EMD F7 in SCALE

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Steggy
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Re: EMD F7 in SCALE

Post by Steggy »

EMD F7 in SCALE
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ENGINEER’s CONTROLS: Part 1

Back when I started this build log, I listed some construction goals, which included:
  1. Power and speed solely controlled by a notched throttle with idle and eight power positions.
  2. Separate reverser with neutral, working independently of the throttle.
Along with figuring out how to get the F7 to emulate the full-sized model’s behavior, I put a lot of thought into devising appropriate engineer’s controls. In a series of exercises involving staring out in space, scribbling on my sketch pad, slurping coffee and pretending that I actually know what I’m doing, I conjured something that appeared to be workable. The questions, of course, were: Could I build it? Would it be presentable? Would it work? Am I nuts? (Don’t answer that last one!)

Before launching into any construction details, see below for a photo of the result:

engineer_controls01.jpg
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In the above photo, the longer of the two levers protruding to the right is the throttle; the other lever is the reverser. The reverser has three positions and is in neutral in the above photo. The throttle is “notched,” with eight power positions plus idle—it’s in the idle position in the above photo.

In between the throttle mechanism and the reverser is a seven-segment indicator that tells the engineer in which notch he has the throttle; it will display 0 when in idle, or 1 through 8 when in a power position. That indicator is mounted on a small printed circuit board that has the electrical bits and pieces required to make the indicator indicate what it is supposed to indicate. :o

That round, white thing in the upper right-hand corner is a super-bright, red LED that is an alarm indicator. In the current circuit design, it will illuminate if prime mover oil pressure is lost. I may also devise an overheat circuit to wire into it.

Visible are some fuse posts protruding to the right. They are only for local circuit protection. The loco’s control “panel” has multiple fuses to protect its circuitry.

The two gauges at the top of the box are a voltmeter and a dual air pressure gauge.

gauges01.jpg
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The white needle in the air pressure gauge is the main reservoir pressure and a yellow needle that is hidden under the white needle is the brake pipe pressure. The reading on the voltmeter will reflect the main control voltage supplied from the loco(s) to the control (riding) car.

One of the significant control design challenges was in devising a notched throttle control:

throttle01.jpg
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The fun thing about the throttle mechanism is I designed it while recovering from surgery back in early 2008. I had already designed the circuitry that controlled the prime mover’s throttle servo, so I knew what was electrically required. The mechanical part eluded me for a while, but gelled while I was subsisting on hospital food and watching mindless nonsense on TV—and listening to the gentleman in the other bed moaning and groaning from a gunshot wound received while trying to relieve another gentleman of his valuables. :D

Working on the cheerful assumption that I would survive the surgery (it was pretty major), I had packed my trusty sketch pad, some pens and programmable calculator along with other stuff that went with me to the hospital. In between being subjected to an assortment of indignities by the nurses and an occasional doctor—not to mention the friendly phlebotomists who seemed to have an insatiable appetite for my blood, I had soon created a sizable pile of scrap paper as I scribbled away trying to design something. Once transferred to CAD, the result looked something like this:

throttle_control_cropped.jpg
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Key components of the above are some stainless steel gears obtained from McMaster-Carr, a four-deck, 12-position rotary selector switch made by Electroswitch and a precision-machined baseplate to hold the mess togther. The gearing has an 18:60 ratio, with the 60-tooth gear having been reworked into a sector that is rotated by the throttle lever. This arrangement rotates the switch at 3.3 times the rate at which the throttle lever is moved. As a bonus, the 3.3:1 ratio amplifies the detent force of the switch, which causes the lever to positively snap from one position to the next, sort of like the way it works in the real F-unit.

The selector switch is a make-before-break design, which feature ensures a smooth change from one power setting to another. Of the switch’s 12 positions, nine are used for the idle and eight power notches, resulting in a total lever swing of 72 degrees from idle to notch 8. A four-deck switch in this application makes it is possible to produce a binary-coded decimal (BCD) output without use of any electronic logic. Hence only four trainlines are needed to transmit throttle position information to the locomotive(s). In the locomotive’s control panel, the BCD input from the trainline is translated into discrete voltage levels that, working with a signal whose frequency varies directly with prime mover RPM, manipulate the prime mover’s throttle servo. Ergo the prime mover is electrically governed in proportion to throttle position, in approximately 200 RPM steps.

