Electric rideable kid's car soft start - Page 1
Forum section: electronics / projects

author: VelOBor
author_rank: Contributor
timestamp: May 17, 2018, 01:38:08 pm
content: Hi guys and gals! Need some advice on soft-starting a 12V DC motor, unfortunately don't have the exact specs.Got my kid one of those power-wheels type cars, though this one is not the "brand-name", but a fairly decent copycat. The motors looked like regular brushed drill motors when I was putting the thing together, but didn't look closely.The problem is this - when the thing starts moving, the motors kick into their full speed (they do have two settings selectable by a switch - slow and fast) and the car lurches forward (or backward if reversing). What would be the best way to give it a soft-start, say a ramp-up time of maybe 1 or 2 seconds until it reaches full speed. Preferably without using a microcontroller, although I have considered that option. Any advice is welcome! Thanks in advance.Can provide the specs on everything if necessary. Photo of the toy attached, roll of e-tape for scale...

author: Marco
author_rank: Super Contributor
timestamp: May 17, 2018, 02:02:19 pm
content: There are motor controllers on ebay with programmable soft start. The problem is how to wire it into the existing control system, which presumably already has it's own PWM full bridge.

author: VelOBor
author_rank: Contributor
timestamp: May 17, 2018, 03:34:01 pm
content: I haven't cracked her open yet, but I presume it has no PWM. The car is either GO or NO GOHas a remote control with buttons (forward-back, and left-right). Has a switch on the "door panel" for slow and fast. I'm guessing just switches between low and high resistance paths to limit motor speed. The remote control is so that the parent can drive the car with the kid in it, if the child hasn't quite figured out the controls. The gas pedal inside is also GO or NO GO. The steering is a little weird too, it doesn't seem to be a proper servo. A single press of the "right" button will cause the steering wheel to turn all the way right, a press "left" will return it to center and another press "left" will, you guessed it, turn all the way left. There is no way to turn a little. Either all the way to one side or dead-center. Rudimentary controls. I have considered buying a proper RC kit and upgrading the whole thing, and installing a proper throttle gas pedal with analog control, but I was thinking a quick and dirty "capacitor and resistor" type thing for the time being. Cheap, quick, easy.And I'm not too keen on waiting for Ebay (sometimes deliveries to Russia take a month and a half).Maybe I can get a similar ESC at my local Chip n Dip (Radioshack-type store in Russia)...

author: Gyro
author_rank: Super Contributor
timestamp: May 17, 2018, 04:36:05 pm
content: Youmightjust get away with a low value power resistor in series with the motor. Not enough to significantly affect its operating speed but just enough to reduce the motor voltage when starting from stall. The value would need some experimentation but possibly in the region of 50 - 200m ohms or so, depending on motor size and gearing. It would get very hot under continuous stall conditions though.There is a lot of ex-Soviet military stuff on ebay from your part of the world, so you could hopefully pick up some decent sized wirewounds cheaply to experiment with.

author: Fred27
author_rank: Unknown
timestamp: May 17, 2018, 04:45:55 pm
content: When I looked at one of these the slow setting connected the motors in series (so they see 6V each) and the fast setting connected them in parallel. Very cheap, but it works.

author: Paul Price
author_rank: Super Contributor
timestamp: May 17, 2018, 05:29:51 pm
content: PWM controlling power to the motors in parallel with a suitable high current MOSFET on a large heatsink would be best and a 555 timer and give you best battery operation with highest efficiency. You would take a string and spring to hook up the pot shaft to the gas pedal.Depending on battery voltage. You might need a simple voltage boost circuit to drive the MOSFET gate so it is always turned fully on doing PWM so working most efficient doing PWM control.Sounds like something fun to play with to put together a circuit to do this, but this may not so hard to do if you consider you can get help from people on this forum with any circuit ideas you can come up with. There is already a lot of circuits and info on the web about brushed motor control running on batteries.A MCU would do a better job and provide the ability with cheap thermistors to measure motor temperature and some very simple tach circuit to detect motor stall conditions that could damage the motors by excessive current or high temperatures.

author: bob225
author_rank: Frequent Contributor
timestamp: May 17, 2018, 06:02:16 pm
content: It sounds like it just uses a micro switch for the throttle a simple on off, what voltage is it ? 6 or 12VA Pwm controller seems the way forwardCode:https://www.allaboutcircuits.com/projects/upgrade-your-power-wheels/

