4x4 electric buggy motors need to go at same speed

Thread Starter

Matt W

Joined Oct 16, 2020
5
Hello I am seeking advice on a 4x4 electric buggy I am making, basically from scrap electric wheelchair parts. The problem is front and rear motors are from different machines, and I need to make their 'road speed' the same, or the motors start to 'fight and scrub'. I have wired in a PWM to control all 4 motors (24volt DC brushed), this works OK but I need to somehow step down the rear motors power (voltage, I suppose) to get them at the same speed. Can I just wire in a second PWM in series using the first PWM output as supply voltage to the second PWM? I wonder if this would ensure running the same speed, at all voltages from the first PWM. I am unfamiliar with complex electronics, only have basic know how on this stuff. Ideally Im looking at an off the shelf device with easy wiring. Maybe there is a suitable 4-way motor speed controller with central speed controller? Hope this makes sense. Any advice most welcome Thankyou Matt W
 

jpanhalt

Joined Jan 18, 2008
10,463
Welcome to AAC, Matt.

1) Are you sure you want all 4 motors turning at the same speed? Do you know why cars have differentials?
2) The only way to ensure the same speed is with a sensor at each wheel, somewhat like the ABS system in modern cars.
 

AlbertHall

Joined Jun 4, 2014
10,536
2) The only way to ensure the same speed is with a sensor at each wheel, somewhat like the ABS system in modern cars.
But they will all run at the same speed (assuming the wheels are all the same diameter) unless the wheels slip. It seems to me that you really need to measure the torque at each wheel and individually adjust the PWM to each motor to match the wheel torque.
 

jpanhalt

Joined Jan 18, 2008
10,463
But they will all run at the same speed (assuming the wheels are all the same diameter) unless the wheels slip. It seems to me that you really need to measure the torque at each wheel and individually adjust the PWM to each motor to match the wheel torque.
Of course, but the TS described it as a modified "wheelchair." I have never seen a wheelchair with equal diameter front and rear wheels. There is also the matter of turning. Very few people navigating populated spaces want to travel straight as an arrow. So, you have 4 wheels, and for smooth operation, they really do need to operate at different RPM's, unless they have "differential" transmissions.
 

AlbertHall

Joined Jun 4, 2014
10,536
But this 'buggy' has four individually powered wheels, and presumably it has some sort of steering on either front or back wheels for going round corners. If the torque is matched then the wheels on the outside of the corner will 'automatically' turn faster. A differential is needed only when one motor drives two separate wheels.
 

jpanhalt

Joined Jan 18, 2008
10,463
But this 'buggy' has four individually powered wheels, and presumably it has some sort of steering on either front or back wheels for going round corners. If the torque is matched then the wheels on the outside of the corner will 'automatically' turn faster. A differential is needed only when one motor drives two separate wheels.
Where did the TS state that obvious conclusion? He wants them at the "same speed," but has not made it clear whether that is circumferential speed or rpm. Even if circumferential, that does not accommodate for turning. Clearly, the TS needs to evaluate the geometry and come to a more realistic plan.
 

Lyes

Joined Nov 25, 2019
11
What is the spec of the two motors or reference?
if the diameter of the wheel is not the same even if the motor speed is the same the perifiral speed will be different
if you have to turn left or right the speed of two wheels is different as well (differential in cars) even if the two wheels are not driven by the same motor
if the control strategy is V/f=Cte the speed is proportional to the voltage so you have to measure the voltage and the speed (4 times)
 

Alec_t

Joined Sep 17, 2013
11,730
I wonder if an off-the-shelf controller for a quad-copter could be adapted? I've never used one, but I assume they give individual PWM speed control of four (brushless) motors.
 