The key component of the reverser is a single-deck rotary selector switch with a stop that limits it to three positions. The switch produces two mutually-exclusive outputs to set the direction of travel. The prime mover cranking circuits in the loco are interlocked with the reverser circuit to prevent starting unless the reverser is in neutral.

Accessories other than the horn are toggle-switch controlled:

switches01.jpg
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These switches are mechanically anchored to the box and soldered into a printed circuit board that interfaces them to the rest of the electrical system. Two switches warrant some explanation:
  • TRANS — Transition control.

    When off, this switch disables the automatic propulsion transition function—the consist remains in low transition.

    When on, automatic transition is enabled, which is controlled by the consist’s speed. The setting that seemed best during testing is forward transition at scale 30 MPH and backward transition at scale 25 MPH. The speed signal that triggers forward/backward transition is generated by a Hall-effect, gear-tooth pulse generator mounted in one of the loco’s trucks.
  • HDLT — Headlight control.

    This is a three-position, center-off switch. In the bright position (switched forward), both nose and door headlights are operated at full voltage.

    In the dim position, the nose headlight is extinguished and the door headlight is operated at reduced voltage.
The key switch in the lower left corner is the master switch that energizes the consist. Separate controls on each unit are used to crank the prime mover and operate the choke during a cold start.

This mess is assembled into a modified Hammond 1550J cast aluminum box, which includes a snug-fitting cover that adds some structural integrity to the box. Below is a drawing of the box as received from Hammond.

box_1550j.pdf
(222.72 KiB) Downloaded 100 times

The box’s cover serves as the mounting interface to the engineer’s control stand, making it possible to conveniently remove the box from the stand using a screw driver.

The next post will continue on controls. A later post will explain how the control box is arranged on the control car.
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Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
Odyknuck
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Re: EMD F7 in SCALE

Post by Odyknuck »

Pretty cool stuff
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NP317
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Re: EMD F7 in SCALE

Post by NP317 »

I really appreciate the various Blogs here documenting significant projects.
This leaves invaluable documentation for others to follow are needed.
Thanks to you, Carl, Jack and many others.
A talented group.
RussN
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rmac
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Re: EMD F7 in SCALE

Post by rmac »

I really like all the details, especially that you set up shop in a hospital bed! That reminded me a little bit of the guy who developed the Curta calculator while in a concentration camp.
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kcameron
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Re: EMD F7 in SCALE

Post by kcameron »

Rmac,

Those are great little calculators! I used one in high school. It bugged most of the class because they could hear me 'cranking out the answer'. My brother and I also used it for sport car rallies.
-ken cameron
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Steggy
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Re: EMD F7 in SCALE

Post by Steggy »

rmac wrote: Fri Mar 24, 2023 11:58 am I really like all the details, especially that you set up shop in a hospital bed!
Seemed like a logical thing to do at the time, although my wife gave me a little guff about it. She apparently thought recovering from surgery involved sleeping. :shock:

That reminded me a little bit of the guy who developed the Curta calculator while in a concentration camp.
It’s amazing what people can accomplish in the face of adversity. That calculator was one ingeniously-designed gadget.
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Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
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Steggy
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Re: EMD F7 in SCALE

Post by Steggy »

kcameron wrote: Sun Mar 26, 2023 9:08 am Rmac,

Those are great little calculators! I used one in high school. It bugged most of the class because they could hear me 'cranking out the answer'. My brother and I also used it for sport car rallies.
Back in my day, we didn't have calculators in school—portable ones didn’t exist. You were expected to be able to do the math the old-fashioned way. In eight grade, I learned how to do square roots on paper using that ancient device called a pencil. :D Alas, I no longer remember how.

That said, I did have a nice K&E log-log slide rule given to me when I started high school, although I wasn’t allowed to use it in math classes. I used that slide rule until the latter 1970s, when pocket calculators capable of handling exponentiation and trig functions became available. Before that, I'd get the trig ratios from a reference book.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
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Steggy
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EMD F7 in SCALE: Controls

Post by Steggy »

EMD F7 in SCALE
————————————————
ENGINEER’s CONTROLS: Part 2

Making the engineer’s controls started with preparing the box.  Naturally, I worked from a CAD drawing to lay out the holes that would be required.  Below is a top plan view drawing:

box_rough.jpg
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All hole-spotting was done in a mill to avoid layout errors.  Hole-finishing was done in a drill press:

box_machined.jpg
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The rectangular opening at top-center is for the throttle-position display module.  Some holes were finished with a piloted counterbore to maintain accurate diameter and roundness—for example, the toggle switch holes, which are 15/32".  Holes into which the gauges and throttle control assembly are mounted were hole-sawed to finished size.

