On-site Integration

[TerryJC]

This Topic has been started to cover the activities to be carried out on site when the Underground Railway Control System is Set to Work. This is expected to take place on Tuesday 2019-03-12.

I will bring the control hardware, with software installed, and also the following tools and equipment:

1. Monitor and Keyboard.
2. 12 V, 5 A PSU brick to power the system.
3. Hardware Test jig.
4. Two Multi-meters to measure voltages during testing.
5. Drawings.

We will need the wiring for the Traffic Signals, the IR Sensors and the track power installed before we begin. If possible the IR Sensors should be positioned under the track by measuring the distance taken to slow down from full speed to stop in 2 seconds. It is appreciated that this might be easier when the Control System is connected, but if this is not practical then it will have to be done on Tuesday. In that case, we may find that the overall job is considerably lengthened and run out of time.

[peter]

Terry
we have all wiring in position ready to fit
all we need do is connect to signal lights from Terminal blocks and ditto to IR lights

Geoff will be there as well as myself so see you tomorrow

Peter

[TerryJC]

Sitrep:

There was a bit of a setback this afternoon. After this morning's work by Geoof and Peter to complete the wiring and install the hardware under the layout, I returned this afternoon to carry out initial testing. I plugged everything in OK, (including the Monitor and Keyboard), and I was about to commence testing of the Traffic Signals when there was a 'Phut' noise followed by a slight smell of burning. Upon investigation I found that the power brick was dead.

I did some checking out of the PSU input circuits on the Pi board with an Ohmmeter, but couldn't find anything apparently amiss. As a result, I had to abandon work for the time being as I didn't have another PSU brick to hand. In any case I hesitate to connect a new brick to the circuit, in case there is something causing a high current draw. I will return tomorrow morning with my Bench PSU and try running the system from that. This has a current limit which I set to 1 A during testing here in my workshop, but for the most part the current was nearer 300 mA than being likely to trip the supply. A fairly high draw of around 500 mA or so was observed at startup but that soon fell away to the quiescent current noted above.

If the system functions OK tomorrow, I shall risk putting another PSU Brick into the circuit. This is the first of these bricks that I've used (and I did use it during testing here), but it's possible that this cheap Chinese import was just too cheap.

[TerryJC]

Sitrep2:

This morning I ran the system using my Bench PSU with a current limit of 1 A. There was no sign of a short and the system performed (almost) perfectly (more about that later).

I then substituted the Bench supply with a 12 V 5 A PSU brick (not one of the Chinese ones, but one my wife found sculling around here). The system booted and ran fine. I will investigate whether or not I can get a refund on the 6 PSU bricks I bought from China nearly a month ago

I then ran the tests that I had intended to do yesterday and everything seems to work OK, apart from the Traffic Signals, which were lighting in the wrong place! In my software, I had assumed that the Traffic Signals attached to the entrance to the tunnel as the train travelled 'Out' from the Store End of the track would be the 'Out' lights and the ones at the other end of the tunnel would be the 'Back' lights. The LEDs , as installed, seemed to assume the other way round. Clearly not a major issue, but we will need to decide which ones are which.

A useful morning!

[peter]

Thursday am Geoff fitted Diodes to track to prevent over running
Tested with manual control at a high speed and stops with room to spare
we believe the Pi speed will be less so we are good to go
Geoff is looking to be in on Tuesday am next week
progress
Peter

[TerryJC]

This morning Geoff and I ran the program on the completely assembled system. It ran extremely slowly, even when the PWM was at 100%. We tried various experiments, but only concluded that the system was providing considerably less voltage than the manual controller.

I returned this afternoon, having realised what the problem was. Back in December, the belief was that it would only take 7 to 9 V to drive the trains at max and min speeds. This wasn't a problem then because I believed that the Motor Drive Board in the system could deliver that. The problem now is that the lighting rails have been added since then and they add a considerable amount of rolling resistance because there are four pickup contacts per carriage and they rely on running against the track. The upshot is that with the Manual Controller attached, the train needs around 10 V to run at a reasonable speed.

