discharge chart for a LiPo will help here).
So for example, if you go 2S nominal 7.4 V, a driver like the Makerverse Motor Driver 2 Channel can run from 3–16 V, which is a much nicer fit than forcing everything through a 5 V solar manager. If you stay with 6 motors, just keep in mind you’d need multiple motor driver channels and enough current headroom overall.
The big gotcha here is that the Solar Power Manager Module (D) is for a single 3.7 V cell, not a 2S/3S pack. So if you move to 2S/3S, I’d stop thinking in terms of “one charger per cell” and instead look for a solar charger/controller that explicitly supports the full pack voltage.
While less in the spirit of your revised rover, it might be worth powering on (pun intended) with the battery setup and retrospectively adding solar.
(Sorry the last diagram wasnt as clear in places)
(and please excuse the MS Paint diagram, it goes together quickly )
My pick would be 2S rather than 3S for this rover — simpler voltage conversion, easier match to small motors, and less wasted power stepping down for the brains of the bot.
That sounds great! It could be a good side project to try and design an MPPT charger/controller suited for 7.4V, since all the ones I can find are for 3.7V only. Since you recommend 2s rather than 3s, is there a 2S version of the BMS you linked earlier? I couldn’t find one when searching for it.
No 2S BMS on the store at the moment (and the 3S needs all cells populated), I’ve added it to the todo list, but might be a while till we loop back around to batteries and BMS’s.
If you come across anything in your searches you love the look of, feel free to send it through and we can take a look at sourcing it!
Liam
Hi Euan.
Google “2S BMS” and you will get pages of them.
Included is pages on Amazon. If you subscribe to “Prime” on some of them is free next day delivery. And is it next day, I have ordered an item sometime after 7 PM on one day and had it delivered to my door before noon next day. It seems as long as you order before midnight it will be next day (if it says “Prime Tomorrow” on the web page"
Cheers Bob.
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