Designing model train layout I/O and PWM speed control

[Ian.M]

Should be, but for breadboarding one, it doesn't really matter if you use 74LS with its higher power consumption or 74HCT with its poorer noise immunity.   Just avoid original 7400 and high speed parts like 74AS or 74AC.

For the AND, there's no problem (except slightly worse propagation delay) with using half a quad NAND with the second gate to de-invert the first gate's output.

You may need a tiny bit of low pass filtering on the current sense OPAMP so turn-on transients cant immediately trip the current limit.  Don't over-do it or it will overshoot the set current limit.

[ilium007]

One more question tonight, what’s the fastest I can PWM the L298’s ? I’ve been running the MC33926 at 20kHz and don’t get any motor noise.

[Ian.M]

The datasheet says the L298 can commutate at up to 40KHz although 25KHz is typical.  If it can commutate that fast it can PWM via enable at the same frequency. However for high PWM frequencies you'll need Schottky catch diodes (at least 1A) to shunt the motor current when the bridge is Hi-Z.  Low frequency PWM can get away with Silicon catch diodes, though Schottkys are preferable.

N.B. the turn-off and turn-on delays are asymmetric so don't slam from forward to reverse without going through a stop mode in between or shoot-through may damage it.

[ilium007]

The datasheet says the L298 can commutate at up to 40KHz although 25KHz is typical.  If it can commutate that fast it can PWM via enable at the same frequency. However for high PWM frequencies you'll need Schottky catch diodes (at least 1A) to shunt the motor current when the bridge is Hi-Z.

N.B. the turn-off and turn-on delays are asymmetric so don't slam from forward to reverse without going through a stop mode in between or shoot-through may damage it.

Ok. Thanks again.

[rrinker]

 Buncha observations, from the point of view of a fellow model railroader. I'm building my own signal/detection/control system, though my layout is DCC. I looked into various bus systems and decided I did not want to deal with the complexity of things like CAN - so I am using RS485 with the CMRI protocol. Yes it is single master - inputs are read by polling the controller to see the current state, not by the input pushing data to the master. This works just fine, and is plenty fast enough - even on some REALLY huge model railroads. The protocol is dead simple - and I'm taking a few shortcuts so that some of the ID and config packets are not needed, by making each of my nodes identical (in software anyway - I am not populating a PCB with 4x 16 bit output latches for a node that only needs 16 bits of output. However, the PCB and the software will accept 64 bits of data, just only the first 16 will actually appear on physical outputs for the smaller node.
 There is in this design something more complex than a micro as the master - it's a full blown PC. That software will be written in VB.
 In the original CMRI articles in Model Railroader magazine back in the mid 80's, there were two versions of loco control for standard DC locos (there was no DCC at the time, but there were some command control systems available). In one, the computer drove a set number of throttles, say 4, which had their outputs switched to various blocks as the trains moved about. You could also use user controlled throttles, with the computer and the CMRI nodes applying a given throttle's power to the appropriate blocks. Option 2 had a computer controlled throttle for each block, similar to the one design posted above. This allowed for both full computer automation or the user could run a simple 'throttle' device that fed in to the computer which then applied power to the blocks as needed.
 Note that the power is not applied to the next block at the moment the train crosses the block boundary - this happens well before the train gets there. You do not operate trains without at least a one block separation. If a train is in block 1, the system should already determine that block 2 ahead of it is clear and assigned power to that train's throttle. If the block ahead is occupied, then the train is stopped - well before the electrical boundary. There should be no issues with one truck (bogie) being across the gap and the other behind it, passing power via the loco's internal wiring.
 The data protocol is open - you would have to make your nodes correspond to known designs if you want to use commercial (well, written by others) control software, or you can do what you like and write your own (as I am doing). There is an Arduino-based node available commercially, the code is open and the schematics are published - I am altering that design to use the SPI version of the IO Expander (MCP23S17 vs the commercial product's MCP23017 I2C version) and also fixing which chips are inputs and which are outputs (eliminates the config packets in CMRI). For actual point movement, I have a separate design that I posted here previously that has both control input from the node as well as local pushbuttons, the button lockouts being active low so that if I don't turn on the master computer and the control nodes, I can run trains by myself and just use the local buttons to line the points. Mine drive a servo, but it could be adapted to a stall motor or solenoid. It also drives a relay to switch power to each frog for maximum reliability.

