Boostcontrol With Target Based Reference

BoostcontrolWithTargetBasedReference

New Boostcontrol strategy allows even easier configuration since firmware 1.2.9

Overview

reference DC can be configured in a 2 dimension table: as a function of RPM, boosttarget
with this "target based reference PWM duty", PID boost is easier to tune for very quick reaction and small overshoot
- if the hardware actuator is good

PID can be configured to switch to PD when boost is way too low, or to PI when boost is too high

actually, configurable thresholds for different strategies (operation ranges), as MAP grows:
boost off
max-DC (to help spoolup)
ref-DC
ref-DC + PD
ref-DC + PID
ref-DC + PI

One can of course configure an extremely large window for the PID range

even to use PID strategy all over the place, and not use any other strategy at all.
the strategy is selected based on the configured thresholds wether it's optimal for the actual system or not

Mattias Sandgren was kind to contribute some documentation embryo - will be moved to [VT help] at "birth-time"

Begin by setting the "Boostcontrol PID integral limit" to 0 and adjust the "refDC Table vs boost target" so that you get a good starting point for enabling the PID regulated boost controller.

Apply max DC until

To shorten turbo spool time you can apply the maximum dutycycle allowed. This can only be done up to a certain limit below the boost target, set this limit here. This must be kept low enough to avoid an initial boost target overshoot that the PID regulation will not be able to stop.

PD control (far from target)

PI control (close to target)

PID overlap range

Boost P, I, D

Boost control PID integral limit

Set how large the integral is allowed to build up. This value is what is added/subtracted to the reference dutycycle in order to reach boost target.

Setting this to 0 disables the I term (important note: NOT the boost control PID regulation !) completely.

Boost PWM maximum value

Limit the maximum dutycycle the boost control valve receives.

Boost PWM minimum value

Limit the minimum dutycycle the boost control valve receives.

Copied from IRC discussions :

Overlap is PID mode, where its far enough from target to still might need the D term

PD is for not winding the integral up when too far from target

PI is close to target (its now control range)

when close to target, PI only is used, as D wouldnt make any big difference anyway

so it can be used with higher settings from far away, still not messing with close to target control

What is the buildup range and control range?

Buildup integral is frozen and control derivate is disabled. It means "too far from target, probably not physically possible to reach, don't build up a integral error" and disabling D term "close enough to target, error for P is small, so no D needed".

Overlap is PID

P is not active between max dc and buildup

Buildup = I frozen outside this range

buildup enables PD

control is PI

between buildup and maxduty there is referece only

Future ?

the buildup range might go, just enabling PD from maxduty

its easier to find the optimal P if its scaled in kpa / %

it's possible to configure that way, of course so that PD starts as early as the buildup

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(may2013 FPhil)This is what I get from above, I hope that is correct

DOMAIN	zero to maxDC	maxDC to BuildUp	BuildUp to Cntrl	Cntrl to Target	Target to ? (Overlap)
STRATEGY	DC	??	PD	PI	PID
PARAMS		PWMmin & max	id & I limit	id & I limit	id & I limit

What happen if for some reasons boost enter the Cntrl to Target domain at fast increase? one can then expect a large overshoot.

he needs to tune better then. The Boost-buildup depends on the settings, eg maxDC and the ref-PWM table values.
- The good strategy is to tune from low-boost (and low values in REF-DC table), only climbing as the control is tuned and confirmed to be consistent.
- or you mean "what if some tube comes off, resulting in unexpected boost sky-rise" ? Yes, that is a concern.. even worse, in many cases the control could hardly do anything about it (or it would be very time-consuming to tune the parameters/thresholds to notice that some tubes came-off and limit boost some other way, eg. with intentional misfire - maybe feasible for 1 million engines but possibly more risk for a few engines than it saves)

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My way is start tune with quite large PID values and low boost with slope RefDc table where bottom row is W-gate minimal pressure and top row - aprox 30Kpa above my maximum boost expectations. Bottom line filled with 0...10, top line - 60...100 - spring dependant, Interpolation inbetween. With PID enabled it is easy to fill Ref DC table with correct values just by reading the log file. Wise way is match row boot value with current target - then actual BoostDC represent DC values for table. Overshoot or slow buildup can be achieved by table increase or decrease respectively.

What is good in new strategy - it allows much larger Integral values.

First WOT run with boost valve off [minimal pressure] is helpful for further tuning. GintsK.

