Tuesday, March 08, 2011

Variable Authority

Up to now in my career, I have flown airplanes that I steered on the ground through pure mechanical linkages powered by my own muscles. I was using levers to physically shove the nosewheel into pointing the way I wanted to turn the airplane. The Screaming Whippet one is too big for me to kick around by myself, or they don't want to make levers that big, so it uses electrically controlled, hydraulically activated, variable authority nosewheel steering. I have to remember that description in case I'm asked about it on the ride. (Ride is pilotspeak for flight test). I also have to remember a whole lot more about how it works.

First pass, what I remember and understand from the class and the reading without looking anything more up:

Nosewheel steering is on the non-essential DC electrical bus. It must be armed and activated to be used. You arm it with a toggle switch, the location and position of which I can't picture now and it becomes activated when the squat switch indicates that you are on the ground and one of the following things happens. Either a) the right speed lever is moved into the low position or b) a button on the side of the left power lever is pressed and held. (Yes, I have both speed and power levers: resolving units, you'd think you'd get a power lever by forcing the speed lever, but that would be a geeky joke and not the reality of this airplane). Once the system is activated, you steer with the rudder pedals, just like old times.

Moving a rudder pedal sends a signal to a potentiometer. There's a pile of potentiometers, I think it was four. One of them represents the input you gave to the rudder pedal, one the position of the nosewheel, one the target position and one for good luck. So maybe it was three. The difference between the target and the position is sent to a selector valve which shuttles the appropriate way to send hydraulic fluid to a piston operating rack and pinion steering to pivot the nose gear in the appropriate direction. The channels in the shuttle valve go straight through for left turns and criss-cross to reverse the pressure for right turns. When the commanded steering meets the actual steering, the system holds that input until a new input is received or the system is disarmed. In the later case it smoothly recentres. The maximum steering in this mode is ten degrees either side of centre, but you can get up to sixty-three degrees deflection in park mode, by pressing and holding the park button. This is for manoeuvring into a tight parking spot. I really hope I don't often have to parallel park this airplane.

There's an annunciator for the system being armed, an annunciator for it activated and a flashing one if it fails. Because it's an electrical system, there is a chance of transient weirdness, and transient weirdness is not something you want to see in large vehicle steering If there's a three degree disagreement between two of the potentiometers, representing some form of where it's requested to be and where it thinks it is going, the system disconnects. You can still steer with braking and power.

Now let me see if by re-reading the chapter I can fix that up, and add things I forgot.

There is a command potentiometer (my "input you gave the rudder pedal") on the left side of the rudder pedal linkage, and a fault protection monitor potentiometer (my "target position") on the right side. There are two more potentiometers on top of the steering actuator, a control follow-up potentiometer (my "position of the nosewheel") and another one for the fault protection monitor circuit. So I was right, there are four, and if monitoring faults is good luck, I got them all right. Well, let's see.

There is a "Nose Steer Computer/Amplifier" which "contains Servo Valve Solenoid drive circuitry to initiate steering based on Rudder Pedal Potentiometer and Nose Gear Follow-up Potentiometer Position." (Why yes, this training manual is apparently so old that it predates the late eighteenth century English move to distance itself from its Germanic roots and not capitalize all nouns).

Okay, reading the quasi-incunabulum more closely, I surmise that the follow-up potentiometer simply turns with the nosewheel steering column, and has the same type of electrical connections as the rudder pedal potentiometer. When the "wiper resistance," a term I am not familiar with, but which I'll read as "the combined effect of the electrical connection and the turning," of the two is the same, the steering is deemed to have turned as far as commanded. The hydraulic supply ports close, so the wheel is locked in the position it was steered to.

I think the Rudder Pedal Monitor Potentiometer must be at the same wiper resistance as the Rudder Pedal Potentiometer--perhaps one potentiometer can only be compares with one other potentiometer, so the latter needs the former to shadow it. My manual names former, and the fourth potentiometer, the Hydraulic Actuator Monitor Potentiometer only once, in the sentence, "In either case, if a difference greater than approximately 3 degrees exists between the Rudder Pedal Monitor Potentiometer and the Hydraulic Actuator Monitor Potentiometer, the protective circuitry within the computer will shut down the system." I can't quite follow that all the way, but I know it means that if the monitor circuit thinks that the commands sent to the hydraulic system are incorrect, it will disconnect the steering to avoid having an electrical irregularity steer an airplane off a runway.