Thread Starter

Matt W

Joined Oct 16, 2020
5
Hello, thanks to all for opinions on my 4x4 buggy project. The buggy has Ackerman-style steering at the front, and yes, front wheels are smaller than the rear. Rear motors are 350 watt, front 180 watt. Im aware of the Ackerman principle of differential circumferential speeds, my hope is that a small amount of scrub on turning would be acceptable, as the buggy is aimed at off-road use where ground would 'slip' slightly, but in continuous forward motion would create what used to be called 'winding up' in locked differential vehicles (like the old US Willys jeep), though I am assuming this would manifest as ever-increasing current drain as the resistance of the slower motors would effectively drop, approach stalling. Once I can adjust rear and front motors, my aim is to change the wheels to four identical quad-style ones, hence needing a flexible voltage adjuster which I can 'tune in' manually with the buggy on blocks. The main aim of this project is to find out how much power and traction is needed to carry camping gear on a potential expedition, I only need a walking speed buggy. From these tests I hope to discover if such a buggy could be continually solar powered, though my instinct and very basic knowledge so far tells me such a buggy would just need too much power to be 'solar practical'. Thanks for any further advice.
 

jpanhalt

Joined Jan 18, 2008
10,463
Your total wattage rating is 1060 W, which if used with 2, 12 V batteries in series would require about 40 A, excluding starting from a stop, which would be higher. Given that solar cells put out about 150 W per m^2, solar power while running does not seem practical. Maybe while parked it could be used for recharging.
 

Thread Starter

Matt W

Joined Oct 16, 2020
5
Yes, thanks for your feedback, my present very basic set up can switch each motor individually, selecting just enough power to move the buggy, so If two motors turn out strong enough, I can ditch the other two and save weight. I was planning to use two large fold-out solar panels, as you say charging batteries for most of the day, then driving I would guess a few hours at dusk, estimated range per day about 15miles. Before I start spending cash, though, I just need to get this basic test buggy running properly, so if it proves to need an unfeasible amount of power I can try something else. Thanks Matt
 

Bernard

Joined Aug 7, 2008
5,680
Back in 2008, I modified a wheel chair carcus , 2- 250W 22V gear reduced motors. F-R via DPDT SWs, speed PWM, 2 linear pots, power 2- 12V 74 Ah batteries in series. I believe a 200 lb. load would be OK. Speed ,about 7.5 km/hr. I advanced throttles too fast and melted 6- 60A FETs. There was an overcurrent ckt. but I think that it was over filtered resulting in slow action.
Its trailer was sold, I dropped project. Starting current around 10 X operating current.
An interesting read " Falconry Lure", Aug. 4 2014, look under Search Forums.
 

Thread Starter

Matt W

Joined Oct 16, 2020
5
Back in 2008, I modified a wheel chair carcus , 2- 250W 22V gear reduced motors. F-R via DPDT SWs, speed PWM, 2 linear pots, power 2- 12V 74 Ah batteries in series. I believe a 200 lb. load would be OK. Speed ,about 7.5 km/hr. I advanced throttles too fast and melted 6- 60A FETs. There was an overcurrent ckt. but I think that it was over filtered resulting in slow action.
Its trailer was sold, I dropped project. Starting current around 10 X operating current.
An interesting read " Falconry Lure", Aug. 4 2014, look under Search Forums.
Thank you for that information, it will help me with my project, I was also concerned about start up current burning out the cheap circuits I was trying, there are so many variables and formulas for both electrical and mechanical devices I decide to just try it out in the real world and see what happens. No fires so far, but only using one 12 volt battery to get the basics right. My instinct at present is that a solar buggy like mine is impractical due to power management and supply, hence not wanting to spend fortunes on something that will probably destroy itself in tests... I will keep posting once I find out!
 

Thread Starter

Matt W

Joined Oct 16, 2020
5
Maybe the easiest/simplest way would be to have one main throttle pot, then a "balance" pot to adjust what percentage of the throttle voltage goes to front and rear controllers. Like the volume and "pan pot" on an audio mixer (pan control is a linear pot with the wiper grounded). See Figure 27.19 on this page:
http://machineryequipmentonline.com/electric-equipment/recording-consolesaudio-mixer-circuitry/
Yes thanks, I had thought a second PWM module instead of a balancing pot, only because as an amateur I can just easily wire them in, as Im not able to design my own circuits, also I think pots might waste energy as heat (?), and PWMs maintain torque rather than reducing the voltage. Im waiting for an ammeter before I test the buggy further, only so I dont burn it up accidentally by overloading it. I will test front and back motors separately then post my results and maybe some pictures, this buggy is 'scrap heap challenge' quality, as is my know how on electronics but am willing to learn new stuff. electric bike wheels would be better but they are aimed at speed not torque, and ideally I need to crawl up slopes on low (maybe solar) derived power. I previously built a 6x6 buggy for children, only 240 watt max, but it carried me up a 1:5 slope though did struggle.
 
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