author: VelOBor
author_rank: Contributor
timestamp: May 17, 2018, 07:11:06 pm
content: WOW! Awesome! I've been reading this forum for a while (and some serious binging on Dave's YouTube channel too), but this response rate is awesome! Thanks for the ideas, guys!It has two 12V dc motors (generic kind, NSD B motor RS550S), each is wired through 2 relays (SARB-S-112D4U), I'm assuming for bi-directional control. The steering "servo" (although I haven't gotten that thing apart yet) consists of something similar, because the control board has two more smaller relays (SARM-S-112D4), which are wired in the direction of the front... I ASSUME (and we all know that when someone ASSUMEs they make an @$$ out of U and ME) that it is basically a DC motor with a couple of end-switches to tell the "brains" that the steering wheels are turned one way or other.I've gotten into it superficially today, just to have a look-see and now I'm thinking a full-on tuning job on this thing. Like PIMP-MY-RIDE type thing. Proper remote control with ESC for the drive motors, servo for steering (I'm thinking one of those 1:5 scale types, with about 20kg/cm of torque should do fine), battery upgrade (higher capacity, not power), perhaps some protection/monitoring circuitry for the battery compartment and the motors. Now 2 questions remaining - do I build from scratch (Arduino-based, with something like an NRF24L01) and motor drivers and all that business OR should I cheat and just buy a RC kit (transmitter/receiver, ESC and blah-blah)...I'm leaning towards the first option, considering that I can get help with all the circuitry.This is going to be more or less a summer project for me, and I've been looking for something like this to do for a while now. Considering that I have a half dozen different Arduinos and bare-bones MCs laying around...This is going to be fun.Attached the photos of what I have discovered so far. Seems like this is pretty much the brains of the operation. I'll have to dedicate some time to actually figure out how and where all the switches come from and to, and all the LEDs on it and all the other stuff. There are two tactile buttons on the steering wheel, one honks, the other turns on and cycles through 4 tracks (one of which is Gangnam Style... go figure). There is a volume knob, an AUX IN 1/8" jack. A "gearshift" for forward and reverse (SPDT switch?), a lights on/off switch, manual/rc switch and slow/fast.Oh, and a micro switch for gas pedal (motor on/off).Doesn't seem too complex.Wow, this is a lot of words. Sorry guys. Just excited about this project

author: amyk
author_rank: Super Contributor
timestamp: May 18, 2018, 04:05:20 am
content: The simplest "soft start" would probably be a suitably large inductor in series with the motors... although finding a "suitably large" one might be a bit difficult.I'd recommend giving it an actual proportional accelerator pedal too, and agree with the others about a simple PWM using a 555. No MCUs, just do it all with good old fashioned analogue electronicsIs the steering "wheel" also a 3-position switch, or does it have an actual steering column and linkage to the wheels like a real car?

author: Zero999
author_rank: Super Contributor
timestamp: May 18, 2018, 08:32:41 am
content: I managed to find a basic data sheet for the motor. It seems like a fairly standard, inexpensive part.https://www.robotshop.com/media/files/pdf/rbban64-banebots-DC-motor-specs.pdfHow about adding a resistor (a car headlight bulb may do but these also have a considerable inrush current) in series with the motor and bypassing it with a timer relay? The resistor doesn't need to have a massive power rating, because it won't be operated for long.

author: ebastler
author_rank: Super Contributor
timestamp: May 18, 2018, 09:20:05 am
content: I would forget about the remote control, and focus my energy on a PWM-based soft start. Remote-controlling your kid‘s powered ride just seems wrong to me. If he (or she) can drive the thing himself, he will hate to be controlled by you. If he can’t drive it yet, it is to early to give him the car.

author: rstofer
author_rank: Super Contributor
timestamp: May 18, 2018, 01:17:13 pm
content: I had one of these RC cars back about 15 years ago and the controls were nearly identical to what is described here. On-Off, hard-left hard-right, etc.I never did solve the steering problem but I did change out the controls to an H-bridge on the drive motors. I used an 8051 style uC but these days it would probably be an Arduino.So, you need to measure the RC pulse width which should vary between 1000 ms and 2000 ms. We consider 1500 ms as OFF, 1000 ms as full reverse and 2000 ms as full forward.Now that we know which direction and how fast to spin the motor, all that remains is to generate the direction and PWM signals for the H-bridge. This is the place where you add ramping.There are a lot of similar projects on the Internet.A brief overview:http://blog.solutions-cubed.com/using-radio-control-transmitters-for-motor-control/

