Subject: Re: A way to calc the starting speed of a Stepper?
From: Jim Tuttle
Date: Thu, 26 Sep 1996 00:59:05 -0700
Paul Dowd wrote:
>
> I am trying to design a motion-control system using steppers,
> and I need to be able to predict roughly what the starting
> speed can be for my system. The design is pretty open right
> now and I am flexible on the various parameters, like the
> inertia ratio between the load and the rotor.
>
> On the one hand I have been told that you can start a stepper
> from a stop to 800 - 1000 full-steps / sec without any accel
> profile. Also, a three to one inertia ratio is supposed to be
> good. On the other hand, however, if one calculates the
> dynamics of the system by taking the Torque, Inertia, and
> target Speed, one sees that it would take far further than one
> full step to acheive top speed. Am I missing something here?
>
> I understand that stepper systems are very different and
> dependent on the level of damping and drive type, however, there
> must be a way to at least get a rough idea for starting speed.
>
> The other paradox is what about microstepping? If you can start
> at 4 rev/sec in full step mode, can you still start at that speed
> when microstepping at 10000 steps/rev? It would seem that in
> microstep mode you would be asking the system to accelerate to
> your top speed in that much shorter of a distance.
>
> If anyone has any practical experience with how these variables
> affect starting speed, I would appreciate it.
>
> Rotor Inertia
> Motor Inductance
> Drive Voltage
> Inertia Ratio
> Microstepping
> Damping
>
> -Paul
>
> paul@engineering.com
> http://www.phantom.com/~pauldowd/
Paul,
The stepper motor manufacturer will specify a "base speed" for a particular motor and a
particular drive. This theoretically means the speed (read frequency) at which you can
supply an input pulse train and have the motor follow, with no ramp up. They will also
tell you the rate of ramp up you can subsequently have, or can have from rest. Typically
they supply curves.
These are usually optimistic figures. Anybody who tells you you can hit ANY stepper with
800 - 1000 steps / sec is simply unexperienced. It'll just sit there and hum, unless
it's pretty tiny and unloaded. Whatever the "book" figures are, conduct some experiments
in YOUR application and allow yourself some margin. I was never able to get within 20%
of the "book" values, with no load other than relatively trivial inertia.
To see my application, check out my web page, sub page "some of our design projects".
Item number two (the 35,000 rpm positive feed drill) utilized a Superior Electric
stepper the size of an auto starter. I spent extensive time with an optical strip chart
recorder and time expanded pulse train analysis to find out it simply wouldn't do what
was claimed, even with a trivial load. Maybe with excess voltage - but not with what
came out of the box.
Practical experiment will be your best guide. If you've got to work from the "book",
take the curves supplied and cut back at least 30%.
--
Best regards,
Jim Tuttle
e-mail: jtuttle@productivitypartners.com
www.productivitypartners.com
Subject: Re: austenitic steels
From: Jim Tuttle
Date: Thu, 26 Sep 1996 01:11:48 -0700
Fred Hendrix wrote:
>
> what are the unique properties of austenitic stainless steels? i
> believe that they are identified by significant amounts of nickel, but
> how does the nickel affect the characteristics with regard to wear,
> corrosion, weldablility, yield strength, tensile strength, fatigue,
> etc.? thanks in advance.
Fred,
Somewhat universal unique properties are a very high degree of work hardening, and
relatively quite high corrosion resistance except in crevice corrosion, where many are
unbelieveably susceptible. Other than that, it depends on the alloy. 304L welds nicely,
303Se is very poor. Some have nice high temperature characteristics, others will develop
a carbide precipitation at certain temps that makes glass look ductile. I suggest you
contact Carpenter and get their latest "Carpenter Stainless Steels" book. It discusses
all the alloys they supply (including numerous austenitics) and gives great data on
virtually all the parameters you discussed.
