Lever control with potentiometer - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: August 03, 2014, 07:46:52 pm
content: I'm trying to design a speed controller for some Scalextric. Having found that the old resistor controller suffers badly from wear I thought it would be best to make an electronic PWM controller. So I need a mechanical input, I would need some kind of lever with a potentiometer attached and perhaps a spring back mechanism.Is this something readily available or should I just make my own from scratch.

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2014, 08:03:24 pm
content: Can you replace the guts of the existing controller and keep the mechanical parts?I think it's possible to get linear potentiometers. You could replace the existing rheostat with one of these perhaps?

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 08:06:19 pm
content: linear pot + 555 pwm generator - extremely simple and should fit into the original casing.

author: Simon
author_rank: Unknown
timestamp: August 03, 2014, 08:30:49 pm
content: Quote from: IanB on August 03, 2014, 08:03:24 pmyep corrected title. I don't think i can fit the potentiometer into the existing controller. I just want to start from scratch. Yes of course you can get a linear potentiometer.What I'm thinking is a lever with a potentiometer coupled to the pivot point with a spring for the lever to push against. It's going to be used mostly by kids so rugged and perhaps with enough travel would be ideal.

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 08:50:32 pm
content: Hmm, just get some encoder + cheapest MCU with pwm and a bit of very simple coding.

author: Simon
author_rank: Unknown
timestamp: August 03, 2014, 08:51:30 pm
content: how many steps will i get from an encoder over 45 degrees ?

author: wraper
author_rank: Unknown
timestamp: August 03, 2014, 08:56:05 pm
content: Highly depends on encoder model, need to read datasheet for particular model. You can get as low as 6 steps per turn and as high as over 2000.

author: miguelvp
author_rank: Super Contributor
timestamp: August 03, 2014, 09:20:33 pm
content: Hall effect sensor? The new XBox One controllers use them for the triggers.

author: rob77
author_rank: Super Contributor
timestamp: August 03, 2014, 09:41:34 pm
content: i'm just wondering how we tend to over-think and over-design things (including myself in many cases)it's a toy - it will be destroyed in months even if you use a industrial grade (read: expensive) encoderfurthermore you would need a high resolution encoder to get a good response over the 45 degree angle..so i think my suggestion to rebuild the controller with a pot + 555 PWM generator is the cheapest solution even if you will have to change the pot twice a year

author: IanB
author_rank: Super Contributor
timestamp: August 03, 2014, 09:54:58 pm
content: The existing controllers probably wear out by erosion from the high current load. It would probably be possible to make an existing controller last longer by sensing its varying resistance electronically and using that to drive an external power controller (PWM or even just a pass transistor).

author: Andy Watson
author_rank: Super Contributor
timestamp: August 03, 2014, 09:58:39 pm
content: Quote from: rob77 on August 03, 2014, 09:41:34 pmexactly! Isn't part of the fun trying to get the controller to work reliably? Or palming-off the dodgiest control to your opponent?How about: A cheap, cordless drill with speed control? Gives you the mechanism and transducer in pistol grip (just like the original scalectrix) plus, you might be able to re-purpose the electronics of the drill to directly control the car.

author: David Hess
author_rank: Super Contributor
timestamp: August 03, 2014, 10:27:40 pm
content: Bourns makes linear potentiometers for position sensing applications:http://www.bourns.com/ProductLine.aspx?name=contacting_linear_motion

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 03, 2014, 10:32:45 pm
content: Quote from: Simon on August 03, 2014, 07:46:52 pmThese are slot cars, yes? I assume the old controller was merely a wirewound low-resistance power resistor feeding the slot of your particular car?I guess the control unit is a simple hand-held trigger thingy?Depending on the voltage the slot car runs on, I'd look into BRUSHED speed controllers (ESC) on eBay, they're cheap. What's the voltage?I wonder if the slot itself would be a good radiator of EMI? I guess it looks somewhat like a diff pair but not a very good one.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 03, 2014, 11:48:42 pm
content: Quote from: miguelvp on August 03, 2014, 09:20:33 pm^^^^^^Not so much Hall as pots, e.g.https://www.sparkfun.com/products/retired/10314Damn, they don't have them anymore. I don't see them at a couple surplus sites either. Maybe eBay some..?Heck, if the Hall type are more common now... go for it...Tim

author: miguelvp
author_rank: Super Contributor
timestamp: August 04, 2014, 12:15:41 am
content: Quote from: T3sl4co1l on August 03, 2014, 11:48:42 pmSeen them with my own eyes, the new controllers use ratiometric hall effect sensors for both triggers.The trigger only moves a magnet closer and farther from the sensor.I'll try to see if I can find pictures.

author: miguelvp
author_rank: Super Contributor
timestamp: August 04, 2014, 12:35:34 am
content: No pictures but unfortunately the only video I found is from Ben Heck and we all know how much the OP likes him.But I'm linking it anyways, since I can't find any other reference:Don't know if I have the time set right, it's at 6:03

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2014, 04:18:52 am
content: Quote from: miguelvp on August 04, 2014, 12:15:41 amHall effect based angular and linear transducers are pretty neat. The only thing they lack is the reliability and maybe precision of a variable differential transformer but they are a lot easier to use and understand.I have seen a linear potentiometer like the one I linked used in a high power RC motor speed control where a linear servo actuated the linear potentiometer which was part of the pulse width modulator circuit driving the motor. It was crude but effective.

