Railroad locomotive control systems ??? - Page 1
Forum section: electronics / general

author: KM4FER
author_rank: Regular Contributor
timestamp: March 07, 2015, 06:46:15 pm
content: After watching a very long train climb a mountain pass a control theory question came to mind that I can't seems pretty amazing to me.As usual the train had two locomotives at the front pulling and there were two locomotives at the rear pushing. OK, basic physics, vector sum of force at front + force at rear results in total force in uphill direction. Making the simplifying assumption that the tracks are a straight line there are three options possible:1. The pulling force is just enough to supply a slight pull on the rear engines. No advantage to this as the rear engines aren't helping at all.2. The rear engines are pushing the whole train plus a little on the front engines. Then why have the front engines?3. Assuming a uniformly loaded train the front engines are pulling the front half and the rear engines are pushing the last half meaning the net force at the exact middle of the train is exactly zero. Tricky balance.Obviously option #3 is the only one that provides any benefit but how do they regulate the power output of the locomotives? Wireless communications? A signal wire running through each car? Engineers skill only?Now throw in that the train is not running on straight track but tight curves and not on a linear incline and it gets more interesting.And then, just for fun, throw in that there were four more locomotives in the middle of the train, all 8 locomotives cranking out lots of power.How do they do that??earl...

author: chamod
author_rank: Contributor
timestamp: March 07, 2015, 06:59:50 pm
content: Now this is something even I was wondering about. Waiting to see an explanation.

author: cimmo
author_rank: Frequent Contributor
timestamp: March 07, 2015, 11:57:01 pm
content: My understanding is that most diesel electic locomotives only have throttle 'notches' (maybe as few as 8 ) - not infinitely variable.Given this constraint, as long as all the locos are in the same 'notch', then the forces would roughly balance?A relevant para from wiki:Locomotive power output, and thus speed, is typically controlled by the engine driver using a stepped or "notched" throttle that produces binary-like electrical signals corresponding to throttle position. This basic design lends itself well to multiple unit (MU) operation by producing discrete conditions that assure that all units in a consist respond in the same way to throttle position. Binary encoding also helps to minimize the number of trainlines (electrical connections) that are required to pass signals from unit to unit. For example, only four trainlines are required to encode all possible throttle positions.

author: xrunner
author_rank: Super Contributor
timestamp: March 08, 2015, 12:02:11 am
content: There's a lot of slop in the coupling between all the cars. Hence the forces that the pulling locos encounter and the pushing locos encounter are not as exact and constant as you might be thinking. It's all constantly changing a bit all through the train. As long as the locos are set to run pretty much the same speed it all evens out.

author: rollatorwieltje
author_rank: Unknown
timestamp: March 08, 2015, 08:33:02 am
content: One of the systems used is GE Locontrol. It uses wireless links to control the other locomotives. There is still a single brake line running all the way through the train, so in case of problems the driver can always stop it.

author: G7PSK
author_rank: Super Contributor
timestamp: March 08, 2015, 10:17:11 am
content: There used to be here in the UK something called rail buses that consisted of one motor unit that had 2 diesel engines and a car unit all carried passengers these would be coupled together to form trains of three or more motor car and carriage units all couple electrically so that one driver controlled the whole. The drive to the wheels was mechanical via a pre selector or semi automatic gear box so the power of each engine had to be balanced with the others so just 3 units (a unit was power car plus carriage) had six engines. I don't think there was much in the way of electronics as these things were designed in the 50's and built into the 60's and 70's everything was electro mechanical. The early ones had a pneumatic governor fitted to the diesel engines manufactured by Leyland and AEC.I have used the wrong term here. I should have said Rail-cars or multiple units as the rail-bus was just that a bus adapted to run on the rails and they only came about in the 1980's

author: Whales
author_rank: Super Contributor
timestamp: March 08, 2015, 10:18:06 am
content: I don't think it's necessary for each loco to supply an equal amount of force (to a certain extent). Each will supply whatever it wants (they'd run them at the same 'throttle' so it would be in the same magnitude). The engines are neither constant-force or constant-speed. Each driven axel provides a force that (ignoring the elasticity of the train) pushes every part of every carriage in the train equally.

author: G7PSK
author_rank: Super Contributor
timestamp: March 08, 2015, 10:39:26 am
content: If you were to drag a diesel faster than it was set to run it would start to act as an air pump and dramatically effect the overall performance and fuel efficiency of the whole train so each power unit needs to be fairly close in output.

author: SeanB
author_rank: Super Contributor
timestamp: March 08, 2015, 10:53:27 am
content: Diesel locomotives are all constant speed engines driving a generator or alternator, with the drive to the wheels being via an electric motor, either a DC motor with a control system or an AC induction motor with a variable frequency drive, depending on the age of the unit. There are very few direct drive diesels, they are all very low power units, confined to a shunting yard and low speed, used to move cars around where the volume of cars is not enough to warrant the use of a larger unit. Another use is in sidings to move the decoupled trains for loading and unloading, where the small size and low torque is fine. Small so you can lift them off the rails and move with a forklift if needed. the only other direct drive diesel is a modified Toyota or Isuzu truck which has been modified to have rail wheels that are able to lift the vehicle to run on rail, with the drive to them being via friction from the regular drive wheels driving an idler wheel. Used for rail inspection and for personnel transport to rail work sites while the line is closed for maintenance, with the advantage of being able to drive off the line to clear it for trains, and to drive to collect parts.