--
Best regards,
Jim Tuttle
e-mail: jtuttle@productivitypartners.com
www.productivitypartners.com
Subject: Re: 4 cycle vs. 2 cycle engine
From: kirchweg@sztms.tu-graz.ac.at (Gerhild Kirchweger)
Date: 26 Sep 1996 08:12:48 GMT
Rey Ramirez Sangueza (rrs@saba.kuentos.guam.net) wrote:
: as far as standby generators are concerned....could anyone out there
: explain to me the difference between a 4 cycle diesel engine versus a 2
: cycle diesel engine??? (comparing fuel efficiency, emissions, which is heavier
: duty, which is more reliable)
Well, as far as I know, all bigger diesel engines (for generators as you
mentioned, but also for those bigger ships) are two-stroke diesels, simply
because of fuel efficiency (you get power out each turn, not just every
other).
Reliability is maintained by small rpm, and operating the engines at loads
that are fair below design borders (compared to small engines).
Of course 2-stroke engines get hotter and have to stand the huge
combustion forces every turn, but then again, you don't make an "empty"
(i.e. without power-output) turn for each "full" one.
You really can't compare big diesel-2-stroke engines to small
(motorbike-type)-gasoline-2-strokers, as they share only the way the air
comes in (fuel is injected, well, it's a diesel, and there are exhaust
valves), so emissions shouldn't be too different from 4-stroke engines.
Having said that, I don't think there really are many big (I mean really
big) 4-stroke engines.
--
oo_oo_oo_oo_oo_oo_oo_oo_oo Gerhild Kirchweger oo_oo_oo_oo_oo_oo_oo_oo_oo
|____ Department of Internal Combustion Engines and Thermodynamics ____|
|__ Graz University of Technology/ Austria ___Tel. ++43 316 873 7212___|
oo_oo_oo_o http://fvkma.tu-graz.ac.at/~gerhild/gerhild.html _oo_oo_oo_oo
Subject: Re: Removing bearings from blind housings
From: Sehall@Ptbo.Igs.Net (Scott E. Hall)
Date: Thu, 26 Sep 1996 04:01:14 GMT
On Wed, 25 Sep 1996 23:13:56 +1000, Andrew wrote:
Not sure if I understood the question fully, but here's a simple,
cheap and easy way of doing the job. If the bearing is sealed it will
work, but if it isn't forget it you'll get more problems than leaving
it in.
1. Find a shaft that is just a little undersize of the bearing bore.
A dowel pin will work if you can get the right size. Try wiggling it
into the bearing bore. It has to be just able to slide with a little
force.
2. Get a nice stiff grease any kind will work. put a nice glob into
the bearing bore and slide the pin in. keep doing this until the bore
is almost totally filled with grease.
3. Slip the pin in and cover everything with a towel/rag. Give the pin
a sharp blow remove rag and take a look. If everything worked out
right the bearing should have moved out a hair from its housing.
This should work for you, but it varies with each case. If you try it
your only out some grease and a dowel pin not a $300 dollar puller.
Hope this helps.
Scott
Scott E. Hall
**************************************************************
* al842@Freenet.Carleton.Ca * Sehall@Ptbo.Igs.Net *
**************************************************************
* Carleton.University - Department of Mechanical.Engineering *
* Ottawa, Ontario, Canada . *
**************************************************************
* IGS - Net Peterborough, Ontario, Canada . *
**************************************************************
Subject: Pipe tolerances
From: John Burns
Date: Thu, 26 Sep 1996 10:02:33 +0100
A net impaired colleague has asked me to post this request:
The American specification, API 5L of 1 April 1995, for seamless steel
pipe
allows for quite large tolerances on wall thickness, but apparently the
actual
thicknesses produced by the pipe makers are generally much better than
those
specified.
Am studying the effects of these variations on specific welding
techniques,
and would be grateful for any statistical data on: the actual versus the
specified wall thickness of API pipe, and/or on the eccentricityu of the
bore
relative to the OD. Am particularly interested in the 6in, 8in, 10in and
12in
nominal diameters of pipe -- but any info welcomed.
Please reply via email to john@seaway.demon.co.uk and I will forward
your comments.
TIA.
--
"It is better to remain silent and be thought a fool than to open your
mouth in a meeting with the users and prove it." (Ancient programming
proverb)
NB: As I am a gibberring idiot any views expressed here are NOT the
views
of Stolt Comex Seaway Ltd. john@seaway.demon.co.uk, Aberdeen, Bonny
Scotland
Subject: Combustion efficiency based on ambient temperatures?
