Throttle Control Philosophy

[electrosteam]

I have recently built a 2-person ride-on trolley and a 600W 0-B-0 loco for 5 inch gauge with simple PWM (pulse width modulated) battery drive.
Both units operate perfectly OK.

The throttle control is a potentiometer that simply adjusts the mark-space ratio of the PWM and the driver trims the setting by the "seat-of-the-pants".

I have designed a new controller using a multiposition rotary switch to provide 4 settings of torque (tractive effort, DC motor current) as is used frequently on the full-sized railways.

The controller measures the motor current and regulates the PWM mark-space ratio to maintain the current at the selected setting.
The settings available would be Off, 25%, 50%, 75% and 100% motor current.
I have visions of a 'beautiful' long-handled control switch with positive 'clicks' at the notch settings.

The prototype has a large mass low friction environment, whereas the model has, in comparison, a low mass high friction environment.
The driving requirements, then could be quite different.

Has anyone got experience of driving with 4 steps of torque (tractive effort) ?
Should I give up on the tractive effort controller and be content with a mark-space controller ?

Happy machining,
John.

[jackrae]

I've no experiience of your particular requirement but my philosophy would be "never give up on anything unless you've tried it" Half the fun is trying. If someone has a good idea there is no shame in using their technique (learning from experience), but just because they may have failed doesn't mean it's impossible.

jack

[steam4ian]

G'day John

Interesting approach.

What you propose is slightly different from the "real thing" because I am assuming you do not have series motors. Series motors inherently slow down with higher current/load which is what makes simple series/parallel and field weakening in say 4 steps able to control an EMU; normally there are more acceleration steps in the series connection.

Also "real" controllers for diesels have a traction curve for each setting which not only gives a maximum current but also a maximum voltage. Together with series traction motors 8 steps (notches) are sufficient to give good control, the steps are set at semi logarithmic intervals.

I would suggest that you basically have the controller set the mark space ratio but include a current limiting feedback loop so as to get a drop in speed with increased load/current. Each step will allow a higher motor voltage and a higher current setting. You may want to fiddle with the steps to get something more like 15%, 30%, 60% and 100%.

Regards
Ian

[electrosteam]

Ian,
Thanks for the contribution to the subject.

I agree, the diesel prototype frequently has a power setting of the engine for each notch accomplished by simply controlling the fuel rate.
The electronics for the engine control, which these days is likely to be 3-phase AC, then control the motors within that power envelope.

I played with the idea of a power controller, but decided the first step should be tractive effort alone.
In each step the motor can come up to the balancing (maximum) speed at the 24Vdc battery voltage.

What I was hoping for from the forum members, was any comments by members on the "feel" characteristics of driving a loco with tractive effort control.

My concern is that 4 notches are insufficient to "tune" the control setting to the train demand on 5 inch gauge.
This would mean constant adjustment as the train accelerates and then decelerates about the desired optimum setting.

In the spirit of experimentation, I think I will make a temporary tractive effort controller and try it out.
Results will be posted in a month or so.

Happy machining,
John.

[jackrae]

That's my boy !

I'd be interested in seeing your circuits if possible

jack