author: VelOBor
author_rank: Contributor
timestamp: May 18, 2018, 04:02:53 pm
content: Lots of awesome ideas flowing in, guys!The thing came "stock" with a RC unit (rstofer - we're definitely on the same page here), so that the kid can get used to sitting in the car first, then figuring out the controls later. My son is just over a year old (13 months right now), so steering the thing himself is a little early. I'm sort of leaning toward the idea of getting a NRF24 type unit pair, a cheap-o RC car control and hacking the whole deal to have proper control over the thing. Plus having an Arduino for the "breains" of the operation would open a bunch of doors to tuning the thing. Headlights, audio-system, proper accelerator pedal etc. ebastler - as of right now the car is either in full manual or full RC selectable by a switch inside the car, and my thoughts are to (perhaps) have parallel control just as a precaution, an additional safety level, so the boy doesn't run into someone's car or whatever.PWM control for motor speed is the way to go, thanks guys, you have definitely convinced me of this. The only way to go if I want proper accelerator pedal. Question - PWM vs varistor (rheostat?) for this particular motor? Maybe PWM for RC mode and varistor for manual mode? Or if I use a micro (Arduino) - use a varistor for signal to uC and then the uC uses PWM for actual motor control. Now this begs the question - which motor controller will provide enough current without blowing up? I do have some heatsinks laying around and PLENTY of computer fans for cooling, but if the motor wants say 6A of current, it seems like no amount of cooling will prevent a L293 from releasing the magic smoke... Am I wrong on this one? What are the options? My plan is to next take the motor out and hook it up to my bench power supply (hope it can handle the current, the specs say 0-30V 0-3A) and see what kind of current it draws at start and operation. Put a little load on the output and see what it draws. Looked up datasheets on this motor and stall current is 85A. That seems a little nuts for this motor, but I guess makes sense. Although the car has a 15A fuse just after the batteries, so maybe the motors aren't that high. Hope that my power supply is protected from overcurrent. Would hate blowing it up...Also I want to test the slow/fast speeds of the stock brains and how it works. Whether it's PWM or connecting motors in parallel/series for 12/6 volts. Idea is this - hook up a voltmeter across the motor terminals and put it in drive. Next step - hook up my scope to motors and drive them (with the wheels lifted up off the ground) in fast and slow mode. See if I get a clear PWM type signal or just straight up 6V or 12V DC.What said earlier about the relays on the board I take back - the forward and back are controlled by a "gearshift" next to the steering wheel.I will post updates when I get any further on the reverse engineering of this. Again - I'm leaning for a full-on rebuild, with microcontroller and full RC, rebuilding motor drive circuitry from scratch with PWM control, possibly headlights and tail lights, maybe a built-in MP3 player and all that.Step-by-step though. First I plan on getting an NRF24 kit and two arduinos and see if I can get them to communicate. I've found some resources on that already (schematics, programming and all that). Should be fairly easy. Then see where I can go to next.

author: rstofer
author_rank: Super Contributor
timestamp: May 18, 2018, 04:45:53 pm
content: The maximum inrush current will occur at 0 RPM so you can just measure the DC resistance of the motor and plug it into Ohm's Law.There are a number of H-Bridges that could handle the current, some are for single motors, others handle two:https://www.robotshop.com/en/medium-power-motor-controllers.htmlETA: If you can't measure the low resistance, you can measure the current when applying full voltage with the armature locked. Be quick, the motor is getting hot!

author: VelOBor
author_rank: Contributor
timestamp: May 18, 2018, 07:45:09 pm
content: Awesome, thanks rstofer. I'll try that. Seems easier to just unplug and pull the motor rather than try to measure while it's running. Thanks for the link, I'll see what sort of chip I can come up with. I'm not a fan of ordering online due to the wait time, but Moscow is pretty decent at having pretty much anything you'd want. Except some really, like REALLY, specific stuff... But if you're willing to break into your piggy bank, you can get it too. But most electronic components are readily accessible. Found a TB6612FNG bare-bones chip at my favourite shop's site, says it can do 3A peak. First I have to see what the motors REALLY draw though, before going further. Maybe I'll have to shove my whole "don't want to wait" thing and buy online.Thanks for the link!EDIT:And why do I need a driver anyways? Why can't I just build an H-bridge with suitable MOSFETs or relays?

author: Gyro
author_rank: Super Contributor
timestamp: May 18, 2018, 08:03:01 pm
content: QuoteI think you'll find that the motor current will be much more than that. 12V at 3A is only 36W, I doubt that will move anything. I would expect more like 15A , 180W, 1/4HP, with higher surge current at startup (the fuse will take more than 15A for short surges). As suggested in your edit, I think you are looking at discrete power MOSFETs if you want to do PWM.Edit: Corrected figures

author: VelOBor
author_rank: Contributor
timestamp: May 18, 2018, 08:13:29 pm
content: Thanks Gyro.I'll get some measurementsdone and post the actual data here.Need. Solid. Data.I'm going to try to get into the thing tomorrow if it's not in use by the youngun. He can't drive it himself, just sits in it and pushes the horn and music change button. The wifey is doing all the driving.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 18, 2018, 10:54:30 pm
content: These things usually have a large ratio step down gear chain following the motor. While the electronics solutions will be more elegant, a purely mechanical solution may take enough of the "jerk" out of startup to do the job for you. Add a flywheel (rotational inertia) on the motor end of the gear chain. This is roughly the equivalent of adding a series inductance as mentioned above. Depending on the exact configuration and your resources this could range from very simple to nearly impossible, but if it works it will be extremely reliable.