Meanwhile, the toggle switch module had been assembled:

ctl_stand_switch_assy_top.jpg
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Due to the confined interior of the box, I decided that using a printed circuit board (PCB) to “wire” the switches would make for a tidier arrangement, as well as eliminate potential assembly errors.  As long as the PCB was there for switch wiring, I also decided to use it as an general interface point for other connections in the box.  These connections are made to labeled wire holes around the PCB’s periphery (holes visible in next photo).

In order to assemble the switch module, all switches were mounted into the box and carefully oriented.  After double- and triple-checking alignment and position, I set the PCB—which already had some parts attached—on the switch terminals, again double-checked things, and went to work with the soldering iron.

ctl_stand_switch_assy_bot.jpg
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Above is a view of the bottom side of the switch module after soldering.  In the center of the module is a 24-pin receptacle that connects the control car trainline cable to the PCB.  The round, black object near the top-left corner of the PCB is a Schottky rectifier diode, which is a component in the automatic transition enable circuit.  Note the wiring holes on the PCB.

With the switches soldered, the module could be bench-tested for potential problems.  Testing successfully completed, the next step was to trial-fit some other parts:

test_fit01.jpg
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In the above, can be seen the switch module, the throttle-position display module (object with the small heat sink mounted immediately to the left of the switch module) and fuse posts.  Above the throttle-position display module is the reverser switch assembly and above it, the alarm indicator.  In the lower right corner are the engine run key switch (larger switch) and horn push button.  As will be seen below, the angled corner of the PCB is to clear the throttle control assembly.

With trial fitting done, it was time to complete the assembly:

ctl_box_assy01.jpg
ctl_box_assy01.jpg (206.75 KiB) Viewed 3484 times

The pigtail with the small plug hanging loose near the end of the throttle lever connects to the control car’s carbody wiring harness.
Last edited by Steggy on Tue Nov 07, 2023 2:17 am, edited 1 time in total.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
Odyknuck
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Re: EMD F7 in SCALE

Post by Odyknuck »

The PC board really cleaned it up. Nice job. Did you make or buy the board?
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Steggy
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Re: EMD F7 in SCALE

Post by Steggy »

Odyknuck wrote: Sat Apr 01, 2023 5:40 am The PC board really cleaned it up. Nice job. Did you make or buy the board?
I had the board made to my specifications.

There are numerous companies that specialize in small-run PCB fabrication—I use a company called JLCPCB. The design is submitted—usually via E-mail—as a set of machine-readable files referred to as “Gerbers.” Each file in the set describes one layer of the board as vector graphics (boards can have multiple layers), plus one file describes the hole layout, which is in the form of a table of coordinates, sizes and features. The board house will use the information in the files to program the machines that prepare blank board stock for etching, drilling, laminating and silk-screening. As this process has become highly automated, the finished product will be exactly as designed, within the precision limits of the equipment used to process the PCB. Errors are virtually unheard of at the production level.

Most board houses have a five-piece minimum, so I’ve got four unused boards in this case. However, costs are such that it’s a “don’t” care situation. I eventually will use one of the remaining boards to fabricate another control box to be used in the shop for testing. It won’t, however, have the notched throttle, just the rotary selector switch with a knob on it.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
Andrew Pugh
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Re: EMD F7 in SCALE

Post by Andrew Pugh »

Could you give us an example of something you did a half-ass job on? ;D

Impressive work! Very nicely done.
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Steggy
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Re: EMD F7 in SCALE

Post by Steggy »

Andrew Pugh wrote: Sat Apr 01, 2023 11:47 amCould you give us an example of something you did a half-ass job on? ;D
I could, but I usually don’t post about it.  Another recent post I made was about an emergency puller hacked together out of a piece of “scrop” and a bolt.  Dunno if I’d call it a “half-ass” job, but it definitely was not museum quality. :D
Impressive work! Very nicely done.
Thanks!  Where possible, I sit on my entire derrière while working so the project doesn’t turn out half-assed.  :lol:
Last edited by Steggy on Tue Feb 27, 2024 12:38 am, edited 1 time in total.
———————————————————————————————————————————————————————
Music isn’t at all difficult.  All you gotta do is play the right notes at the right time!  :D
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