VemsTune

Since vemstune knows the runtime integral, it can display the contribution of I term by multiplying with configured pid_ki value (and a constant):

boost_integral_effect= config.boost_pid_ki * (runtime.boost_integral-128) / 32
- boost_integral += error / 128 every time (eg. 40 times per sec) where error is in 1/4 kPa units. So for 10 kPa error and 40Hz configured, boost_integral changes with a rate= 10*4 / 128 *40 = 12.5 / sec (inside the ECU it's 128 times higher precision than logged, but that's not interesting as far as VT concerned).
boost_P_effect = (boost_target_raw*16 - MAP) * config.boost_pid_kp / 4 / 32 *100/256 need update and benchtest

(MAP - boost_target_raw*16) * config.boost_pid_kp / 8

The unit is not kPa but 1/4 kPa for MAP (and 4 kPa for boost_target_raw, that's why the *16 - only applies locally, *16 MUST not be applied to MAP !)

So for kp=100, with MAP 10 kPa below target: ((190-200)*4) * 100 / -2048 = 1.95 (/256 of full range, so 0.76 %)

perceived : KPA_ERROR * kp / 100 / 32 , when Error is 10kpa, kp is 100 , then output P is 0.3125%

Where did you see 0.3125 % ? Although internally higher resolution, don't you see output DC in 0.39% steps ? Maybe you mean 31.25 / 256 = 10 * 100 /32 / 256 = = 12.2%

Similarly for IAC:

iac_integral_effect= config.iac_pid_ki * (runtime.iac_integral-128) / 16
iac_P_effect= config.iac_pid_kp * (RPM - target_rpm) / 16

Since the actuator is known (eg. boost_DC and iac_duty )

contribution of D term follows : output = reference + P + I + D

(although D term is "transient" by nature, so I compensates in the long run).

[VT playground] for testing with new variables for gauges that can be selected ( also, the custom variables can be used to test new formulas in ini file - very useful ).

Gunni is writing a complete boost control guide for VEMS´s new setup, but have found the following:

"I" value influences the Integral limit and thus the total possible DC% that the I value is able to give.

(A+l) * I == AI + L has completely same effect if L = l*I so it's just a configuration question
- if sufficiently high integral limit is configured, then integral will be limited (to provide integral windup) as the min_duty or max_duty is reached so integral limit should not have a big significance (maybe it should be removed and not be configurable ?)
- We are aware of "I_limit would better be final contribution DC % instead DC / ki". But changing would break compatibility and hundreds of tuned boost/iac setups (and noone would be able to tune the PID who cannot tune this way). Yes, it would be nice if integral limit would be duty % (not changing when changing ki value), but with the automatic integral limit it matters none. The non-invasive solution now is more likely displaying integral_limit * ki * constant in duty percentage, which represents the final DC% and updates automatically if either integral_limit or ki reconfigured. Alternatively integral_limit could be automatically adjusted when ki changed - maybe a bit superfluous.

I have detailed so far the operation of getting to a final boost target in a flow chart and then getting to a final BoostDC% but need more information about the PID values

Benchtesting shows clearly

P_DC = KpaError*kp / 32 ( KpaError*kp / 32 *100 /256 when viewed in % )
- exactly: kPa is 1/8 kPa inside boostcontrol (MAP target), and *8 / 256 = /32 so (after correcting some likely typos) theory and measurement matches fine

I value:

the update rate is as configured
- eg. 40 Hz boost_out PWM frequency is 40/ second that is 25 msec; internally the integral is updated every period (error is added)
See boost_integral_effect above for how the contribution effect can be calculated from 128 based boost_integral value

D value contribution:

pid_kd * feedback_change / 4096
feedback is 1/8 kPa for the MAPtarget (boostcontrol), 1 RPM for iac
update rate is as configured

PID 0-255 scaling:

Ideally would be removed and engineering scaling implemented instead even if it´s a drop down menu with 255 pre selected values or Vemstune automatically rounds down or up to the nearest allowed value like is seen in many places already.

Boost PID output is logged (boost_DC and iac_duty).

feedback is logged (MAP, or RPM for iac)
Integral (the only internal "memory"/state that does not follow from actual ).
- the applied duty cycle can be represented as 0-100%, or 0-255 it's the same thing, with a constant factor (like kPa or PSI - both arbitrary choice; 0-100 is also arbitrary choice, 256 being 2's power is more natural) Boost duty DC is historically displayed as 0-100% - likely to stay that way, maybe not reasonable to waste time on that.