author: rstofer
author_rank: Super Contributor
timestamp: May 19, 2018, 01:16:31 am
content: But the flywheel works both ways. It may take longer for the motor to accelerate but it will also take longer to decelerate. Whether that is a problem is going to take a little experimenting. The relay based controls that come with these toys doesn't have dynamic braking (IIRC). The relay simply opens the circuit and the motor eventually costs to a stop.These toys probably don't have brakes - at least I don't think mine did. OTOH, mine wasn't a riding car.

author: james_s
author_rank: Super Contributor
timestamp: May 19, 2018, 03:40:35 am
content: I just went through this exercise with a cordless electric lawnmower. The original mechanical switch had failed so I replaced it a while back with a mosfet. Recently I replaced the SLA batteries with LiPo packs and the much lower ESR caused huge inrush that blew the mosfet. I built a simple soft start circuit using a ATTINY25 8 pin microcontroller diving a beefy mosfet. You can buy speed controllers ready to go if you wanted though, they make them for electric scooters and RC cars and such.

author: VelOBor
author_rank: Contributor
timestamp: May 19, 2018, 10:09:35 pm
content: I wanna steer away from mechanical solutions. For one - I don't have any machining gear to make a proper flywheel and looking for one that will fit the motor axle is too much of a bother. I'm fairly certain that the motor has plenty of torque, so in order for this flywheel solution to be worth it - it would have to be a heavy flywheel, but the gearbox is fairly tight. I think in my case (considering all the criteria that I have put in place) an electronic solution will be best. Unfortunately I was unable to get into the guts of her today and measure the motor resistance, so I'm pretty much at a stand-still.Will a MOSFET be able to handle PWM? So that I don't have to worry about a beefy enough motor driver/ESC. I don't need regenerative braking or anything of the sort. I'm thinking MOSFET-based H-bridge, an Arduino UNO should have plenty of pins to drive everything (again - considering all the options that I want, the headlights, temperature monitoring, RC, etc.) and still have a few to spare.Okay, am I getting this right - if we number the MOSFETs in the H-bridge 1-4, like this:13H42, then I just need 1 pin for each direction? Just wire 1 and 3 to one pin, 2 and 4 to the other. Then I only have 4 pins occupied for the two motors. Furthermore - if I wire the two drive motors in parallel onto the same bridge, I cut the pin number in half. So I only take up two pins for forward and reverse.james_s - I've considered LiPo packs, but I don't trust myself enough to put something like that under my kid's butt. The lead/acid batteries provide enough current, and the specs say 2 hours of continuous run time. I'm thinking an hour is more realistic. I will be testing that also. One of the mods I have in mind is adding two more of the same batteries in parallel to increase run time, but that is optional for now. Maybe after a couple of years I will get more powerful motors and then upgrade the power supply also, but I do appreciate the idea though!

author: james_s
author_rank: Super Contributor
timestamp: May 19, 2018, 10:52:04 pm
content: Well my mention of LiPos is only because that's what I used, the SLA batteries were shot and I already had a pile of LiPos for my RC planes, may as well put them to use. The speed control will be the same regardless of the power source.It's a tad more complex than that for the H-bridge since the high side mosfets need to be driven differently than the low side. There are lots of H-bridge motor drive circuits out there though so that part isn't too hard to work out. The thing you have to watch out for with PWM control of largish motors is the spikes that you get each time the transistor turns off can easily kill it. You often need a snubber across the mosfet. This should give you a few ideashttp://www.masinaelectrica.com/dc-motor-speed-controller-pwm-0-100-400hz-3khz-freq/Also look for projects involving large robotics, many of them use brushed motors with H-bridges. Some of the larger RC car speed controllers might be suitable too.

author: rstofer
author_rank: Super Contributor
timestamp: May 19, 2018, 11:06:30 pm
content: Just buy a commercial ESC, you aren't going to build a better one. One thing you need to watch out for is 'punch through' where the top and bottom MOSFETs (on the same side of the H) are on at the same time.The bottom MOSFETs will be N-channel and you might be able to drive them with logic levels but usually not (and get them into saturation). The top MOSFETs could be N-channel, in which case you will have a complex issue driving them (you need to develop a gate voltage some 10V above the top rail) or P-channel which is easy to drive but more difficult to select. That's why they invented H-Bridge driver chips.You will drive the top MOSFETs with a solid direction signal and the bottom MOSFETs with PWM. Some commercial controllers have logic on board to cut the number of signals down to 2 instead of 4 (per motor).If you can get by with a 10A controller rated 30A peak (30 seconds), it will only cost $24.The PWM frequency for MOSFETs is usually pretty low (like 1 kHz or so) and the reason is that the current waveform is going to look like a trapaziod. The problem is that at the leading and trailing edge, the MOSFET is working in a linear region and developing heat. The width of this edge, for a given controller, is fixed and the fewer the better. At a high frequency a much higher percentage of time is spent in the linear region.MOSFETs can definitely control the motor but without knowing the inrush current everything is pretty much a guess.Your idea of wiring MOSFETs 1 & 3 gates together is exactly how punch-through occurs. During the transitions, both devices are on (slightly) shorting out the power supply. When this occurs, the MOSFETS self-disassemble.

