Here's the link Fuel Pump (DC MOTOR) Tuning Basics
WeaponX_Perf said:The internet and industry is full of presumptions and so much mis-information that many times it is hard to sift through most relevant information out there.
The main points concerning vehicles are electrical diagnostics, tuning and understanding of the electrical components that make up a modern day vehicle. Today, the modern day mechanic often doesn't have the technical background to understand the intricacies of todays modern engine management systems. The only people with enough knowledge would be individuals with a deep degree of electrical engineering and sound electrical principals who are the individuals designing these systems, not the individuals debugging or fixing them.
With that being said, many interesting emails have passed my way concerning one common mis-conception I would like to clear up which is fuel system tuning. During my time as an EE and teaching at a post secondary level there are many details that must be looked at when tuning these systems that are often overlooked. Here are some small excerpts from relevant notes taken from a EE with masters degree in electronics.
DC MOTOR SPEED CONTROLLERS
The purpose of a motor speed controller is to take a signal representing the demanded speed, and to drive a motor at that reference speed.
Open Loop Speed Controller
Electrical power supplied to a motor, machine is allowed to operate without any automatic adjustment.
No compensation for changes in
● load
● unintended changes in the supplied power
● changes in the ambient temperature
● changes in the operating temperature of the machine
● or anything else that might cause the output to vary during operation
Output of the machine is determined by the response of its design characteristics to the operating conditions.
Feedback Speed Controller or Closed Loop Speed Controller append - (LATE MODEL VEHICLE FUEL PUMP [PID] CONTROL)
- desired output of a system is called the reference
- output variables of a system need to follow a certain reference over time (fuel pressure) and measured by feedback devices (in this case fuel rail pressure sensor)
- controller manipulates inputs (voltage tables) to obtain the desired output
......
TYPE OF CLOSED LOOP CONTROLLER = PID
● proportional
● integral
● derivative
*note - there is no such thing as a PURE feedforward controller as PID is required to find setpoint.
Proportional term
-determines the reaction to the current error and known as “gain”
-determines majority of the output gain
-large gain causes large changes in output
-faster response to large error since greater feedback is used to compensate
For example when gain is 2 TIMES
- when input is a constant 5 volts from reference, proportional term outputs 10volts in direct action against the difference.
- when the system is changing 1 volt per second from reference, proportional term outputs 2 volts per second in direct response to the change
Integral term
- determines the reaction based on the sum of recent errors and known as “reset”
- proportional to magnitude and duration of the error
- steady state errors are eliminated quicker
- tradeoff is larger overshoot giving positive error before reaching steady state
For example when reset is 1 TIME
- if error is steady state 5 volts from reference integral term will output 5 volts per second in response to the error
- if system continues to increase in error change to the output becomes exponential in order to compensate for the increasing error
Derivative term
- determines the reaction based on the slope of error over time known as “rate”
- greater derivative term slows overshoot rate but slows down transient response of a system
Example when rate is 1 TIME
-if the system has a steady state 5 volt error derivative term does nothing because there is no change in error over time
-if system error increases at a rate of 2 volts per second the derivative term outputs a steady state 2 volts in response to the error
REAL WORLD EXAMPLE
As a human controller, one decides roughly how much to change the tap position (Manipulated Variable) when allowing fuel to an engine. After one determines the Process Variable Difference, and therefore the error the first estimate is now the equivalent of the proportional action of a PID controller.
The integral action of a PID controller can be thought of as gradually adjusting the manipulated
variable (fuel pressure) in order to reach preferred setpoint. Without integral action a PID controller would not achieve setpoint successfully.
Derivative action can be thought of as noticing if the system variable is getting closer or further, as well as how fast, anticipating further change and tempering adjustments for a soft landing at the desired setpoint.
The main problem with altering voltage input at the fuel pump is that the PID controller typically adjusts output depending on voltage reference, or voltage read at the PCM. If voltage at the PCM does not match voltage at the pump the PID controller effectively cannot work in harmony with the DC motor.
EXAMPLE
Previously PID proportinal term outputs X volts (total gain) based on feedback from the FRPS (fuel rail pressure sensor) and voltage reference read at the PCM.
If voltage is artificially inflated to, for example, 28 volts at the pump, and the PID proportional term is calling for a gain of 2x from the PCM reference X which now becomes a 4X multiplier.
Why is this?
System PCM reads 14volts as nominal, 28 volts is supplied to the pump having a gain of 2X. PID controller calls for another gain of 2X effectively increasing gain above and beyond the working range.
Disadvantages of this? Increased system overshoot, blown fuel rail pressure sensors, increased ringing in the system due to system overshoot causing ruptured diaphragms on sensors.
This is just the example for the proportinal term. Integral and Derivative term will also be improper for the system.
So how do you adjust your PID controller for smooth system response.
Typically there are 2 methods but this is the most common.
If the system must remain online.
One tuning method is to first set INTEGRAL and DERIVATIVE values to zero. Increase the PROPORTIONAL TERM until the output of the loop oscillates, (system ringing) then PROPORTIONAL should be left set to be approximately half of that value for a proper "quarter amplitude decay" type response. (quick to react but with minimal overshoot). Then increase INTEGRAL until any offset is correct in sufficient time for the process. However, too much INTEGRAL will cause instability. Finally, increase DERIVATIVE, if required (not usually for fuel systems), until the loop is acceptably quick to reach its reference after a load disturbance. However, too much DERIVATIVE will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly; however, some systems cannot accept overshoot, in which case an "over-damped" closed-loop system is required, which will require a PROPORTIONAL setting significantly less than half that of the PROPORTIONAL oscillating setting.
