by Michael S. Kaplan, published on 2008/11/27 16:31 -05:00, original URI: http://blogs.msdn.com/b/michkap/archive/2008/11/27/9148204.aspx
Once again, a blog for your reading pleasure that is technical though not on the usual subjects. If iBOT crap bores you, then please skip gracefully!
Prior blogs in the series here and here and here and here and here.
It was actually in a response to that last one that regular reader and long-time friend and colleague Tony Toews asked:
So I gotta ask a basic question. Do the batteries last all day when you're spending large parts of the day in balance mode on two wheels?
Now of course we know from past experience that it is the basic questions that can have some of the most complicated answers!
Now at one level it is simple --they did manage to last that long while I was at PASS.
And also simple at a more technical level, as the provided documentation for the iBOT describes it all.
Of course the usefulness of that text:
When tested in conformance with ISO 7176-4 (1997), the theoretical distance range while driving steadily on flat, level ground exceeds 15.5mi (22km) range in Standard, 4-Wheel, and Balance Functions.
is somewhat obviated by the disclaimers of the following sentences:
This data should only be used for comparison purposes with other power wheelchairs. The actual distance you can travel will depend on total weight carried, weather, surface conditions, driving style, other power usage (e.g., transitions between functions), and climbing stairs.
and really it kind of tends to ignore the actual question Tony is asking -- which is a very reasonable question to have.
Now ISO 7176-4 (Wheelchairs -- Part 4: Energy consumption of electric wheelchairs and scooters for determination of theoretical distance range) is useful as far as those things go, though it ignores some crucial information such as the vastly different characteristics of the Ni-Cad batteries of the iBOT from the Lead Acid Gel batteries seen in most power wheelchairs -- differences that will really impact actual usage. even 7176-4 says as much on its introduction:
Distance range is also strongly dependent on the way in which a wheelchair is driven, and a single value for theoretical range can be insufficient to provide an understanding of the performance of a wheelchair. Two methods for determining theoretical range are provided in this part of ISO 7176, for driving and for manoeuvring. These values are intended to facilitate wheelchair comparison in a manner analogous to the extra-urban and urban fuel consumption figures published for motor vehicles.
Funny, that's what I was about to say this was as useful as. :-)
So why is it that a document that claims that various modes (Standard, 4-Wheel, and Balance) all will allow for analogous ranges?
Well, technically it doesn't -- it just says they can all go THAT far, wihout claiming any one can go farther. Even if it can.
Is there truly no power cost to the more heavy-duty 4-wheel mode even when it is on flat ground, analogous to the way that a car in 4-wheel drive mode needs more gas than one on 2-wheel drive mode?
Maybe -- because as observers have noted, the wheels do not work independently; all four always turn even if two are off the ground.
But is the power cost of the iBALANCE functionality (used in Balance mode and to a lesser but still present extent in 4-wheel mode) truly so negligible that the three modes are considered to be the same in tems of power cost?
This just seems wrong....
Every taximeter I have ever seen while sitting in a taxi charges for both the driving time and the waiting time by some unknown (to some) formula that one could clearly use to one's advantage if one wanted to charge too much. It would be easy to think the iBOT above such petty nickel-and-diming as the taximeter and such criminal designs as the one who would actively subvert one to make more money.
But I have seen my battery meter decrease while I was just standing in balance mode in a presentation or at the bus stop waiting for the 545 to Seattle.
So I know that such a thought is baseless.
Even more than I "know" that the whole Ni-Cad memory effect is probably a crock since it hasn't really been proven and I am the sort of person who thinks that anything people "know but can't bother to prove" is more likely to be a crock than not. More on my opinions here in a blog post some other day....
Because this one, I know through direct observation, through admittedly suspect measurements.
I trust the battery meter on my iBOT like I have never trusted a battery meter before -- because this is the first time it ever really mattered that the gauge looked accurate enough that it could give me useful information.
And that meter showed some cost to iBALANCE in the iBOT's Balance mode.
Not a lot. But some....
Could the decreased cost of some other factor cancel out the increased cost of the iBALANCE stuff, making all three modes really have the same range? Hmmm...
This is, by the way, the kind of thing that irks me some. I mean, all of the testing and work they do and they can't just provide some data on average power usage of iBALANCE that could be compared as amount of time one could have instead been driving if the human dignity thing wasn't as important as the distance?
Perhaps the next time I am reading a book I'll get in the iBOT in Balance mode and read until the battery drains two ticks on the battery meter, then move to 4-wheel mode and read until the battery drains two ticks on the battery meter.
If either takes an unconscionably long time, I'll apologize for my the part of my irk not related to the fact that they have almost certainly has done this kind of testing but never bothered to publish the results, and be happy with the original non-information they provided. And if either time is significantly short enough, I'll be at full irk mode and try to look into a fuller, more formal amount of data to be provided....
Until then, the question is still pending, Tony. Stay tuned. :-)
This post brought to you by ♿ (U+267f, a.k.a. WHEELCHAIR SYMBOL)
Tony Toews - Access MVP on 27 Nov 2008 10:01 PM:
Oh great. Blame me for this blog posting. And what a geeky answer. <chuckle> Of course I expected no less.
My understanding is that the NiCad memory effect only applies to satellites in some orbits which have a very fixed time in and out of the earth's shadow. But who knows how true that is.
And of course as the batteries age they will hold less and less of a charge.
Hmm, I recall reading somewhere that the more you drain the batteries the shorter the life. That is if you only drain the battery 10% you could get 5,000 charges out of it but if you drain the batter 50% you could get only 50 charges out of it. OTOH that may have been urban legend and applies only to lead acid batteries. Who knows.
OTOH it may be worth plugging in your chair when at your office desk to keep those batteries at peak performance. OTTH doing this may not give you enough warning for those occasional days when you spend a lot of time moving.
OT4H I hope the batteries are some industry standard and thus cheapish to replace.
John Cowan on 28 Nov 2008 2:27 AM:
Taxi meters in NYC are actually smarter than that. The flag-drop fare is $2.50 including the first unit (plus any time-of-day-related surcharge), and then it's 40 cents per additional unit. A unit is 60 seconds if standing still, or 1/5 mile if moving. But if the taxi is moving at less than 6 mph (not uncommon in rush-hour traffic), the driver actually would get less per minute at the "moving" rate than at the "standing still" rate, so such low speeds are treated as "standing still".
Andrew West on 28 Nov 2008 4:29 AM:
15.5mi = 15km ?
Michael S. Kaplan on 28 Nov 2008 1:57 PM:
Whoops! Fixed now...
Brent on 17 Dec 2008 10:42 AM:
Actually, the IBOT "iBalance" technology is mostly software, and the hardware is never actually "off" during non-dynamically stabilized functions. The averaging of power consumption is generalized so greatly due to A) the significant environmental variables involved, and B) the reduction of speed in the various functions. Standard Function is the fastest, therefore uses the most power, and 4-wheel speed is slightly reduced giving is increased efficiency (though only on flat level surface). Balance Function speed is reduced even more, therefore it is effectively the most power efficient. Also, NiCd memory effect is easily proven with a plotted voltage curve. Thanks....
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