From: Will Stewart
Date: Thu, 26 Sep 1996 06:56:02 -0400
DaveHatunen wrote:
>
> In article <52bujf$5d5@mane.cgrg.ohio-state.edu>,
> David Heisterberg wrote:
> >DaveHatunen (hatunen@netcom.com) wrote:
> >: In general, once warmed up, an ICE works MORE efficeintly at low
> >: ambient temperatures. This is because thermodynamic efficiency is
> >: dependent on the difference between operating temperature and and
> >: ambient heat dump temperature. Since the operating temperature of the
> >I don't think so, Dave. The temperature difference that counts is
> >that between the combustion and that of the gases at the time the
> >exhaust valve opens. Once the valve opens the gas ceases doing any
> >useful work for you, unless perhaps you run it through a turbocharger.
> Not quite. Efficiency is also affeted by the backpressure of the
> exhaust system, and that is a function of ambient temperature (among
> other things).
You're clutching at straws, Dave. Just admit that you are wrong.
> BTW, one of the first things a flying student learns is the dleterious
> effects of warm air entering the carburetor. I damn near got killed
> once when my carburetor heat cable broke, and carburetor heat was left
> on; I cleared an obstruction at the end of hte runway by about 6
> inches.
Power and efficiency are not one and the same. Warmer air is less
dense; an ICE operating in warmer air uses less fuel.
> >: Most people have noticed that their car has more pep when it's cold out
> >: than when it's hot out.
> >
> >That's because the air is denser and you get a greater mass of air and
> >gasoline in the cylinders for the same volume (displacement).
>
> In any case, you get better efficiency at colder temps, once the engine
> is warmed up.
Still wrong. Talk to an engineering professional.
--
William R. Stewart
Member American Solar Energy Society
Member Electrical Vehical Association of America
"The truth will set you free: - J.C.
Subject: Re: Respectfully disagree with your woolly headed psuedoliberal commentary
From: billd@cyberia.com (Bill DeFelice)
Date: 26 Sep 1996 13:07:01 GMT
>
>Tell that to my friends who graduated in mechanical a few years ago
>when there were no jobs, in the middle of the recession. I chose my
>major based on my likes, but also with an eye to the market. Hell if
>I'd done what I REALLY wanted to do I would have been a philosophy
>major. But I didn't see many advertisements for philosophers in the
>classified section of the newspaper.
>
>The point is that many of us are multitalented. I could have done
>electrical, mechanical, civil, computer science or any number of other
>things; and I liked them all too.
>
>But unfortunately we live in a market driven economy. You old hands
>out there who have been employed for fifteen or twenty years may not
>realize it, but it is not that uncommon to find an engineering
>graduate who is in an out of favor (ie marketwise) discipline flipping
>hamburgers. Aereonautical is notorious for conferring this status on
>grads in bad years. So rather than offer up well intentioned, but
>perhaps ill considered advice to "follow your heart", how about
>offering these guys what you as people in the industry probably have a
>better handle on than anybody else: Which disciplines seem to offer
>the best opportunities due to shortages or projected demand.
>
My advice is to follow your heart!
When I had to choose which engineering major I would want I looked at a
couple of factors. Of course as a Know-it-all 18 year old I looked at
starting salary and job availability. In 1981, Petroleum Engineering majors
were starting at $36,000/yr. and everyone was getting two job offers.
So that was my choice. Now I was lucky because I had always loved math,
geology, and chemistry (weird kid;)). So I did fairly well in that area but
in 1985 when I graduated, out of the 45 in my class 2 got job offers in the
Petrol field. One of my best friends with a 3.8 gpa was delivering pizzas
three months after graduation.
I found a job as a mechanical/manufacturing engineer in a heavy
wall pressure vessel manufacturing plant. Now 11 years later I have been
promoted to a Business Development Manager in charge of opening the
Petrochemical Markets for my company. This may be a bit of serendipity but I
think you make your own breaks by keeping your eye on the target and that is
to be responsible for your own actions and be a good all around person.