author: Simon
author_rank: Unknown
timestamp: August 04, 2014, 05:57:10 am
content: The linear pots look interesting, I think they might just fit in the old style controller.I had thought of using the original controllers as the input, sadly the old style controllers no longer work and the only others I have are the horrible new ones. Most of the children playing with our track at an event yesterday with the new controllers were holding them upside-down because of the stupid shape of the new micro scalextric controllers.The controllers will see quite a bit of use as this is to drive a large micro scalextric track that we take around to outdoor events for children (of all ages) to play on.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 04, 2014, 01:08:12 pm
content: I wonder if it would be worthwhile making the output an adjustable current rather than voltage. The car may be easier to control.You can get really fancy and measure the motor speed by measuring the back emf generated by the motor and comparing the actual speed to the setpoint.Nice project.

author: Simon
author_rank: Unknown
timestamp: August 04, 2014, 01:32:32 pm
content: It doesn't really need to get that complicated. I'm no motor expert but current control to me sounds akin to voltage control and as far as i know PWM will get better torque at low speeds and scalextric cars hardly need full power most of the time.Maybe one day I'll add sound effects and current measurement, a sudden loss of current draw after a certain value would be a very accurate indicator of the car flying off the track so I could play a crash should not that I have any idea of how to do that yet.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 05, 2014, 05:30:27 am
content: SimonMotor torque is directly proportional to motor current, (F=IBl) so if you configure the controller as an adjustable closed loop current source rather than an open loop PWM generator it may give you better control over the car. I remember Scalextric cars are very twitchy and difficult to control and this control strategy may make them more managable. Just a thought.

author: Simon
author_rank: Unknown
timestamp: August 05, 2014, 08:48:08 am
content: Right, maybe that would be a good idea. It shouldn't be too hard to make a SMPS current source or sense the current going in on PWM and it seems to make sense. So if parts of the track are dirtier than others and there is a bit more resistance in there it won't matter within reason.I've nmot been able to find nice lever controls without spending a fortune so i guess I need to either adapt the old one or make my own.

author: Richard Head
author_rank: Frequent Contributor
timestamp: August 05, 2014, 09:09:40 am
content: Just to test the concept of closed loop current control I would just use an LM317 configured as an adjustable constant current source. It won't be as efficient as an SMPS version but a hell of a lot quicker and easier to get operational.

author: IanB
author_rank: Super Contributor
timestamp: August 05, 2014, 02:42:08 pm
content: Tread gently where constant current power supplies are concerned. They can cause damage by delivering excess power to any weak part of the system they are connected to. This is illustrated by the power equation,W=I2R. Since the current is constant, the local power delivery goes up in any part of the system where resistance increases, for example a poor contact or dirty connection.I saw an illustration of this when I tried to run a "hot wire" arrangement with a constant current source. I wanted the wire to act as a heater and wanted to control its temperature. I found that with a constant current supply it was impossible to make the wire glow evenly red along its length. The wire would inevitably develop a local hot spot, go white hot and melt at that point.

author: Simon
author_rank: Unknown
timestamp: August 05, 2014, 02:49:09 pm
content: The voltage will be limited to no more than it was ever meant to be so no more power than what was already meant to be. The situation is quite different from yours. The problem with the old rheostat controls is that with a constant trigger position no two places on the track will deliver the same power as the resistance constantly changes. I've have plenty of kids constantly crashing cars on the same spot over and over because one bit of track was dirty and slow and then they hit a good bit and the car fly's off to exclamations of surprise and dismay. I measured the resistance on the track and it varied considerably.

author: pickle9000
author_rank: Super Contributor
timestamp: August 05, 2014, 03:36:10 pm
content: Hey Simon,Just as a slight side note. Pot wipers usually have a bit of slop. Bournes makes a small series of pots that have little to no slop and are intended for position sensing. Not that you need such a thing for this purpose but who knows if it will come in handy one day.String pots like thishttp://www.unimeasure.com/lx.htmare an example. You can see the blue pot hanging out the back side. Super easy to implement.

author: Simon
author_rank: Unknown
timestamp: August 05, 2014, 03:40:59 pm
content: Do you mean rotary pots ? If I can hack the original controllers I may use a "straight pot" but looks like I'll make some nice control levers with a rotary pot that will do fine.

author: pickle9000
author_rank: Super Contributor
timestamp: August 05, 2014, 03:55:24 pm
content: A string pot is an assembly that uses a multi-turn rotary pot with a spring and a spool to measure distance. The pots themselves are specially made to reduce positional errors. They are widely used on custom heavy equipment because of the ease of installation.A straight pot or slide pot will probably be more effort than it's worth. It would probably fit OK, the mechanism on these are not very stable. A regular rotary pot is the way to go if you can fit it

author: Simon
author_rank: Unknown
timestamp: August 05, 2014, 04:55:05 pm
content: At the end of the day it's just a controller for a toy, I'm not using it to precisely control safety critical lifting gear. I'll try to make it reasonably rugged as it could see much rough use but the mechanical side should handle that with no strain going onto any sensing component.Looking at the old controller it may lend itself to have the original rheostat replaced with a sliding pot and something to replace the original cursor can be a lever to move the slider pot wiper.