RC Car Electronic Speed Controller  - Page 1
Forum section: electronics / projects

author: jack_elec
author_rank: Contributor
timestamp: May 05, 2016, 05:02:58 pm
content: Hi everyone,I'm looking to build a speed controller for a RC car I have, to replace the old controller which blew up due to too much current (it was massively under-rated).My plan is at the moment, to take the speed/direction control PWM signal from the RF receiver, and pass it into an Arduino to convert the pulses output by the receiver to a set of direction & PWM control outputs for a H-bridge to control the motor. I plan to use a micro to do this as it will provide a lot of scope for tinkering with the performance of the car all in software, and I can add extra features such as battery & temperature monitoring/logging later on.The main focus for the moment is designing a suitable H-bridge, which is simple yet reliable. I plan to use all N-channel MOSFETS on high side and low side, so will need a suitable gate driver for the high side devices.So far I have found the Maxim MAX620:http://datasheets.maximintegrated.com/en/ds/MAX620-MAX621.pdfwhich seems to fit the bill. It operates down to Vcc=4.5V (the receiver will have cut out by then anyway) and can drive all 4 FETs from logic level inputs. The only problem I have with this chip is it's cost (£7.81/pc on Farnell) and potential obselesence, meaning a re-design of the H-bridge should it fail.A few general requirements/specifications are:Battery Voltage (fully charged) = 7.2V (NiMH)Able to operate down to < 6V (the receiver cuts out around 6V according to the datasheet - I think it is slightly lower in practice)Motor Current > 10A (largely determined by the FETs I know. Here are the ones I plan to use:http://www.nxp.com/documents/data_sheet/PSMN2R0-30PL.pdf)Low operating/quiescent current draw to increase battery lifeSimple & robust and able to handle the harshness of being in an RC carDoes anyone have a potentially *better* way of going about this? Using N and P channel FETs? Different method all together?I appreciate your suggestions, thanks in advance!Other Specs:Receiver: Acoms AR-2/27Motor: Team Orion Orbital 2R Mach Mod 1x21TBatteries: Reedy Black Label 2 NiMH 7.2V (unknown mAh)Car: Tamiya Dirt Thrasher (all standard apart from motor & batteries)

author: CM800
author_rank: Frequent Contributor
timestamp: May 05, 2016, 05:11:30 pm
content: I would personally suggest going a route of getting one of these:http://www.ebay.co.uk/itm/261898428792then put that with your Arduino to do the PWM control, but in that single unit is your whole power stage and logic level control.

author: macboy
author_rank: Super Contributor
timestamp: May 05, 2016, 05:45:51 pm
content: Quote from: TCWilliamson on May 05, 2016, 05:11:30 pm50 A won't nearly cut it for an RC car, especially one that has admittedly already blown up a controller. Average current is likely 30 to 40 A and peak is much higher. By 'peak' I don't mean a millisecond pulse but a continuous surge for several seconds as the car accelerates.

author: CM800
author_rank: Frequent Contributor
timestamp: May 05, 2016, 05:53:28 pm
content: Quote from: macboy on May 05, 2016, 05:45:51 pmMy suggestion was simply based on what he had said, if I can get better numbers, I can give a better solution.

author: sca
author_rank: Regular Contributor
timestamp: May 05, 2016, 05:54:06 pm
content: OK, first of all, take this as possibly the ramblings of an old fart.... the RC business moves quickly - but I do still keep in touch with a few friends who are still seriously good!I was last seriously involved in 1/10RC over 20 years ago, and started in the early 80s when Electronic Speed controls were just starting to become available - names like Demon, Novak, Nosram etc. I designed a few myself.I know a lot of stuff these days is Brushless DC, but AFAIK there are still a lot of simple brushed DC motors out there, and yours seems to be in that group.ESCs for racing were generally forwards only - mainly to save the losses of half a bridge but also to save space. That could still be useful now. It certainly makes the control much simpler.Think about the current draw you're going to see - you say 10A, but even back 25 years ago we were draining a 2.7AH battery in 5 minutes when racing - an average current way more than 10A if you do the sums. Battery capacities have gone up since then.Even back then you could get single MOSFETS rated at 50-60A, but in an RC car the main problem is keeping them cool enough. Solution was perhaps 5, 6 or more FETS in parallel, mainly to keep the RdsOn heating down to manageable levels. Even then they ran bloody hot. These days I think there is a lot more active cooling (think fans / heat pipes), and RdsOn levels are much better, but you're still in a very hostile environment, close to two heat sources, and in a very small physical package (we had to do it all through hole as well!).On a forwards only ESC you can also keep the braking FETs smaller / fewer than the forwards ones, wheras with a full bridge you'd generally want it symmetrical. You also then only need a single high side driver with appropriate boost. You could probably get away with logic level FETs on the brake side these days, simplifying the drive further.Food for thought.scaedit - add a missing word!

author: stmdude
author_rank: Frequent Contributor
timestamp: May 05, 2016, 07:32:12 pm
content: Quote from: sca on May 05, 2016, 05:54:06 pmLast I checked (been two years since I raced professionally), the usage of reverse was _absolutely_ forbidden in a race.Nice to meet fellow RCers on here.

author: TheSteve
author_rank: Unknown
timestamp: May 05, 2016, 08:09:19 pm
content: I think I'd be tempted to use N and P channel fets to keep it simpler. Years ago P channel fets sucked, now it is easy to get good ones.I have raced RC for 25+ years. Started with Tamiya, then moved to the Losi JrX2. Used to pay for the hobby by fixing Novak and Tekin ESC's. Had multiple Victor Engineering Hi-IQ's for charging(still have a few). Also ran 50 MHz as I had a ham license. Even spoke to Bob Novak directly and he sent me the parts to convert 27 MHz receivers to 50 MHz. Had a home brew 2.4 GHz controller running before they were common, still using it today. Was sponsored by Castle Creations when I switched to brushless. Much fun has always been had in RC. Maybe we should start a separate thread to compare notes!