author: VelOBor
author_rank: Contributor
timestamp: May 20, 2018, 05:41:48 pm
content: Thanks for the input guys!Did a little measuring today (haven't tested for PWM with the scope yet, but will do shortly, although I STRONGLY doubt that it's speed controlled, just straight 6V or 12V depending on speed setting, you never know with the chinese though... also will test for ACTUAL current with multimeter) with a multimeter and got this:the motor resistance is in the 0.6-0.7 ohms vicinity, which gives us about 10A, 60W @6V (slow speed setting) and 20A, 240W @12V (fast setting).So I am going to need a commercially available ESC that is capable of handling that kind of power. If I want to be able to get the speed out of the car that it is designed for, I will have to get an ESC rated for 20A current, with higher peak performance, right? If I get one rated for 10A and 30A peak, and I'm running it at 20A (fast setting, throttle all the way down) I'm running the risk of burning it out. Or can I just provide it with ample cooling (heat sink and fan) and not worry about it?

author: james_s
author_rank: Super Contributor
timestamp: May 20, 2018, 05:50:22 pm
content: Is that under load? With a kid in it going up a steep hill the current is likely going to be substantially higher.My friend's daughter has a similar kiddie car that he retrofitted with a 4 cell LiPo, it will do a burnout nowOk so the solid plastic tires provide very little traction so that helps too.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 20, 2018, 06:25:56 pm
content: Just get a heavily over-rated ESC. I see one rated for 320A for under $20. You will still be dealing with the fact that the foot control is a switch. The commercial ESC's expect a standard servo control PWM input. An Arduino or other microcontroller can provide this with whatever ramp up you find appropriate, including non-linear things like square law. Setting up a proportional throttle can be a mechanical challenge, but look into sewing machine foot pedals. Some are electronic and based on a pedal operated potentiometer. Under $20 and an easy hack to microcontroller input.

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2018, 06:52:33 pm
content: Quote from: james_s on May 20, 2018, 05:50:22 pmIs anyone else slightly uncomfortable with the idea of retrofitting LiPos intosit-inkids plastic toys? Granted, SLAs can be a pain with Sulphation, weight, etc. and they can source a lot of current, but properly fused, they're not known for their spontaneous flammability.

author: ebastler
author_rank: Super Contributor
timestamp: May 20, 2018, 07:06:01 pm
content: Quote from: Gyro on May 20, 2018, 06:52:33 pmWell, as james_s stated, the "burnout" is actually the point of this conversion...

author: Gyro
author_rank: Super Contributor
timestamp: May 20, 2018, 07:30:51 pm
content: Hmmm, If they're lower impedance and sourcing more current than the original SLA then it sounds as if they could well be bare cells (unprotected).Edit: Sorry OP, I'm drifting off topic.

author: james_s
author_rank: Super Contributor
timestamp: May 20, 2018, 09:04:17 pm
content: I'm not advocating the use of LiPos but those of us who have been using them for years generally have a pretty good sense of how to use them safely so it's not really an issue. I've seen fires caused by lead acid batteries too, not the batteries themselves but they can deliver enough current easily to set wiring on fire. LiPo fires are almost always due to improper charging where one or more cells gets overcharged. Mechanical damage can also do it, we had a guy crash a large electric RC heli at the field I fly at, the thing smashed into the ground at high speed and the battery erupted into a fireball, it was pretty impressive. Fuse the output, charge them properly and don't smash them and they're fairly safe.Heck when I was little the lucky kids had gasoline powered go-karts and scooters, fires due to fuel leaks or spills have got to be as common as LiPo fires. Years ago one of my neighbors set his lawnmower on fire trying to pour fuel into it while it was hot.

