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Posted:
Tue Oct 27, 2009 5:19 pmby Sandra
I am building a regulationsystem to my rolling road with a thelma retarder. I am working on my software to control the brake and have some questions that I hope someone can answer.
How much should I accelerate for the best result? How many RPM acceleration do you recommend?
How often should I regulate the power to the brake? And how much should I regulate? The brake is controlled by power from 0V to 230V, how much voltage should I change for every regulation?
Of course I will try and error a lot but I hope that I can save some time with your help.
Posted:
Wed Oct 28, 2009 2:54 amby Tony
Sandra, the first thing you need to do is look at the tachometer output pulses coming from your rollers, and work out how that corresponds to road speed and/or engine rpm.
That will depend on roller diameter, gear ratio, wheel diameter, and how many output pulses there are per roller revolution.
Then you decide what sort of speed range, and acceleration rate you want to have during your dyno sweep test. That is entirely up to you.
Now let's imagine you have figured it all out for your specific dyno, and your particular vehicle application.
And you expect your dyno to accelerate up from 430 Hz to 2,350Hz in five seconds.
The actual figures could be very different from that, but let's assume this is the goal to test run your vehicle on your dyno.
Now what you must do is constantly control the dc power into the Telma retarder so that the speed of your rollers are controlled very closely to your desired acceleration rate, REGARDLESS OF POWER.
The way to do this is to generate in software a model of the acceleration you want using a software timer. This software timer needs (in this example) to very smoothly sweep upwards from 430Hz to 2,350Hz in maybe five seconds.
You then compare the pulses coming out of the software timer to the pulses coming off the actual rollers.
If the rollers are running ahead (fast), increase the dc current into your Telma to increase the load.
If the rollers are running behind your acceleration model, reduce the dc current to the Telma.
Obviously a 50Hp motor needs a lot less retardation than an 850Hp motor. so the software looks at what the rollers are doing, compared to what they SHOULD be doing, at any instant in time, and very quickly adjusts the dc current fed into the Telma retarder.
So in software, you need to write (and tune) a PID closed loop control program to compare the real world tacho pulses coming off the rollers, to the ideal perfect acceleration model you have created with your software timer.
This is all far from easy, especially getting it to be very smooth and respond quickly, and be completely stable.
But that is the way it is done.
Hope that helps.
Posted:
Wed Oct 28, 2009 2:30 pmby 49-1183904562
Tony,
Could you please explain how this correlates to engine load? I follow the timer sequence, but is your model using torque feedback also ?
Rick
Posted:
Wed Oct 28, 2009 3:33 pmby Tony
All you really require, is for the dyno control system to provide sufficient loading to hold the rollers at whatever speed they are supposed to be.
At fixed constant rpm, the roller speed is compared to some software target reference speed, and current through the retarder set by a PID control loop to hold that exact set roller speed.
A sweep test works exactly the same, but the software target reference speed glides upwards at a set known rate.
The PID loop follows torque humps and dips and modulates the dyno load to keep the roller speed increase exactly on track.
The dyno CONTROL system does not know or care about torque, only about roller speed. It adjusts the roller loading (by whatever means) to correct and hold the exact desired target roller speed, at any instant of time.
So your dyno controller should be able to hold the road speed, (or engine rpm) constant at any, or varying throttle opening. That is how the engine is tuned at part throttle settings at particular rpm for EFI engine mapping.
It is the dyno MONITORING system that measures roller torque, roller rpm, and calculates power. The dyno control system, and the dyno monitoring system are quite independent, and share nothing, except perhaps the roller speed sensor.
A 4WD dyno is just two separate dynos that reference the same software speed reference. So each set of rollers are speed locked to the reference.
Front and rear drive torque can be very different, depending upon front/rear torque split. But the roller speeds should be held absolutely identical if the control system is working well.
It should also be realized that the PID roller speed control loop will also compensate as your load absorbing device heats up, as it most surely will.
Friction brakes and eddy current retarders require considerably more applied control input effort to hold a given torque load (brake fade) as things start to get really really hot.
The PID speed control loop will compensate automatically as things really warm up.
It is the only possible way of running consistent sweep testing at widely varying power levels, and absorber temperature levels.
Water brakes are a bit different, but exactly the same principle holds. The dyno control system controls the roller speed by adjusting water flow into the power absorber.