[ilium007]

Buncha observations, from the point of view of a fellow model railroader. I'm building my own signal/detection/control system, though my layout is DCC. I looked into various bus systems and decided I did not want to deal with the complexity of things like CAN - so I am using RS485 with the CMRI protocol. Yes it is single master - inputs are read by polling the controller to see the current state, not by the input pushing data to the master. This works just fine, and is plenty fast enough - even on some REALLY huge model railroads. The protocol is dead simple - and I'm taking a few shortcuts so that some of the ID and config packets are not needed, by making each of my nodes identical (in software anyway - I am not populating a PCB with 4x 16 bit output latches for a node that only needs 16 bits of output. However, the PCB and the software will accept 64 bits of data, just only the first 16 will actually appear on physical outputs for the smaller node.
 There is in this design something more complex than a micro as the master - it's a full blown PC. That software will be written in VB.
 In the original CMRI articles in Model Railroader magazine back in the mid 80's, there were two versions of loco control for standard DC locos (there was no DCC at the time, but there were some command control systems available). In one, the computer drove a set number of throttles, say 4, which had their outputs switched to various blocks as the trains moved about. You could also use user controlled throttles, with the computer and the CMRI nodes applying a given throttle's power to the appropriate blocks. Option 2 had a computer controlled throttle for each block, similar to the one design posted above. This allowed for both full computer automation or the user could run a simple 'throttle' device that fed in to the computer which then applied power to the blocks as needed.
 Note that the power is not applied to the next block at the moment the train crosses the block boundary - this happens well before the train gets there. You do not operate trains without at least a one block separation. If a train is in block 1, the system should already determine that block 2 ahead of it is clear and assigned power to that train's throttle. If the block ahead is occupied, then the train is stopped - well before the electrical boundary. There should be no issues with one truck (bogie) being across the gap and the other behind it, passing power via the loco's internal wiring.
 The data protocol is open - you would have to make your nodes correspond to known designs if you want to use commercial (well, written by others) control software, or you can do what you like and write your own (as I am doing). There is an Arduino-based node available commercially, the code is open and the schematics are published - I am altering that design to use the SPI version of the IO Expander (MCP23S17 vs the commercial product's MCP23017 I2C version) and also fixing which chips are inputs and which are outputs (eliminates the config packets in CMRI). For actual point movement, I have a separate design that I posted here previously that has both control input from the node as well as local pushbuttons, the button lockouts being active low so that if I don't turn on the master computer and the control nodes, I can run trains by myself and just use the local buttons to line the points. Mine drive a servo, but it could be adapted to a stall motor or solenoid. It also drives a relay to switch power to each frog for maximum reliability.

Thanks for the detailed reply ! I’ll take a look at CMRI although I’m pretty certain I’ll go down the CANbus path as I can use the same protocol and design in later projects I have planned.

There is an Arduino-based node available commercially, the code is open and the schematics are published

Do you have the name of this ?

[rrinker]

 Yes, the commercial Arduino-based CMRI node is called cpNode from Model Railroad Control Systems

http://www.modelrailroadcontrolsystems.com/cpnode/

 They have a bunch of other interesting things as well, including equipment to do RFID.

[ilium007]

Thanks. They are the same ones I found when I went searching.

[C]

Think you might want to look at some LED drivers/
WWS2801
https://cdn-shop.adafruit.com/datasheets/WS2801.pdf
On amazon you can get these in strips or and single connected by wires
There are others
SK6822 WS2813 APA102 SK9822

Track blocks
With no motor controller on engine you could use a lot of blocks.
lan.M
has suggested motor controllers with current sense.

Three or more blocks in series lets you tell what block and what direction train is moving on block crossing.
A test loop could have large number of blocks
ABCABCABC

You could know that a engine is in B but not which B.

If you add a block D the computer could then tell you what block of the B blocks the engine is in after passing through D.

Think of the rules
  More then one engine in same block get same power & motor controller has to supply all.

  Move from block to UN-powered block.
  Move from block to like powered block.

Do not move from one block to different powered block.

With more blocks letters and a good time schedule you could have many engines on a loop.