author: VelOBor
author_rank: Contributor
timestamp: May 24, 2018, 09:14:02 pm
content: Sorry for not replying earlier. Busy time here!Did not measure current through the motors directly, just the voltage across motor terminals and plugged it into Ohm's law. I am now looking for ESCs on Aliexpress, they have acceptable pricing. Looked around my local electronics shops and anything they have over 30A is "commercial grade" stuff - for industrial motor controls and such. And priced accordingly. Will keep looking locally, as I am not a fan of waiting for a month for my goodies to arrive... However the price wins here. So far the plan is - hack a cheap-o car remote and retrofit with arduino and NRF24 (with pair in the car) for RC, that will give PWM to the ESCs for proper speed control. Retrofit the stock steering with a proper servo (I'm thinking around 20kg/cm torque should be more than enough), but if I do something like that - I need to consider some sort of power steering (I doubt that turning this servo by hand will be possible for a small child) for when the RC mode is off, in manual mode so to speak. This is easy enough (a pot mechanically linked to the steering wheel and controlling the arduino's output to the servo), of course there is risk of loss of steering with controller failure. But then again - if the arduino kicks the bucket, the whole thing will just stop. As for the gas pedal - I'm thinking a slide pot mechanically linked to the pedal. I'll have to stick a spring in there to have the thing return to "idle", but that's easy enough. I'm okay with the mechanical skills. As far as the battery goes - I'm on the fence. If I go with LiPo - I will have to make sure to design proper protection, not too keen on the idea of a kid riding on top of a firecracker. If I stick with the stock lead/acid pack - I'd definitely want to boost capacity by adding a couple more (stock version has two 6V batteries in series and with 15A fuse, I'd just get two more sixers and parallel them up to the stock ones, but deal with the doubled re-charge time).So far that's it for the mechanical side. The rest is pure aesthetics. Headlights, perhaps MP3 player, as it already has some audio thing on-board, but it's garbage - has 4 tracks recorded in 10-15 second loops that play for a minute and a half, and cycle through the tracks with a button... The aesthetics are probably going to be left for the winter-time tuning though.Sorry about the long-winded post, but the second white russian in me is taking its toll.So far so good. I'm starting to get a solid understanding that this is going to be a fun project, and I just need to get all the parts together in one place (preferably my workshop) and get started.

author: james_s
author_rank: Super Contributor
timestamp: May 25, 2018, 12:43:44 am
content: Look at the RC hobby suppliers like HobbyKing and such, they should have everything you need, the speed controllers will be a "brushed ESC". If you really want radio control, I highly recommend a radio system made for RC models rather than trying to hack together Arduino type stuff. A friend of mine is into making remote controlled robotics like R2D2 replicas and such which he takes to hacker/maker/robotics club events. He's another RC modeler so that was his natural choice for the radio link and he said it's always the guys trying to use various radio modules with Arduinos and such that have problems. Meanwhile a $25 radio set for an RC car or boat can be remarkably dependable.Robotics suppliers are another good place to look for motor drives that are cheaper than industrial stuff. I'm not in that scene but I'm pretty sure I've seen ~100A dual motor drives for big robots. Pololu is a place that comes to mind, not the cheapest but they have a good selection of stuff.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 25, 2018, 03:37:43 am
content: I will repeat my suggestion of a sewing machine controller for the foot pedal. They contain a pot and all of the springs and mechanics to make it move and to make the pedal follow your foot up and down. You throw away or disconnect the light dimmer style electronics, bolt or glue the controller to the floor of your car and wire the pot into whatever your control mechanism will be. These controllers are widely available on EB and thrift stores for very low prices, often because they are not working. The pot is sometimes the failure mechanism, but more often it is not. Even new they aren't all that expensive.

author: james_s
author_rank: Super Contributor
timestamp: May 25, 2018, 04:49:39 am
content: Just make sure it's a modern one. Older sewing machine pedals were typically a wirewound rheostat that directly controlled current to the motor.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 25, 2018, 03:50:13 pm
content: Quote from: james_s on May 25, 2018, 04:49:39 amEven the older pedals will work. Add a series resistor on each end and you have the voltage divider you need. The ground end series resistor can be several times the rheostat total resistance of the rheostat and after the A-D you can scale as required. The other resistor is chosen to make the top of the rheostat match the high end of the A-D range (usually 5 or 2.5 volts). The two added resistances reduce the current in this leg a few percent of the motor current, thus not high power resistors and not a big drain on the battery. Not ideal, but when you are going for cheap you look where you can.Making a foot pedal operated variable resistor that is reliable and resistant to users is a bigger project than the whole rest of the build put together, which is why I suggest avoiding all of that mechanical work.

