> An average flight of stairs is closer to 3 meters...
> 10 meters of stairs in 5 seconds would be pretty impressive.

Watts =3D N.m/s =3D kg.g.m/s ~=3D 10 x kg x m / s

For an 80 kg body

    Watts =3D 10 x 80 kg x 10 m / 5 seconds =3D 1600 Watts.

Definitely impressive.
And, that's the actual work done in lifting the "body".
Actual power required would be somewhat more - say 2 to 3 kW.
Even VGSW would have jelly legs after that :-).


> I dunno if I'd trust a "Watts output" from an exercise machine to be
> technically accurate, or even consistent between different vendors...

To be taken seriously exercise machines need to be certified to comply
with relevant standards relating to accuracy.
Figures escape me now (quite some years since ...) but there was a
rating above and below 50 Watts for exercise bikes and elliptical
trainers. May have been 5% below 50 and 10% above (or the opposite or
something else) ... .

Any equipment being used in a gym has a high likelihood of needing to
meet such standards.

Getting adequately accurate results using an electronically controlled
load was "annoying" at least. Use of a torque arm and speed
measurement would have made life easier but the client didn't want to
pay what that would cost. Use of alternator current and voltage and
speed was a poor substitute due to temperature effects on alternator
windings,  magnetic saturation with current and speed and airgap and
...., and more. Much tearing of hair needed to meet standards. Doing it
again - and I certinly never wish to if avoidable -  I'd really try to
be able to use true torque.

Mark Rages may have some good things to say about this.

_______

Re the load control discussion. I'd personally try very hard to use a
FET as they are beautifully easy to control and manage in this context
compared to a BJT. Low side current sense and drive power become non
issues.
There are enough very cheap FETs available (MJ?3055's if you want to
feel linked to the past)("3055" is burned forever into my brain)(along
with eg '26 BNE')  that having to parallel a suitable number is not  a
great problem. As noted, load sharing is easy, and if desired the cost
of an opamp and sense resistor per FET is not going to be cost
prohibitive in most applications.

Use of a modulated resistor (as per my and others' previous posts)
with a FET as switch would serve many needs.
Placing a largish capacitor (onl of Olins N mFs would do well) at the
hot end of the load resistor would allow peak currents to be largely
carried only by the R with the source largely feeding the capacitor.
RC time constant need sto be "somewhat" greater than switching
frequency. Watch ripple current ratings.

eg a 1 ohm resistor plus (FET) switch can be made to look like 1 ohm
on up. If the source was say 15 volts max power =3D 200+ Watts. At say
30 Watts mean source current is 2A but switch current is either 15A or
0 A.
For a 1 mF filter cap the time constant =3D RC =3D 1 x 0.001 or 1 mS. PWM
at say 10 kHz or above would do a fair job of letting the source see
an effective 7.5 ohm mean load (at 2 A ) and 50 kHz + would be very
fine.

1 ohm 250 Watt wire (or ribbon) wound resistors are liable to be
available from surplus outlets for about $0. In this example a 15A +
FET also approaches $0 compared to time to build etc.

Nobody is liable to send you free samples of 1 ohm 250 Watt resistors
:-). [ YMMV ].



                  Russell
--=20
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