author: bson
author_rank: Unknown
timestamp: May 05, 2016, 09:11:09 pm
content: If the motor is brushless you need a bit more understanding to effectively control it without destroying it in the process. Googling "quadrature encoding for brushless motors" came up with this which looks like a good intro:http://www.nxp.com/files/dsp/doc/app_note/AN1961.pdfI'd buy the cheapest comparable motor you can find on ebay, and experiment with that until you get it running smoothly and efficiently. Only then move on to more expensive equipment.A brushed 2-phase DC motor on the other hand is trivial. A single high-side PWM switched power source with overload shutoff is all you need. An NTC thermistor can be used for thermal shutoff.

author: bson
author_rank: Unknown
timestamp: May 05, 2016, 09:26:54 pm
content: We engineers did quite a bit of racing out in the parking lot at a previous place of employ. Here's an XRay T4 built to that end... All stock off-the-shelf parts though.

author: jack_elec
author_rank: Contributor
timestamp: May 05, 2016, 10:47:05 pm
content: Thanks for all the replies guys, I'm glad I asked on here! My interest in RC cars is very much just hobby-based, my father built the car as a kit a while back and has kind of passed it down to me to get going again. I'm in my first year studying EE so feel like I'm actually starting to get an idea of how to approach the project.Quote from: TCWilliamson on May 05, 2016, 05:53:28 pmSorry I didn't make it clear about the current demands, they were quite vague. The old controller was binned long ago, so I can't find out what current capability that had, however suffice to say 50A surge is probably a minimum. I always remember the old controller running scorchingly hot until it died, so it obviously was under-rated. It had 4 FETs on a tiny heatsink and could do forward & reverse so I assume it was a full-bridge configuration. The motor is brushed DC.Regarding forward and reverse, I understand having both will impact power dissipation & complexity, however I would like to design both in for now. If I wanted to do forward only I could always have interchangeable H-bridge boards and re-program the micro to suit (I was leaning towards a modular-ish design anyway at least for prototyping - I'm not too critical on size for the moment). As mentioned earlier, I am mainly a hobbyist and don't do any racing so the inclusion of reverse isn't a problem for now.As for using P channel FETs to do away with the gate drivers, I will have another search around for some adequately rated N channel devices and their complements - the quick search I had done before showed that the Rdson values for the P channel devices was quite a bit higher which had me worried with dissipation at such a (potentially) high current.If I do stick with full-bridge with all N channel FETs, what are people's thoughts on using the MAX620 as a driver? It seems to do exactly the job I'm looking for but ideally I would find a cheaper alternative. I have looked into the Intersil HIP4081A (similar device, but only operates down to ~8V Vcc, but does have integrated dead-time protection) and also at photovoltaic isolators, but these proved to be quite expensive too and slow with high gate capacitances.Once again, thanks for all your help.Intersil HIP40981A Datasheet:http://www.intersil.com/content/dam/Intersil/documents/hip4/hip4081a.pdf

author: sca
author_rank: Regular Contributor
timestamp: May 05, 2016, 11:04:18 pm
content: Quote from: TheSteve on May 05, 2016, 08:09:19 pmThanks for that, I was trying to remember the 5 letter manufacturer beginning with T!Quote from: stmdude on May 05, 2016, 07:32:12 pmReverse was permitted back then in the UK - though it was more common on the old style servo driven switch style speed controllers.I think Futaba were the first to come out with an all FET reversing ESC. Before that most used a relay to provide the other half of the bridge. It was notoriously easy to blow though.sca

author: sca
author_rank: Regular Contributor
timestamp: May 05, 2016, 11:18:29 pm
content: Quote from: jack_elec on May 05, 2016, 10:47:05 pmYou would probably want to at least add a zero to that when considering the surge rating - as macboy said the currents can be extremely high under acceleration for an appreciable time.Keeping that RdsOn low and providing for getting the heat away is the major aim though.It's a nice project for a first year student - theoretically reasonably straightforward but plenty of gotchas especially when you shrink it down small enough to go in the car - one of those few times when the cheap 5cm x 5cm PCBs are too big. Be prepared for plenty of interesting post mortems when you really start abusing it.Good luck!sca

author: mubes
author_rank: Regular Contributor
timestamp: May 06, 2016, 07:15:13 am
content: You might want to take a look athttp://vedder.se/2015/01/vesc-open-source-esc/. The CPU is hugely overrated for the job it has to do, but the design is reasonably nice (but not built for cheap).RegardsDAVE

author: jack_elec
author_rank: Contributor
timestamp: May 06, 2016, 09:43:59 am
content: Thanks for the advice sca, it will definitely be a good challenge to build something robust enough that can handle the current required. My plan is to do a few protoypes first, perhaps breadboard the micro circuit for now, and throw together a quick and simple PCB with some beefy traces for the H-bridge. Once I'm happy with the design I will try to shrink it all down as much as possible (we are able to do SMT stuff here at uni so that should help).For now it looks like the MAX620 is the best option, it can drive 4 gates, provides buffering of the MCU outputs and doesn't need many external components. I would definitely like to find a cheaper alternative however, but using the parametric search tools on the likes of Farnell doesn't seem to bring up anything suitable.