author: VelOBor
author_rank: Contributor
timestamp: May 30, 2018, 09:49:20 pm
content: Yeah, I think I'm convinced of using "tried-and-tested" gear such as ready-made RC controllers and sewing machine foot pedals. Makes sense not having to reinvent the wheel. I haven't done much RC modeling since high school (and that was nigh 20 years ago), but if I remember correctly the controller basically has PWM outputs that you connect straight to ESC or servo. That makes things easy. I just like my arduinos and want to find use for themThe main problem is - the cheap-o remote that I found comes without the receiver module, and I'm too cheap to pay more than I found initially. Although by the time I hack the 'duino into the remote, the whole kit will run me at least as much as a decent 2-channel ready-made RC set...Now I just have to sell this idea to the wife, and I'm set... She's not too keen on me spending on my "toys". And let's face it - this project is as much of a toy for me as it is for my sonPretty much found all the bits and pieces from local suppliers (as far as ESCs go and all that stuff). The "gas" pedal will be the only thing left.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 30, 2018, 10:22:32 pm
content: You are right, that is how most of the RC servos work, PWM just like the Arduino. The receivers put out this PWM.You will have to decide whether you want to incorporate the remote control idea. Steering will require a high power servo ($$$) and some pretty extensive hardware mods. You may also want to check whether you are breaking any local regulations putting a human payload in a remotely controlled vehicle. Any laws that exist won't have been aimed at an application like yours, but that won't help when some busybody gets involved.Check out the used market for RC gear. The technology has gone a long way over the last two decades and older gear is often given or thrown away. Just have to ask the right folks. I have some like that, but I suspect shipping costs to you would be more than buying new locally. PM me if you are interested.

author: VelOBor
author_rank: Contributor
timestamp: May 30, 2018, 10:49:28 pm
content: The car already has RC function, but the controls are "digital". In that they are either ON or OFF, or ALL THE WAY RIGHT or ALL THE WAY LEFT. No analog steering or acceleration. The whole idea started with the car being too jerky when it starts moving, so the idea of ramping up the motors evolved into full-on RC upgrade.And yeah, shipping ANYTHING half-way across the globe will cost more than getting some cheap-o or used gear locally, but thanks for the offer! Really appreciate it.Found this servo from a local electronics shop: KJH KS-3620 for about 20 bucks US (which fits easily within budget for this part of the project), that claims about 20 kg/cm of torque @6V. I'm thinking it's probably closer to 15 kilos, but I think that should still be enough. I will have to power it separately from the control board I'm guessing. Also one of the issues with using ready-made RC module is "local" steering control. When I put the car into "manual mode" (where the child driving it has control of accelerator pedal and steering) - he will have to overcome the servo in the steering. And turning a deeply geared motor can be tough, and also not too healthy on the reduction gears. I was thinking either a servo tester just linked mechanically to the steering wheel and feed the signal into the micro, or use a pot and again feed the signal into the micro.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: May 31, 2018, 08:02:10 pm
content: Or put an electric clutch between the servo and the mechanical system. Deactivate when in local control. Really cheap ones can be salvaged from automotive systems like the air-conditioner, but are usually large and current hungry. I don't have any suggestions off hand for smaller, more appropriate sizes, but there might be something out there for the nitro/gas powered RC car market.

author: VelOBor
author_rank: Contributor
timestamp: June 02, 2018, 04:32:34 pm
content: First of all thanks all for helping me out so much! Y'all are AWESOME!!!And as for the clutch - I'm thinking of staying away from clutches, I'd rather have a servo-driven power-steering thing I think. Seems like it would be easier to implement a pot linked to the wheel, rather than trying to jam a clutch in there somewhere... I dunno, I'll look into it. I don't have a machining shop (just some woodworking tools and such), so hacking that into the steering system may prove more difficult than it seems.Okay, so the project calls for a standalone atmega328 (for some lighting and such, maybe an actual MP3 player...). Now I'm experimenting with powering this thing (and because I don't want to blow it up, I'm using a motor as load instead of the chip) with an LM7805 and a few caps (electrolytics) and a lab PSU.Input: 12V and something like 0.08A with the motor runningOutput: 5VSo far I have everything on a breadboard, and I have an issue. Perhaps several, not the least of which is me being a total NEWBIE!...Let's start with the fact that all of these caps are salvaged from an old PC PSU, which seemed okay, except one transistor having had a very bad day. So bad in fact, that it left a brown skidmark on the PCB and left it smelling toasty.The input capacitor is 16V 1000uF, across the LM7805 (GND and OUT pins) I have a 50V 4.7uF, and the output has a 50V 10uF. All the polarities seem proper. The motor is a chinesium-grade hobby motor out of some dead toy, with resistance measured at around 8 Ohm.I hooked up my scope (don't ask why I have one... had the opportunity to get it basically for free, so I did) to the input (channel 1, yellow) and output (channel 2, blue). And here's what bothers me: the output voltage has a spike at around 10V (as seen in the photo attached, note the timescale). Am I reading something wrong? Is this normal for an inductive load? Is it an issue with improperly selected capacitors voltage-wise? Capacity-wise? Electrolytics, where I should've used ceramics? Dead capacitor somewhere? Or am I just an idiot and should give up this whole electronics hobby and take up knitting and gardening (or perhaps something safer where I can't hurt myself or others)What I'm getting at is I don't really understand capacitors. Not like how or why they work, but rather how to use them properly. Where resistance and current and voltage make sense to me, capacitors not so much. I mean I understand that they are in a way like a battery and a temporary reservoir of short-term power and all that, but the whole thing of which caps (voltage and capacitance, electrolytics VS ceramic VS film) to use in what cases - that's a mystery to me. Photos attached.