author: G7PSK
author_rank: Super Contributor
timestamp: March 08, 2015, 11:32:34 am
content: The ones in the UK like the English Electric delta and the sulza engined ones were not constant speed they were DC electric but the engine speed was increased as the load went up.

author: Ericho
author_rank: Regular Contributor
timestamp: March 08, 2015, 11:33:09 am
content: I doesn't matter if One locomotive outputs more power than the other.You run them both on wathever power setting is most economic or maximum if needed.So if 1 engine outputs 3000kW and the other 1000kw, you put 4000kw in the whole system and that will deternine the speed.You can trow in another small 100kW engine if you want. then the whole train will speed up a little more.

author: German_EE
author_rank: Super Contributor
timestamp: March 08, 2015, 12:31:02 pm
content: Here is a guess:The two locomotives at the front are tightly coupled together electrically and mechanically and operate as one unit, they pull the entire train up the mountain. The rear locomotives are also tightly coupled together but are attached to the rest of the train using a load cell.If the load cell shows that the rear locomotives are being pulled then they increase engine powerIf the load cell shows that the rear locomotives are pushing too hard then the engine power is decreased but with a lower limit, set by the engine power control.The ideal of course is that the load cell shows that the rear locomotives are neither pushing nor pulling and all four locomotives in the train are supplying the same power. Going downhill the roles are reversed with the front locomotives regulating power using their load cell.

author: sunnyhighway
author_rank: Frequent Contributor
timestamp: March 08, 2015, 01:43:30 pm
content: It is the amount of torque which determines how many wagons a locomotive can push and/or pull.Only if the amount of torque of a locomotive is too low to move itself alone at a certain speed, it becomes a drag.

author: electr_peter
author_rank: Unknown
timestamp: March 08, 2015, 03:56:42 pm
content: Quote from: KM4FER on March 07, 2015, 06:46:15 pmSituations 1) and 2) are impossible in practice (except for situations in curves or under braking). Each car and locomotive can be either pushed, pulled or neutral. Locomotives at the front are pulling, locomotives at the back are pushing (as middle cars provide the drag). During constant speed periods push/pull/neutral position goes back and forth between all cars and locomotives.As for 3), load is not distributed evenly - each locomotive does it's best at desired power and total sum of forces pushes the train.

author: Seekonk
author_rank: Super Contributor
timestamp: March 08, 2015, 04:16:57 pm
content: One thing to think about is a locomotive always backs up before going forward with a long load. If all the couplings were in tension, it wouldn't be able to move.

author: Rory
author_rank: Frequent Contributor
timestamp: March 08, 2015, 04:36:02 pm
content: Looked it up on Wikipedia. Interesting.http://en.wikipedia.org/wiki/Multiple-unit_train_controlhttp://en.wikipedia.org/wiki/Locotrol

author: German_EE
author_rank: Super Contributor
timestamp: March 08, 2015, 08:00:39 pm
content: It's amazing what you learn on this site.

author: Rufus
author_rank: Super Contributor
timestamp: March 09, 2015, 05:36:28 pm
content: Quote from: KM4FER on March 07, 2015, 06:46:15 pmThey don't need to carefully regulate anything. Couplings at the front of the train are in tension and at the back of the train in compression. The change from tension to compression occurs somewhere in the middle determined by the relative power output of the two engines.

author: HighVoltage
author_rank: Super Contributor
timestamp: March 15, 2015, 11:20:37 pm
content: This is the same as adding a parallel source of electricity to an existing system.Only if the voltage is going high enough, will an additional current flow in to the loadThe additional amperage will then be supplied to the loadThe engine of the second locomotive has to increase the power output (horse power) until a positive torque (Horsepower x RPM) is applied to the wheels and then participates in moving the train up the hill. I don't think there is a control system needed. Dual train systems are probably older than control systems.

author: Galenbo
author_rank: Super Contributor
timestamp: March 16, 2015, 08:03:48 am
content: Quote from: HighVoltage on March 15, 2015, 11:20:37 pmKinematical this is true, but you will run into dynamical behavour, because there's a lag between wanted power and given power.Resulting in push-pull oscillation in certain conditions.A control system must be added/modified/switched over, to reduce/prevent/damp this excessive continuous energy exchange, or change its frequency.(For the pure electronicals here, reduce the Q factor of a LC circuit by adding a resistance, to reduce energy exchange between L and C, keep U and I into safe limits, change the oscillation frequency(s) to a range where it won't occur.)In other systems, like power stations, this is called load sharing, often less easy and intuitive as thought, because there are many changing factors.(rails go up, down, train empty, full, speed, temperature,...)

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2015, 12:34:14 pm
content: When operating a long freight train in the mountains (e.g., the Rockies or Sierra Nevada), the tricky bit for control is when the train must crest a hill. There is a transition where the net load changes from net uphill (force against motion) to net downhill (force along motion) when half of the cars get past the crest. With improper operation, the loss of slack in the couplings can cascade and force a derailment. The operator must keep the train in tension for safe operation