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A new circuit design help

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skeeterb

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I'm trying to design a circuit that will be used in association with a uC to create a cruise control for an EV. What kind of setup would I need to use to allow a floating power input to float until a second input goes high. It would then lock in the power at the level the circuit was at when the second input went high. Before anyone fusses about me asking to design the circuit, I AM NOT asking that. I am just asking for any input that I can use to help ME design this circuit.
 
By locking in the power do you mean lock in the voltage level (sample of the voltage)?

If so then you could do that with a differential instrumentation amp with inputs connected to the the floating supply and the output driving a sample & hold circuit to capture the voltage.

Why is the voltage floating? How high is it floating (what is the common-mode voltage between the two circuits)?
 
It would probably run between 0-5V from a 5kΩ Throttle pot on an EV. The Varied voltage input is because the circuit would be in between the throttle pot and the EV controller. The circuit will also have an input from the VSS (Vehicle Speed Sensor) to allow throttle-up when climbing a hill.
 
Seems like you would want to sample the VSS and then servo to that. Otherwise you could fool the speed control by setting it while coasting or going up hill.
 
Every cruise control I have ever seen captures (samples and holds) the vehicle's speed (not the throttle setting) at the instant the control is set. It then compares the actual speed to S&H speed, and adjusts the throttle to maintain zero error.
This needs to be done digitally. An analog S&H will invariably droop over time.
 
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Every cruise control I have ever seen captures (samples and holds) the vehicle's speed (not the throttle setting) at the instant the control is set. It then compares the actual speed to S&H speed, and adjusts the throttle to maintain zero error.
This needs to be done digitally. An analog S&H will invariably droop over time.

so I would need to read the Vehicle Speed Sensor on the transmission in order to create a cruise control for the EV? Since I plan on using a uC to control it I would probably need to sample the input from the VSS to keep the speed at a constant speed.
 
so I would need to read the Vehicle Speed Sensor on the transmission in order to create a cruise control for the EV? Since I plan on using a uC to control it I would probably need to sample the input from the VSS to keep the speed at a constant speed.
That's what I think. For example, drive a car with a cruise control, and accelerate to, lets say, 65mph. Now remove your foot from the throttle, so you are coasting, When the speedometer hits 60mph, set the CC. You will find that your speed locks in at 60mph, not zero (or idle speed), as the throttle would indicate.
The CC overrides the throttle, and takes no input from it. On my vehicle (and probably all vehicles), the throttle is pulled down, away from your foot, when the CC is set, so that you don't inadvertently accelerate. You have to move your foot down farther before you can override the CC.
Can you drive? Do you have access to a car with CC? This is not new technology, at least at the man-machine interface. Drive a car and see how it works.
 
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I love using the cc on whatever vehicle I'm driving, especially if I am gonna be on the road a while. The Throttle pedal on the EV would not move because there would be no physical linkage between the cruise control and the throttle pedal. To control the speed it would control the amount of power going into the Controller bypassing the throttle pedal. I will have a few safety features like the switch actually controlling the power of the circuit. Turning on the switch would activate a transistor which would allow power to flow into the circuit. I want to add more features in the case of a runaway, but right now I'm just focusing on the cruise control feature.
 
The uC will have to have compensation to prevent hunting or surging of the control speed, such as can be provided by a PID or Fuzzy Logic control loop. (I prefer Fuzzy Logic as I believe the loop is simpler to design and optimize with it, since it basically just uses a series of If-Then-Else statements, but that's just my personal preference).
 
On my vehicle (and probably all vehicles), the throttle is pulled down, away from your foot, when the CC is set, so that you don't inadvertently accelerate. You have to move your foot down farther before you can override the CC.
I have to disagree with you there Roff.
On my car (a Ford Mondeo), when the cruise control is set, the accelerator (throttle in US parlance) just sits at its resting position.
If the driver touches the accelerator the car will speed-up and when the driver takes foot off the accelerator again the car will slow down to the cruise control speed.
If the driver touches the brake, the cruise control is cancelled until the "resume" button is pressed.
This behaviour was the same on other Ford vehicles I have driven recently, a Focus and a Galaxy.

JimB
 
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I have to disagree with you there Roff.
On my car (a Ford Mondeo), when the cruise control is set, the accelerator (throttle in US parlance) just sits at its resting position.
If the driver touches the accelerator the car will speed-up and when the driver takes foot off the accelerator again the car will slow down to the cruise control speed.
If the driver touches the brake, the cruise control is cancelled until the "resume" button is pressed.
This behaviour was the same on other Ford vehicles I have driven recently, a Focus and a Galaxy.

JimB
I won't argue with that. I had a Ford Ranger until last summer (I traded it in on a Toyota Tacoma), but I can't remember the details of the throttle.
All CCs will be cancelled by the brake.
 
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