The problem with that is that the L298N chip in the Motor Drive Board drops anything from 1.8 V and 3.2 A at 1 A current draw and up to 3.9 A at 2 A. The drop we are getting is significantly less than the worse case scenario, because we are only drawing up to 0.5 A during acceleration. However, we only get an RMS reading of about 8.9 V when measured across the track. This afternoon I put my scope on the track and the pulses were only about 10 V amplitude which equates to the aforesaid 8 to 9 V when developed across the motor.

There are a number of solutions to this:

1. Best - Somehow reduce the rolling resistance of the track. It is quite variable; at the two ends the train speeds up and at a point about half-way between the tunnel and the door end, the train really struggles.
2. Increase the power supply voltage to compensate for the volt drop. Today I used my Bench PSU to put 14 V into the system and the train ran pretty well, (but still had a noticeable slow down near the door end. There are two problems with this solution; one of which is easy to fix and which is less so:
   1. We have to be careful that increasing the input voltage doesn't upset the DC/DC Converter in the system; we don't want to blow up the Pi.
   2. We have to protect the train. If there was a failure in the Motor Drive Board, it is possible that the full PSU voltage might appear on the track. We can mitigate this by putting a couple of Zener Diodes across the track so that they will absorb the excess voltage by breaking down. (We need two so that the track is protected both ways and to get the right values we need to know the maximum safe voltage that the train can take. (It may aleady be protected.))
3. Completely redesign the hardware to use MOSFETs instead of the L298N which uses bipolar transistors. The problem with this is that the design for these MOSFETs isn't easy (we tried for the River System Gate Valves and switched to the L298N instead.)

Any other ideas?

[peter]

Terry
Power goes to the track at the end by the Raspberry Pi
If the train is slower at the door end does that mean a drop in power along the rail ?
Does that need a supply going to the door end to offset the power drop?
we get this on the main line and have at least 3 feeds of power around the circuits to offset this problem.
If Geoff is in on Thursday iI will discuss and suggest this.
Peter

[TerryJC]

Peter,

You misunderstood. The train goes faster at the two ends, fairly slow in the section between the Store end and the tunnel and very slow in the section between the tunnel and the door end.

Geoff says that he feeds in the power at two places, (not at the ends though), so I don't think that the problem is the feed points. Also. The train goes faster with the Manual Controller which delivers more voltage.

[TerryJC]

BTW. I'm going in again tomorrow morning.

[TerryJC]

I have been looking into this business of the voltage on the track and the risk of damage to the train motor if 15 V is applied directly (eg in the event of a driver board fault). I've been researching two aspects; the maximum voltage that the motor can withstand and the specification of DCC products. Information isn't exactly forthcoming, although I have seen a number of comments that would indicate that the motor is good for well over 15 V. More relevantly, it is apparent that DCC devices put 15 V on to the track (this is the amplitude of the pulses, not the average voltage). This makes a great deal of sense, because they probably have a similar motor drive chip to the one that we are using, so the volt-drop is likely to be the same. I am therefore of the opinion that we don't need additional protection for the train motor. If a fault did occur, the worst that could happen is that the train races down the track until it passes the break.

This morning I replaced the 12 V PSU brick with a 15 V one (supplied by Penri).

I can now measure a fraction under 12 V on the track when the train has accelerated to full speed and it is clearly faster than before. However, I am still running the PWM at almost 100% so there is no headroom. The speed of the train is still not constant and there are sections of the track which cause particular problems. For example, if the train happens to stop with the engine just inside the tunnel at the Door End entrance, then it doesn't start at all.

So I think that we either have to consider increasing the input voltage by another few volts or get rid of some rolling resistance. I wonder if the LEDs would still work if we halved the number of contacts (ie, one +ve and one -ve, instead of two of each). I appreciate that this would probably result in flashing, but perhaps we could put a large electrolytic capacitor across the voltage terminals in each coach to store a bit of juice when the contacts separate.

Any other ideas?