Absent positive steering, you still have directional control of an airplane through braking and differential power, so this is better than definitive by incorrect steering inputs. The "in either case" above refers to the case of a wheel being locked in place despite commanded steering versus uncommanded steering occurring.

The arming switch is located on the left hand console.

I also need to remember the fault test switch and that the system acts as a shimmy damper when not activated, but left in what is known as castoring mode, and that on retraction it automatically centres because the centre position is the low point in the race ... ah I guess I don't understand that well enough to explain it clearly. Also variable authority. I'll say that again to make sure I remember it.

Today is, I believe, International Women's Day. There is one male on our course and the rest are females, who happened to all be connected. The man had a eureka moment when the women all had connections to one another, either having previously met, knowing people in common, or just the familiarity of being females in aviation. We were all talking about then and catching up. He suddenly realized what it must feel like to not be part of the "old boys' network," and the girls saw it at about the same time. We promised not to exclude him from our network, and of course had no intention to do so, but through the week I noticed that our conversations would drift into discussions of female interest topics like clothing, or career planning in the context of postponing or forgoing pregnancy and he would be unintentionally excluded by dint of not having experience or opinions. It must have been for him exactly as it is for me when my usually all male colleagues start discussing football, or someone getting drunk while hunting and rolling his truck. It's hard to get into a conversation when you can't identify with the decisions being made or discussed. I can sort of feel for the men who feel threatened about women in traditionally male-dominated fields, but I don't think it will hurt them to learn to say "aww" when they hear about strangers' babies any more than it has hurt the women to feign interest in football for social purposes.


Wayne Conrad said...

I love this post. Yes, I'm a geek.

When the steering is disconnected due to a fault, what happens to the nosewheel? Does it become free-castering (I don't know if that's the right term... I'm not _that_ much of a geek), or locked in its current position, or what?

I've had "this is what it feels like to be one of them" moments. Maybe I'm weird, but I've liked them, for the most part: Even if the social part of my brain is freaking out a little bit, the part that loves to learn new stuff is having a party.

Aviatrix said...

Yes, if the nosewheel steering is not selected, or if it disconnects due to a fault, it becomes free castering, exactly as you describe, and that is the correct term.

You're one. :-)

kschendel said...

Re: wiper resistance. The center terminal of a pot is the "wiper", so named because physically it's a little arm that rides on top of the resistive element. As you turn the pot, it wipes along the resistive element, such that the resistance from the wiper to one end decreases, and resistance wiper to the other end increases. (End-to-end resistance is constant, of course.)

The term "wiper resistance" is a little odd, but I assume they're talking about the varying resistance from either end (pick one) to the wiper.

GPS_Direct said...

Keep it up! Your descriptions are always entertaining - cryptic abbreviations, obscure references, and geeky physics jokes included.

(Reminds me of the the old "You might be a Physics Student if..." One of the lines was "You've ever assumed a horse was a sphere, just to make the math easier!")

Not having any large-airplane experience, it took me a while to work things out. For a bit, I thought you were flying the Space Shuttle! It's amazing how system designs get re-used across aircraft... Now, I think I've got it - and I like it! Always thought Whippets were nifty looking planes.

Have fun, just watch those breaks!

TgardnerH said...

I think my favorite "oh, this is what it feels like to be one of them" moments was visiting Provincetown, MA, after having grown up in a city that is very much identified as a liberal bastion. Where I grew up, being gay wasn't a problem, or even really anything to be noticed, just (as is inevitable if you're a minority) not slightly different.
Provincetown in summer is a gay mecca, such that being straight isn't a problem, or really anything to be noticed, just a little different.

I'd never realized how much of a difference being in the majority meant. It's an experience that I think all white men should experience, eye-opening in a really good way.

Anoynmous said...

