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Pulsed DC can loosely be defined as PWM although, I think in this case it has distinctions.
If PWM was used in a system such as this, on would expect that the 24 VDC peak would be a constant value. i.e. a 0-100% square wave havig an amplitude of 24V.
So let's say measurements were made and that the CTI throttle reverted to PWM when the throttle was 10% or less. Further lets assume that the peak was 10% of 24V or 2.4V. If we have a 2.4 V pulse at low throttle, probably the trainwill laugh at the PS, but IF the amplitude was 24 V with a duty cycle of 10% to 0% from 10% to 0% throttle, then the train would move.
Hence, intuitively, the mechanism used for wimpy, low inertia motors is appropriate. For BIG, high inertia motors isn't.
PWM preserves torque.
For instance, as an intuative approach, take a bunch of people trying to push an MG and a mid-side car. In the MG case it would be a case of push, roll, push, roll (like full blown PWM on a small train). With the larger car, full blown PWM would move the car slowly.
Thus, I'd be more likely to advocate PWM over the entire range of control for BIG motors. This is the cheapest but it relies on some intimate details.
Their design wouldact a little better than a traditional power pack in any case. The only way to get true low speed performance is to use PWM at the motor's voltage. There will be the case of setting the power pack at 5% to get the engine moving and then reduce to 3% to keep it moving slowly when starting from a full stop.
It would be in CTI's best interest to allow setting of that point (% throttle), voltage to use for 100% starting at that point and PWM to start at that point. Now you've added complexity.
I found this when searching the web confirming the need to change the voltage and kick. I did not see the ability to modify values in SmartCAB. The documentation does say it does it though.
Thus if CTI did their homework, they would have a way to kick, change the polarity and chage the power supply voltage. If they really did their homework, you would have direct control of this through speed/throttle profiles, thus I truly believe that reverse engineering would reveal signals for:
speed - power supply voltage
kick - logic signal
direction -signal to directional relay.
Inherent in the design would be currrent limit at 3A
Inherent in the design would be short circuit detection
Inherent in the design would be a software lockout for direction changes. Won't necessarily be zero volts because minimum output is 1.2V.
So, you must have a programmable power supply. You cannot kick the programming value because of inherent slow start in most power supplies.
Find out: What does smartCAB do with a short. It may be in the docs.
Power OP amp requires no modifications. Current limit would be inhernt in the OP amp selection. Short circuits need to be reflected back to the input. Unregulated supplies may exceed absolute manimum ratings of the op amp. Approach is straightforward. Probably some IC glue. Suggested add on's: Voltage and current analog meters
Conventional requires reverse engineering: find kick, power supply setpoint, reverse and possibly shutdown Modify power supply: bring out signals. Possible glitch: Processor controls power supply through bus
May require removal/bypass of revering relay.
Use commercial power supply with isolated analog inputs. Prefer form factor or 1 rack unit high, This will usually come with meters and current limit can also be set. A short circuit needs to be fed back. Some glue logic on the analog current out. External reversing relay using the same signal in original module. Probably can let smartCAB handle safety logic. Use isolated analog programming in to set output voltage. Kick needs to be designed. Sometimes the supply has a pin for this.
In both cases I recommend a DIN rail design with rack mount cases.
Power supplies.
For the Conventional method, it's not practical to build a variable output Switching power supply for 15A at 24V. Commercial with isolated programmability is required. The manufacturer MUST be consulted for motor load compatability. Regulation not down to zero is OK. $1500+ is estimate.
For the OP amp method, simple recification, simple rectification with a constant voltage transformer or regulated bipolar supplies. The fist two will likely require a custom solution. CVT's are noisy. Both are heavy. I loved the CVT I used for my audiou amp except for the hum. Regulated high current fixed 24 VDC are available for around $200 each and you need two. It might be possible to modify a fixed supply, but likely won't be easy.
Current limit and over temperature protection - part of design, hopefully
Overcurrent protection - Design is virtually the same in both cases and is essential. An artifact of this is would be it would be easy to add analog readout of current and voltage. Bar graphs migh be cooler.
SmartCAB may take care of the rest. Manual reset requires extra junk, but I have done that before. The current exceeding a threshold for x time causes a trip. Below that threshold causes a reset.
Cooling/Heatsinks Forced cooling will be required for the OP amp model. Should be temperature based. The KICK FET would have to be evaluated. High side drivers are complex.
Glue Logic Lots of glue logic to provide the interface to SmartCAB for the conventional method.
Polarity Reversal - Let Smart Cab be responsible for the interlock there too.
Other notes: Braking would be more effective for the OP Amp model.
the TrainBrain module does all of the commanding to the selected SmartCab. The SmartCab responds to the signals sent out from the TrainBrain, as far as how fast to ramp up voltage, low and high voltage, etc. The SmartCab has a microprocessor to run what the SmartCab says to do. I was trying to get a circuit diagram from them to see about swapping(bypassing) components. However, $100.00 a pop(for a SmartCab or the TrainBrain) just to look is a bit expensive for me. I was hoping for some of the promised support to help, but nothing so far...
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