author: james_s
author_rank: Super Contributor
timestamp: June 02, 2018, 06:17:50 pm
content: For the servo, look at what guys are doing for robotics, if you want I can ask my friend that builds the droids and stuff what he uses for big servos.Inductive loads are notorious for creating spikes, why this happens is beyond the scope of this topic but I encourage you to read a book like The Art of Electronics, that will help you understand what's going on. Looking at your breadboard you have the load way down at one end and the source at the other, this means you have a good bit of resistance as well as parasitic inductance and capacitance. A small ceramic capacitor across the motor as close to the motor as you can get it may help. Generally you also want to keep connections as short as you can, especially ground, ideally all of the grounds should converge at a single point. This is going to be more important the larger the load you start dealing with.

author: VelOBor
author_rank: Contributor
timestamp: June 03, 2018, 10:07:23 am
content: Awesome! Thanks!I'll get that book, been looking into it for a while.Also will experiment later today with some different loads. Perhaps a couple LEDs or something of the sort, to avoid inductance. See what that gives me. Also will try a smaller breadboard and/or shorten my connections and grounds.

author: Circlotron
author_rank: Super Contributor
timestamp: June 05, 2018, 12:44:54 am
content: Quote from: Hero999 on May 18, 2018, 08:32:41 amEven better, use an NTC thermistor with timed bypass relay. The kind of thermistor used in SMPSs for inrush current limiting. The bypass relay will enable the thermistor to cool down once the motor is running otherwise the next start will be a hard start because the thermistor would still be hot and so low resistance.Edit -> changed PTC to NTC.

author: james_s
author_rank: Super Contributor
timestamp: June 05, 2018, 12:48:13 am
content: There's no need for anything like that, if you have PWM control of the motors then soft start can be done in the control. It's already built into most ESCs so you don't strip the gears in geared drives.

author: Circlotron
author_rank: Super Contributor
timestamp: June 05, 2018, 01:43:36 am
content: ^^ Of course. I’m thinking though if the OP doesn’t have pwm. Trying for the least worst solution.

author: WB8PFZ
author_rank: Regular Contributor
timestamp: January 03, 2021, 01:15:11 pm
content: I need to take apart an old variable speed drill and see how they did it.

author: Nicton01
author_rank: Newbie
timestamp: July 03, 2022, 04:59:19 pm
content: I also have the same problem with my granddaughter's ride-on electric car. It is far too jerky both when starting and stopping. I have been considering a number of potential solutions and the one I have decided on is as follows:-I am going to leave all the controls as is apart from the motor drive. I shall disconnect the current relay motor drives from the motors and replace them with a 12V PWM CW CCW Reversible Switch Motor Speed Controller, from ebay. All inputs to the current board will be left intact.I will put together an Op Amp RC circuit between the pedal and and the Potentiometer input to the PWM board. The circuit will have a time constant of 2-3 seconds and ramp down time constant of about a second. I shall connect the ON/Off switch and direction switch tothe new Op Amp board + PWM board.I shall be starting this as soon as all the parts arrive. Has anybody else had similar throughts on this problem?

author: Haenk
author_rank: Super Contributor
timestamp: July 05, 2022, 07:05:18 am
content: "= fun" ?Just a sidenote, some of these can go very fast, even unmodified. And most likely, the kids do not know how to properly handle the toy at higher speed, even less, when panicking. So *please* let them wear a helmet.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2022, 05:06:30 pm
content: Doesn't seem like a big deal to me. When I was a kid I knew people that had little 50cc gas powered trail bikes and four wheelers made for children, hardly anyone even wore helmets back then although I do think those are a good idea. We would ride them around the field behind my friend's house unsupervised and go home when it was time for dinner. Kids are not as fragile as people seem to think, I feel bad for those growing up these days that are forced to live in a bubble and not engage in any potentially risky activities.

author: Haenk
author_rank: Super Contributor
timestamp: July 05, 2022, 06:23:14 pm
content: Quote from: james_s on July 05, 2022, 05:06:30 pmWell, my neighbours' kid did a wheelie and backflip (modified I think with double batteries), he was about 3 then. And it is quite a difference if you do this stunt on a field or on a road made from concrete blocks. That certainly hurt a lot.

author: james_s
author_rank: Super Contributor
timestamp: July 05, 2022, 09:29:53 pm
content: Well I always advocate wearing helmets, they are inexpensive and readily available.

author: Marin
author_rank: Newbie
timestamp: June 27, 2024, 01:07:17 pm
content: Did anyone managed to get "soft start" pedal working?Can someone help me to get it done on my sons car? It's a 12V car. And please consider I have veeery low technical knowledge.THANK YOU in advance!!!