You could have a dumb control system that learns from just driving a engine around track.
  At this PWM this block it X long. You have now measured length of most blocks.
 You have tested track sensors and found order.

Using CAN bus you can time sink all attached devices.
Simple sink gets you to one packet time difference.

For some things you can treat what is happening on track as an AC signal that is floating.

[ilium007]

I was looking through H-Bridge data sheets (thats my pastime at the moment !) and came across a different H-Bridge that seems rather simple compared to the others I had been looking at. While I'm happy to build the L298 based design and now basically understand the OPAMP / comparator / logic circuit required I found the TI DRV8870 seems to have the OPAMP / comparator built in - http://www.ti.com/lit/ds/symlink/drv8870.pdf.

The DRV8870 device is a brushed-DC motor driver
for printers, appliances, industrial equipment, and
other small machines. Two logic inputs control the Hbridge
driver, which consists of four N-channel
MOSFETs that can control motors bidirectionally with
up to 3.6-A peak current. The inputs can be pulsewidth
modulated (PWM) to control motor speed, using
a choice of current-decay modes. Setting both inputs
low enters a low-power sleep mode.

Plenty of current for my needs.

The DRV8870 device features integrated current
regulation, based on the analog input VREF and the
voltage on the ISEN pin, which is proportional to
motor current through an external sense resistor. The
ability to limit current to a known level can
significantly reduce the system power requirements
and bulk capacitance needed to maintain stable
voltage, especially for motor startup and stall
conditions.
The device is fully protected from faults and short
circuits, including undervoltage (UVLO), overcurrent
(OCP), and overtemperature (TSD). When the fault
condition is removed, the device automatically
resumes normal operation.

From reading the data sheet I can design a current trip point based simply on the sizing of the sense resistor.

Is there any advantage / disadvantage to using something like this vs. the L298 ? I know its in a HSOP package which I can get around with a reflow heat plate which I was going to experiment with anyway.

[Ian.M]

Its nice but its not safe to parallel them as will happen when the loco crosses an isolation break, again because its too smart.

See section 7.3.3 - if its in current regulation mode at the moment the wheel reaches the break, there's a risk of the next segment's driver having an upper MOSFET on while the current segment has both lower ones on, resulting in severe over-current and oit tripping its internal protection circuit.

You could get away with using it as a dumb H-bridge with short-circuit protection by grounding its Isense pin (avoiding the above issue), but it still has the disadvantage that you need to apply the speed control PWM to different pins for forward and reverse.

Its advantage is that the MOSFET output transistors it uses will have much lower voltage drip than the bipolars in the L298, so it needs less heatsinking, and the DC bus can be closer to 12v (lower) and still get 12V out, so it *may* be worth the pain of adding external high side current sensing or even true floating load current sensing + external logic for early PWM pulse termination for current control.

[ilium007]

You could get away with using it as a dumb H-bridge with short-circuit protection by grounding its Isense pin (avoiding the above issue), but it still has the disadvantage that you need to apply the speed control PWM to different pins for forward and reverse.

I did think about that, it would be a matter of reconfiguring the PWM pin on motor reversal and setting the other pin as digital LOW - I don't think its a big deal.

Are you saying by taking I_sense to GND it will do its own short circuit protection (is that what they are calling OCP) ?

Still happy to go with the L298 design but I thought this new IC looked pretty good !

[Ian.M]

It does have protection against a hard short - its datasheet claims a 3.6A limit, followed by tripping and auto-restarting 3ms later to see if the short's still present,

However its going to be a PITA to work with - either it needs a lot of PCB area to get rid of heat, or you need to either use an aluminum substrate PCB, or you need lots of thermal vias and the back of the PCB clamped to a heatsink with a silpad in between.   I hope you've got a hot-air rework station with a board preheater - you'll need it.

Its the usual issue - modern H-bridges don't come in hobbyist friendly packages, (which is why Polulu et al can make good money selling them on breakout boards) and doing maintenance on a board with an array of them clamped to a heatsink will be a real PITA - you have to first remove umpteen clamping screws, pry the board off the silpads hopefully without flexing it enough to crack any parts, pre-heat the whole board to replace one, and don't dare run it till its fully cooled off and any compressed or torn silpads have been replaced, and *ALL* the clamping screws are back in and tightened.  If you want to avoid that aggro you'll basically end up creating your own breakout board for it so you can undo a couple of screws and desolder the pins individually like you would to to replace a L298

[C]

There are some aliexpress.com modules based on

bts7960
http://www.robotpower.com/downloads/BTS7960_v1.1_2004-12-07.pdf

might work

[ilium007]

I'll have a look at designs but I'm not using breakout boards. I am going to build something to suit the purposes I require (even if I have to 'lean' on some other designs I find !)

