Open Loop, Closed Loop???
03-09-2005, 07:30 AM
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Open Loop, Closed Loop??? Open Loop and Closed Loop can someone please explain what both of those mean, what they are used for and how they effect the car?
Thanks
~Tom
Thanks
~Tom
03-09-2005, 07:53 AM
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Open Loop and Closed Loop are 2 terms that describe a type of control system.
An open loop system does not use feedback and has no way of determining if the commanded value is the actual value. For example, (in any controls book i've ever read) a toaster. You put the toast in, you select a number for how burnt you want the bread. We'll say you select 3. You put it in and it comes out toasted - simple control system. In this example, there is no way for the toaster to really be able to tell if it got to a level 3 toasted value but it doesn't care. It could have toasted to level 2.75 or to 3.35. There could be variation from toast to toast and the toaster would never know.
In a closed loop system you would have a mega toaster. You put the toast in, select level 3, and as it toasts the computer would continually monitor it and make sure it was toasting to exactly level 3. It uses a feedback control to check, "am I at level 3 yet?" it checks and says "No" and then keeps toasting. It continually monitors the feedback until it has reached your commanded input, 3. The output is a perfectly toasted level 3 piece of toast!
So where do you use it in cars? Well everywhere from controlling the bypass valve in a supercharger to make sure you are getting the commanded boost level, to how much fuel you need to start up your car on a cold morning. Sometimes the PCM uses a simple lookup table based on various parameters and spits out a value (open loop). Other times the value is continually checked to make sure you are at that exact value (closed loop).
Closed Loop control systems are more complex but are more accurate. Open loop systems are easier to engineer (less cost, time, money) but you can't be sure you are getting out the value you desire.
Hope I didn't completly lose you! Ask more Q's if you need!
An open loop system does not use feedback and has no way of determining if the commanded value is the actual value. For example, (in any controls book i've ever read) a toaster. You put the toast in, you select a number for how burnt you want the bread. We'll say you select 3. You put it in and it comes out toasted - simple control system. In this example, there is no way for the toaster to really be able to tell if it got to a level 3 toasted value but it doesn't care. It could have toasted to level 2.75 or to 3.35. There could be variation from toast to toast and the toaster would never know.
In a closed loop system you would have a mega toaster. You put the toast in, select level 3, and as it toasts the computer would continually monitor it and make sure it was toasting to exactly level 3. It uses a feedback control to check, "am I at level 3 yet?" it checks and says "No" and then keeps toasting. It continually monitors the feedback until it has reached your commanded input, 3. The output is a perfectly toasted level 3 piece of toast!
So where do you use it in cars? Well everywhere from controlling the bypass valve in a supercharger to make sure you are getting the commanded boost level, to how much fuel you need to start up your car on a cold morning. Sometimes the PCM uses a simple lookup table based on various parameters and spits out a value (open loop). Other times the value is continually checked to make sure you are at that exact value (closed loop).
Closed Loop control systems are more complex but are more accurate. Open loop systems are easier to engineer (less cost, time, money) but you can't be sure you are getting out the value you desire.
Hope I didn't completly lose you! Ask more Q's if you need!
03-09-2005, 08:10 AM
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Open loop means the PCM reads airflow and calculates
required fuel, using the tables in the PCM without any
real-time correction from the O2 sensor feedback loop.
Closed loop means it's using the sensors to determine
whether it's rich or lean, and trying to straddle the fence.
O2 swings, time-averaged, against a threshold make an
error term which is accumulated to the short term fuel
trims (STFT). High STFTs also will drive a long term trim
(LTFT) value which absorbs the baseline discovered error
to make sure the STFTs have a freer hand and the system
starts up closer to ideal.
As long as your feedback isn't all messed up like some
garage band doing their first mic check. Heh.
The fuel shot mass is commanded fuel air multiplier times
air mass (dynamic cylinder air) times (1+STFT+LTFT)
divided by stoich value (14.628 or whatever). Then the
magical gnomes inside the box have to calculate that into
a driver pulse width, but that's another fairy tale.
In closed loop the multiplier is 1.000 (stoich target, which
for a narrowband O2 is the only place you can work, and
the narrowbands were developed to work, there). The trims
are active.
In open loop the multipliers vary by numerous tables, the
trims are zeroed (unless you have positive LTFTs, then in
PE more these are kept as a safety padding) and the O2s
are ignored. Though, sometimes a little of their PE-smacked
input seems to bleed over onto the fuel trims, I've observed).
Closed loop is the default when the engine is warmed up.
You get into open loop by a number of trapdoors. Engine cold,
PE conditions satisfied, O2 sensor faults are the most common
but I suspect there are a few more.
required fuel, using the tables in the PCM without any
real-time correction from the O2 sensor feedback loop.
Closed loop means it's using the sensors to determine
whether it's rich or lean, and trying to straddle the fence.
O2 swings, time-averaged, against a threshold make an
error term which is accumulated to the short term fuel
trims (STFT). High STFTs also will drive a long term trim
(LTFT) value which absorbs the baseline discovered error
to make sure the STFTs have a freer hand and the system
starts up closer to ideal.
As long as your feedback isn't all messed up like some
garage band doing their first mic check. Heh.
The fuel shot mass is commanded fuel air multiplier times
air mass (dynamic cylinder air) times (1+STFT+LTFT)
divided by stoich value (14.628 or whatever). Then the
magical gnomes inside the box have to calculate that into
a driver pulse width, but that's another fairy tale.
In closed loop the multiplier is 1.000 (stoich target, which
for a narrowband O2 is the only place you can work, and
the narrowbands were developed to work, there). The trims
are active.
In open loop the multipliers vary by numerous tables, the
trims are zeroed (unless you have positive LTFTs, then in
PE more these are kept as a safety padding) and the O2s
are ignored. Though, sometimes a little of their PE-smacked
input seems to bleed over onto the fuel trims, I've observed).
Closed loop is the default when the engine is warmed up.
You get into open loop by a number of trapdoors. Engine cold,
PE conditions satisfied, O2 sensor faults are the most common
but I suspect there are a few more.
