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Forum Index : Microcontroller and PC projects : In the hot seat
| Author | Message | ||||
| Tinine Guru  Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
My client is a producer of coach driver-seats and he wants to incorporate heaters. They intend to fabricate a metal enclosure with a "rugged switch" and have asked me to come up with a small control unit. I don't agree with their suggestion that this should be simply ON for a factory preset time and then OFF for a preset time. My belief is that this will burn the driver's buttski. I'm thinking; one of the tiny Picos, driving several levels of PWM, maybe three levels; low/medium/high. Any opinions on this? I have received a couple of these heating elements and that is basically it, just two wires. I measure 52R element resistance. Craig |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 639 |
G'Day Craig again. As I have said in the past, the hard part is defining the problem. The software is the easy bit. Good luck Peter |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 3496 |
Most heaters are resistive. So in a coach, where battery voltage is typically 24V the 52 ohm would be like 10 watt. Yes you could PWM it, even with temp sensor feedback from a pico. Let the PICO drive a power FET on a PWM output. Read temperature from a DS1818 or similar, and glue the sensor to the seat. Add a variable resistor that you read from an ADC to adjust driver comfort level... That would be around $10 in materials EXCLUDING the connectors.... PicomiteVGA PETSCII ROBOTS |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5705 |
A 556 timer? One half is a "temperature" control that switches the other half. The other half is an oscillator driving an output transistor. So, the second half sets the max temp and the first half fires it in bursts. Or an 8-leg PIC and a darlington transistor. You could have a cheap temperature sensor in the seat (the linearised thermistors are really nice and will give a 0-5V output). Simply maintain the seat at a fixed temp when it's switched on. Similar idea - burst fire. PWM is overkill. I'm not tempted to use a Pico or Arduino here simply because of possible silicon supply difficulties. It may also be an electrically noisy environment. KISS wins. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 1985 |
the temperature levels (however you actually "do" it) need to be fairly rapid in response. Often I have seen solutions that have a constant setting for a given level that is fixed for the whole time of demand, i.e. if MED is 30%, the PWM comes on at 30%. This way it takes ages to warm up. Simple feedback can provide a much better method whereby you come on at 100% for rapid warm up then drop to your required maintenance value as you approach the required temperature - bearing in mind there is hysteresis and the will be some over-run. Probably not critical for a human butt, but precise control e.g. bring a lump of metal to a given temperature in the shortest time, progressively slowing the PWM as you approach the target temperature to a point where the PWM sustains the temperature... and dealing with the inevitable over-shoot as the environmental temperature varies - ask me how I know this without inquiring about the heated bed controller for my 3D printer  |
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| PeterB Guru Joined: 05/02/2015 Location: AustraliaPosts: 639 |
As I was saying  |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
@Volhout: Sorry, typo...that should have been 5.2 ohm  They already know that if it's cost-sensitive, they're talking to the wrong guy Yeah, I brought-up the 24v question. I don't know if they bring that into the cab...waiting for clarification on that. @Cap'n: PID is what I do so no problem there  @Mick: Client needs to be able to play with parameters such as PID gains and whatever timers we come up with. I was also thinking about some form of power-setting indicator. They are talking an initial order of 200 units and so I'll insist on a downpayment that would cover all components. Of course, I got carried away and proposed a mobile-device/Bluetooth controller but I think that was a bit much for them Rough idea:  Craig |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 5705 |
There are already heated seat systems. Loads of them, even for buses I think. Why aren't they looking at those? Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
It's the old game of coming up with something that's their own brand. I mean, this will probably be a stainless-steel enclosure with laser engraving...they have all the equipment for this and any knobs or pushbuttons must be rugged:  Craig |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 1773 |
With engine running - (28V ^ 2) / 5.2R = 150W It's a freckle cooker without reliable feedback! |
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Plasmamac Guru Joined: 31/01/2019 Location: GermanyPosts: 501 |
Inside from the elektric blanked : Medisana Gtx Plasma |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
Oh heck, now I'm in trouble because the client is all excited All I suggested was a single button and few LEDs to indicate the power level. He agrees that all-or-nothing power is not the way to go...so PWM it is PicoMite is overkill but I love it Craig |
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| CaptainBoing Guru Joined: 07/09/2016 Location: United KingdomPosts: 1985 |
it's a $4 component and once you have everything in the software domain it's all a piece |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
IKR? But I have 3 PicoMites linked together as I/O expanders and it feels criminal to do this. OK they handle debounce locally and for me, a "START" signal involves two inputs transitioning simultaneously (lo to hi and vice versa) but I still feel like I'm committing a crime. [checks outside for black SUV] Craig |
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| Tinine Guru Joined: 30/03/2016 Location: United KingdomPosts: 1646 |
Huh! Boringly simple.... But I guess, no need for a thermistor. Craig |
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