[Ian.M]

I'm just saying that a monolithic board with  16x DRV8870 or similar SMD motor drivers on it will be an assembly and  maintenance nightmare.    Some stuff is best made modular, for ease of both construction and maintenance.  The pricing structure for nearly all PCB manufacturers that support small volume prototyping also favours a modular design, keeping individual board size down and board reuse up.

[C]

BTS 7960B
P-TO-263-7
BTS 7960P
P-TO-220-7

A TO-22o with 7 leads
A half-bridge so you need two

[Ian.M]

Nice chip, but its gone obsolete.  Fortunately there is a replacement with only a slight change in specs - BTN7960P, however even that is apparently NRND.

Its got the same sort of current limiting as the other modern H-bridges or half bridges we've looked at but as its nominally good for 47A, in this specific application paralleling them briefly via LC low pass filters for the loco to cross an isolation gap should be OK.  The track wiring will probably melt first, and its short-circit protection should handle the conflict if the over-current protection did get tripped.  However it certainly isn't suitable for hard paralleling in other applications,

It is going to need an external current limit comparator and gating circuit to avoid welding the train wheel to the isolation gap if a software fault lets a train attempt to proceed onto a section currently being driven with opposite polarity.

Its also not particularly cheap, (especially as this application will need 2x per segment vs half a L293) though you get a *LOT* of drive for your money, and the fact its an individual  half bridge makes a lot of sense for PWMed single ended drive applications or for three phase brushless motors etc.

[C]

Some one must have a few or there are clones.

Amazon has boards also.

Search for modules had h-bridge module under $10 on aliexpress and 11-22 on amazon and this is with heat sink.

Searching Aliexpress, some other chips are also used.
 
just thinking out loud so to speak.

Engine should move into an UN-powered block or a block with matching power inputs.  With engine using all wheels on a side you have a transfer of load for that length of time.
With a large number of blocks, control system should be able to prevent worse case which would look like a power rail to power rail short, but to chips an output to power rail that is said to be protected or output to output short.

Just wondering if three phase chips should be looked at. Two three phase chips could give 6 blocks for a section/. Could know position and direction of engine movement.
Would have to allow each half-bridge to be controlled and three current sense would be nice.

I would just get a few per-built modules. Test one then get more.

[ilium007]

I learning a lot out of the content of this thread - don't stop !

I can't believe that this "train goes from one track to another under two seperate PWM drivers" has never been done before but I simply cannot find anything on the internet about others running in to the same problem.

Likewise I can't find any OPAMP/comparator/logic circuits based around the L298. I understand the circuit from Ian.M's description, its just the supporting circuit such as the low pass filtering - I have no idea why I need low pass filters.

[Ian.M]

A large leading edge current spike is very common when PWMing a load.
See fig 2. at https://www.powersystemsdesign.com/articles/flyback-power-supply-development-part-iv/38/6602
Although it is NOT an H-bridge circuit, you will get similar issues with the L298 H-bridge low side current sensing.

The waveform is composed of a massive leading edge spike due to the discharge of the junction capacitance of the reverse biassed diode in parallel with the switching transistor (body diode for MOSFETs, or external catch diode for BJTs) superimposed on the typical inductive current ramp-up.   The raw signal (a) is unusable for current limiting as the spike will immediately trip the limiter.   After proper filtering, you get waveform (c) which is satisfactory for current limiting.  Waveform (b) is the bogus results you get if you use a scope probe with a normal length ground clip lead - the nearby current spike through the sense resistor inductively couples to the clip lead well enough to still swamp the signal.  See photo to the right of waveform (c) with the scope probe grounded by a loop of wire directly soldered to the ground plane, adjacent to the point being probed.

The magnitude and duration of the spike depends on the actual circuit and the slew rate of the H-bridge output.  You need to do some tests to determine an appropriate time constant for the sense waveform RC filter.   

