————————————————
ENGINEER’s CONTROLS: Part 1
Back when I started this build log, I listed some construction goals, which included:
- Power and speed solely controlled by a notched throttle with idle and eight power positions.
- Separate reverser with neutral, working independently of the throttle.
Before launching into any construction details, see below for a photo of the result:
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.
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.
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:
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.
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:
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:
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.
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.
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.