It seems strange to me to that one would consider "wiper resistance" when using a potentiometer. The word "potentiometer" implies that the parameter to be measured is a variable voltage, not a variable resistance. If the wiper resistance is the value of interest, I'd call the device a rheostat. That's mostly a distinction of usage rather than one of essence, similar in spirit to the difference between "cattle" and "beef".

Tom Q said...

I don't know about aviation, but in electronics people tend to call any three-terminal variable resistor a "potentiometer", even when it's not used as a variable voltage divider. Actually, we usually shorten it to "pot."

The things that I see people call "rheostats" are generally heavy-duty or high-power devices.

(I've been reading the blog for quite a while now...keep up the good work!)

Anonymous said...

Is it possible wiper resistance is referring to the balance of pressure in the hydraulic cylinder?

In my experience a "wiper" can be a rubber washer type thing that goes inside a hydraulic piston.

Aviatrix said...

Wiper resistance definitely refers to the potentiometer. kschendel's description is exactly what I wanted. And the manual called them pots, too. I just don't use nicknames until I have more familiarity than that.

Zarniwoop said...

You do not normally read a resistance from a potentiometer but a voltage.
A potentiometer normally has three terminals:
1. Supply voltage
2. Tap voltage (from the wiper)
3. Ground

A supply voltage is applied and the tap voltage varies as the wiper moves due to the potential divider changing (as kschendel described). The tap voltage is what will be monitored by the system.
'Pot' is a very common term used for potentiometer in electronics.

Rhonda said...

I caused the "not part of the group" reaction once...

I was with a large group of computer geeks, and found out one of them had a background in chemistry. He asked what I did for work and I told him, and we chatted a bit about the chemistry I used in my job; the computer geeks around us all got very confused looks on their faces and eventually walked away, muttering "now I know how non-computer people feel when we get to talking..."

DeAnn said...

Really great work here ... this is how I learn as well. I write what I think and read it to correct errors. My research in to "owning" information indicates that committing to an idea is the only way to make the initial impression (and then refine it if need be). A bunch of people seem to want to rehearse an answer for the checkperson ... and I see energy being spent to tweak towards the perceived biases of the examiner ... those tidbits of information aren't stored in our brains in the "things I understand file" ...

You're going to rock this. I am enjoying watching this very much (and even learning some cool airplane stuff - thx).
A comment to the other - my experience is the guys are surprisingly inclusive and supportive. Seems like the most meaningful connections adhere to what we have in common. (love that "free-castering" ... what a fun concept) ~D

kschendel said...

Re object names - "potentiometer" really is a silly name for the device, because it's fundamentally a variable resistor (not a variable potential or voltage), and it doesn't "meter" anything. But that's what it's been called for decades. As TomQ said, "rheostat" usually implies something built the same way as a pot, but typically designed for high power work. Another, much less used name is "variable resistor". (But NOT "varistor", which is a different animal entirely!)

One other note: the pot is fundamentally a three-terminal device (the two ends and the wiper). Frequently one needs to use it as a two-terminal device, meaning that one cares only about the varying resistance between one end and the wiper. In that usage case, you'll often see the nominally unused end tied to the wiper. The thinking here is that if the wiper bounces off the resistive element, the circuit doesn't see a momentary open circuit (which could lead to sparking and erosion of the element, if nothing else). Instead, the jump is to the end-to-end value of the element, which is much less disruptive.

It's been decades since I had to remember any of this, rather nice to know it's still in my head somewhere. :-)

D.B. said...

Have you ever been walking down the street, thinking about something else, when someone stops in front of you and cheerily says "Hi xxx, how are ya doing?" - but you can't remember her name and her face looks only slightly familiar?

That's what this post is doing to me. It's been 30+ years since I've cared about the difference between a rheostat, potentiometer and an elephant. I seem to recall that the elephant is only useful for determining very large voltages, but that's about it. Still, long neglected brain cells are stirring in the recesses on my skull. But a few days of high-tech marketing will lull them back to sleep, I'm sure.

Others seem to know what a "screaming whippet" might be, while I have no clue - and I know my aviation-related brain cells are still in top notch condition (in my youth in the UK I would have said "in good nick" - all sorts of old things are stirring in this head thing of mine). I'll have to do some research on the C- register list.