Fancier chips have leading edge blanking on their current sense output - the internal current sense circuit is momentarily disconnected from the sense resistor when the control signal turns the transistor on.  You could add that externally with a fast CMOS analog switch and a circuit to generate a brief pulse to switch it at the PWM leading edge, but that's a lot of extra complexity for each driver board, and a RC low pass filter with an appropriate time constant is good enough.

[ilium007]

I don't have a scope so it is going to be next to impossible for me to progress on this design.

[Ian.M]

It will be difficult without a 10MHz or better bandwidth scope, but not impossible.
A lot of calculation and simulation will be involved, to get the design right the first time instead of 'suck it and see' experimentation.

Also a lot more detailed testing will need to be done e.g. test a L298 with a resistive load at DC to set up the current sense circuit, and the over-current comparator, then PWM it look at the delay between the PWM rising edge and the comparator tripping by converting the PWM signal and current limit flip-flop outputs to DC voltages by low pass filtering.   If the resistive load is under the limit, the flipflop output should stay high. If it trips due to the leading edge spike its output will be low nearly all the time.   Fiddle with the filtering till it doesn't trip on the leading edge.   Then add an inductor in series with the load resistor.  You know the current must ramp up, so you can now see if the current limit setpoint can shorten the PWM pulse from the gating circuit the way it is supposed to.   If that's all behaving as expected, set a safe current limit (go back to DC to do so) then try just the inductor as a load.   

It will probably take something like ten times longer without a scope and if something unexpected happens, it will be much harder to debug.   Perhaps you should consider getting a scope - I certainly wouldn't want to start a big project involving sensors, motor drives and serial comms without one.   If you cant afford a scope, you must be so close to living hand-to-mouth, that you really shouldn't be wasting money on electronics projects.

[rrinker]

 A train moving from one block to another under different PWM sources happens all the time when you cross booster block boundaries with DCC. Granted, it's not the motor drive power, at least not directly, but the two booster outputs are each a variable width squarer wave signal which is the propulsion voltage plus digital data stream for DCC. It's key that there is a common between the boosters, and both rails are gapped. This then becomes seamless, even when using the type of loco that picks up from one rail by the drivers and the other rail by the tender, in case of a steam loco, or in the older style diesels where the front truck picks up from one rail and the other picks up from the rear truck only. When such locos cross the gaps, you end up with one rail signal coming from booster A and the other half of the signal coming from Booster B. With a common between them, there is no voltage double effect and the loco just glides through. I don;t see why this would be any different if the signal were actually a PWM output connected directly to the loco motor.
 I'd have to go back and look at it, I'm not sure what Bruce Chubb's computer throttle design did for the actual throttles, it wasn't a PWM design but I'm not so sure it was a filtered DC either.
 I'll point you to something else as well, that pre-dated DCC. Ed Ravenscroft's MZL control. This was a non command control (thus no modifications or boards in the locos) system where the 'engineer' only had to wacth signals and control his train speed and direction. None fo that silly toggle switch flipping to assign blocks, power was automatically assigned to a clear block ahead based on the route selected at each turnout. Plenty of contacts and relays, but no active electronics. It didn't automate the movement of trains, it merely automated the "assign power to the correct block" task which was the most non-prototypical aspect of previous cab control systems. By computerizing the relay logic aspect, it could be made into a more automated running system. That's more or less what the Chubb Computer Cab Control (CCC) did - the computer automatically assigns power to the block, the person running the train just has to run the train and obey signals. It's almost all the capabilities of DCC without the decoder in each loco. One possible advantage over DCC - you can;t play Gomez Addams and have to trains hit head on. IN an eastbound train gets into the block of a westbound coming right at it, one of the two will reverse direction.

[Old Don]

I've been following this thread for some time and I've got to say you're putting a great deal of time into it and I hope you solve the problems.

An observation, once you are done you'll have a system that nobody else will be compatible with unless they modify there equipment to run on your track. So over here in the USA a lot of NMRA members visit with each other and bring their own equipment to run on the visiting rail line. That's why DCC has become the norm for our equipment. Warts and all. Locomotive DCC boards are all compatible, but controllers coming along from visitors have to match the DCC system used or they can't be used.