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  #1  
Old 05-26-2008, 05:58 AM
300SD81's Avatar
1981 Mercedes-Benz 300SD
 
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Designing a replacement cruise amp

After reading the thread on the new tach amp, I thought I might give making a modernized cruise amp a try... How much interest is out there if this works out? Based on the wiring diagram, it seems to be a pretty simple job with a microcontroller, with almost everything done in software.

Pin 1, 12 - Power/Ground:
Step down to 5v for microcontroller

Pins 3,6,2,4,8 - Control inputs and Disengage:
step down to 5v and feed into input change pins on microcontroller

Pin 11 - Speed input:
Input pin on controller, type depending on signaling method

Pin 5 - Release clutch control output:
standard output pin + transistor or relay

Pins 7,10 - Motor control outputs:
PWM output to H-bridge

Pins 9,13 - Feedback input:
5v on one pin, other input to ADC

I'll work out a basic circuit tomorrow after some testing for signaling methods, voltages and resistances. I'd estimate approx 25-50 components, and if theres interest, I'll offer a kit or preassembled, but you'll have to be able to solder SMT components if you get the kit, I've given up drilling holes in my PCBs a long time ago...
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  #2  
Old 05-26-2008, 06:35 AM
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good luck.
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  #3  
Old 05-26-2008, 07:58 AM
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Quote:
Originally Posted by 300SD81 View Post
After reading the thread on the new tach amp, I thought I might give making a modernized cruise amp a try... How much interest is out there if this works out? Based on the wiring diagram, it seems to be a pretty simple job with a microcontroller, with almost everything done in software.

Pin 1, 12 - Power/Ground:
Step down to 5v for microcontroller

Pins 3,6,2,4,8 - Control inputs and Disengage:
step down to 5v and feed into input change pins on microcontroller

Pin 11 - Speed input:
Input pin on controller, type depending on signaling method

Pin 5 - Release clutch control output:
standard output pin + transistor or relay

Pins 7,10 - Motor control outputs:
PWM output to H-bridge

Pins 9,13 - Feedback input:
5v on one pin, other input to ADC

I'll work out a basic circuit tomorrow after some testing for signaling methods, voltages and resistances. I'd estimate approx 25-50 components, and if theres interest, I'll offer a kit or preassembled, but you'll have to be able to solder SMT components if you get the kit, I've given up drilling holes in my PCBs a long time ago...

seems like you are opening yourself up to a lot of liability. anything to do with the power systems control is rigerously tested. automotive systems have to be extremely robust due to the very dirty 12v power, dust and vibration extremes, temperature extremes and just really dumb people out there driving. while it seems like a simple system, making it work 100% in the automotive environment is a diffficult thing indeed
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  #4  
Old 05-26-2008, 10:37 AM
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Good luck...
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  #5  
Old 05-26-2008, 10:43 AM
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Why not just install an aftermarket system?
You can buy complete setups for what a rebuilt controller costs.

I've installed ARA systems on 4 cars over the years, and they work as well as any factory unit I've had. I like the ARA line, because they will supply a control stalk that appears to be factory installed. While these are primarily for American cars, they also have a generic stalk that resembles the Mercedes setup.

J.C. Whitney has a large selection of units, some below $100. I know a few friends who are pleased with their *discount* cruise controls....

Jim
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  #6  
Old 05-26-2008, 10:52 AM
Craig
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Why not just replace/repair the correct system? I installed a rebuilt amp and a brand new actuator from GDL for about $600 total, it's not really worth trying to reinvent the wheel. You can also install an after-market system, just leave it reversible so the next owner can take it out and put in the correct system.
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  #7  
Old 05-26-2008, 12:10 PM
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do it- why buy something when you can build it. I woud love working CC,but it would have to be affordable for me........
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  #8  
Old 05-26-2008, 12:22 PM
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Quote:
Originally Posted by mobetta View Post
do it- why buy something when you can build it. I woud love working CC,but it would have to be affordable for me........
And cheap! And easy! If you make a cheap reliable one that I can just put in with out having to really understand how it works... take one out-put one in....I and lots of folks will line up!
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  #9  
Old 05-26-2008, 12:28 PM
Craig
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Quote:
Originally Posted by Alberta Luthier View Post
And cheap! And easy! If you make a cheap reliable one that I can just put in with out having to really understand how it works... take one out-put one in....I and lots of folks will line up!
That's what we have now, the rebuilt amps are just plug and play. I doubt anyone is going to make a replacement much cheaper.
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  #10  
Old 05-26-2008, 12:30 PM
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Having soldered the occasional SMT component, I would advise against a kit, at least until you complete the initial production run and get some user feedback.

Jeremy
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  #11  
Old 05-26-2008, 12:36 PM
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Quote:
Originally Posted by Craig View Post
That's what we have now, the rebuilt amps are just plug and play. I doubt anyone is going to make a replacement much cheaper.
I've thought about this, going so far as to start a PCB design (only up to component placement, though). The biggest problem is the mechanical aspect - pins, pin frame, housing. If you rebuild failed units by reusing the mechanicals and replacing the PCB and electronics, you should be able to do this for under $100.

This is the approach I took for my programmable interval wiper relay - replace the guts, keep the skin.
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  #12  
Old 05-26-2008, 12:40 PM
Craig
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I can see the parts costing less than $100, but what about the labor?
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  #13  
Old 05-26-2008, 01:10 PM
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make the buyer sign a liability waiver.
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  #14  
Old 05-26-2008, 01:32 PM
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Quote:
Originally Posted by Craig View Post
I can see the parts costing less than $100, but what about the labor?
Parts and labor. These modern microcontrollers have so much integrated function that parts count comes WAY down. My first pass at placement used a PCB that was less than one third the size of the original, and component count was about one fifth.

There is also the ability to add function. My 300D is kind of doggy off the line unless I manually switch off the AC compressor. My cruise amp concept included a seven second compressor cutout when the accelerator pedal travel reached a certain limit, or when the accel pedal rate of change was large and positive. All software and almost no components.
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  #15  
Old 05-26-2008, 01:40 PM
ForcedInduction
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The cruise amp is an analog computer, the switch to digital alone will make a huge cut in the parts count.
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