Paul B said...

Take two pots: put one onto an operating lever (rudder pedals), the other onto the device that will be turned (nosewheel). Each pot will have it's "bottom" connected to 0v, the top to "volts" (eg 10 volts). The "wiper" will then have a voltage between 0v and 10v on it, depending on its potition.

If the two wipers are in the same position, they will have the same voltage on them, thus no voltage difference.

But if they are in different postions, there will be a difference in the voltage: the "voltage difference"... you could measure this with a voltmeter.

Depending on which pot is set where, the difference could be a negative or a positive voltage (*), where positive means "I need to turn the "nosewheel" leftwards to make it the same as the control lever, and negative means "I need to turn it rightwards...."

Now, if I connect a motor to the nodewheel, and drive it by that voltage difference (yeah, we'll need a motor "interface" too....) then the motor will turn the "nosewheel" to match the control lever... as it gets closer to the setting, the voltage difference reduces and (depending on that motor interface) it may slow down the motor... so we gently approach the "matching point" when the two pots have the same voltage.

(We could not bother with slowing down the motor: the problem then is it might overshoot, and then we have to start again, in the opposite direction......)

(* if pot1 = 9v and pot 2 = 5v, then the difference is 9v-5v = 4v. But if pot1 = 5v, and pot2 = 9v, then the difference is 5v-9v = -4v)

This is the whole idea behind "servo motors"... the motor "servos" to the controller... and it's 100% what you get in model aircraft servo motors, and sounds like, with a "hydraulic motor", is what you get in REAL aircraft too!

Great blog Aviatrix!

Zarniwoop said...

re: kschendel

I think potentiometer is a suitable name. It is more than a variable resistor (ie just a two terminal device). It is a variable potential divider which meters a voltage potential, therefore, a potentiometer.

Dafydd said...

Little bit off main topic but you have touched on the sisterhood , and today is International Women's Day .
So - recently ,Captain Barbara Harmer died age 57 in UK . Her career was inspirational .Leaving school to begin work as an apprentice hairdresser she graduated into aviation culminating in captaincy of Concorde with British Airways .
A dalliance with Google will reward the interested with more insight into the life of this remarkable woman .

zb said...

Everything and anything you ever wanted to know about potentiometers, maybe even published around the time your particular airplane was engineered:

The Potentiometer Handbook

Published by Bourns, a company that makes these parts. (pdf, 33 MB, really worth the download -- but maybe not when you're hooked up too bad hotel WiFi) I liked the pdf so much that I even bought it in dead-tree-form on the second hand market.

Make sure you don't miss chapter nine, To Kill A Potentiometer. I am not aware of any other application note for electronic parts that would be just half as much fun! This book is a treasure and I think it's wonderful that it is up on the company's website amongst all the shiny new marketing material. You'll even find pictures of very special aircraft somewhere inside (V/STOL).

(The only strange thing is the page order of the scanned pdf document. A bit annoying, but you'll figure it out...)

zb said...

Everything and anything you ever wanted to know about potentiometers, maybe even published around the time your particular airplane was engineered:

The Potentiometer Handbook

Published by Bourns, a company that makes these parts. (pdf, 33 MB, really worth the download -- but maybe not when you're hooked up too bad hotel WiFi) I liked the pdf so much that I even bought it in dead-tree-form on the second hand market.

Make sure you don't miss chapter nine, To Kill A Potentiometer. I am not aware of any other application note for electronic parts that would be just half as much fun! This book is a treasure and I think it's wonderful that it is up on the company's website amongst all the shiny new stuff.

(The only strange thing is the page order of the scanned pdf document. A bit annoying, but you'll figure it out...)

((Sorry if this comment eventually appears twice, it seems like blogger nuked it the first time I hit "publish", so I'm re-trying.))

Michael5000 said...

Somebody was passing out treats for International Women's Day at work, and I thought they were saying "International Human's Day" and asked for my share and felt a little paranoid when I was refused and everybody laughed at me.

Later, I got drunk and rolled my truck, and then I felt better.

Michael5000 said...

That's as may be, Dupont Hydro Industries, but you don't need to go around bragging about it. Besides, I had the better anecdote.