Bill DeFelice
BDeFelice@PCC-York.com
http://www.PCC-York.com
Subject: Re: 4 cycle vs. 2 cycle engine
From: Corey Wills
Date: Thu, 26 Sep 1996 15:33:48 GMT
Gerhild Kirchweger wrote:
>
> Well, as far as I know, all bigger diesel engines (for generators as you
> mentioned, but also for those bigger ships) are two-stroke diesels, simply
> because of fuel efficiency (you get power out each turn, not just every
> other).
>
> Reliability is maintained by small rpm, and operating the engines at loads
> that are fair below design borders (compared to small engines).
> Of course 2-stroke engines get hotter and have to stand the huge
> combustion forces every turn, but then again, you don't make an "empty"
> (i.e. without power-output) turn for each "full" one.
>
> You really can't compare big diesel-2-stroke engines to small
> (motorbike-type)-gasoline-2-strokers, as they share only the way the air
> comes in (fuel is injected, well, it's a diesel, and there are exhaust
> valves), so emissions shouldn't be too different from 4-stroke engines.
>
> Having said that, I don't think there really are many big (I mean really
> big) 4-stroke engines.
Actually, there are large four-stroke Diesel generator sets. I work in
the
railroad industry and we have two primary manufacturers of locomotives.
Electromotive (General Motors) uses two stroke Diesel engines and General
Electric uses four stroke Diesel engines. Both manufacturers are
producing
locomotives in the 4400 Hp range and are developing 6000 Hp units.
Corey Wills
Subject: Re: 4 cycle vs. 2 cycle engine
From: Corey Wills
Date: Thu, 26 Sep 1996 15:33:48 GMT
Gerhild Kirchweger wrote:
>
> Well, as far as I know, all bigger diesel engines (for generators as you
> mentioned, but also for those bigger ships) are two-stroke diesels, simply
> because of fuel efficiency (you get power out each turn, not just every
> other).
>
> Reliability is maintained by small rpm, and operating the engines at loads
> that are fair below design borders (compared to small engines).
> Of course 2-stroke engines get hotter and have to stand the huge
> combustion forces every turn, but then again, you don't make an "empty"
> (i.e. without power-output) turn for each "full" one.
>
> You really can't compare big diesel-2-stroke engines to small
> (motorbike-type)-gasoline-2-strokers, as they share only the way the air
> comes in (fuel is injected, well, it's a diesel, and there are exhaust
> valves), so emissions shouldn't be too different from 4-stroke engines.
>
> Having said that, I don't think there really are many big (I mean really
> big) 4-stroke engines.
Actually, there are large four-stroke Diesel generator sets. I work in
the
railroad industry and we have two primary manufacturers of locomotives.
Electromotive (General Motors) uses two stroke Diesel engines and General
Electric uses four stroke Diesel engines. Both manufacturers are
producing
locomotives in the 4400 Hp range and are developing 6000 Hp units.
Corey Wills
Subject: Re: Combustion efficiency based on ambient temperatures?
From: kirchweg@sztms.tu-graz.ac.at (Gerhild Kirchweger)
Date: 26 Sep 1996 14:28:09 GMT
Will Stewart (wstewart@patriot.net) wrote:
: > Not quite. Efficiency is also affeted by the backpressure of the
: > exhaust system, and that is a function of ambient temperature (among
: > other things).
Yeah, if you're really counting peanuts. Pressure difference between exhaust
gas at outlet valve open and ambient tends to be huge compared to what
pressure difference you get by variations of ambient temperature. At least
for four-stroke engines.
Two-strokes is quite another problem.
: Power and efficiency are not one and the same. Warmer air is less
: dense; an ICE operating in warmer air uses less fuel.
Well, yes. But you have less air in the cylinder to burn it, so you get out
less power. Less fuel divided by less power makes not necesarily higher
efficiency.
: > >: Most people have noticed that their car has more pep when it's cold out
: > >: than when it's hot out.
: > >
: > >That's because the air is denser and you get a greater mass of air and
: > >gasoline in the cylinders for the same volume (displacement).
: >
: > In any case, you get better efficiency at colder temps, once the engine
: > is warmed up.
:
: Still wrong. Talk to an engineering professional.
The crucial part for power increase is the intake pressure (influenced by
ambient temperature, but you get a much bigger effect by going up some nice
mountain roads).
The theoretical effficiency is better at lower intake temperature, though
I doubt that you'd really notice the effect at all.
--
oo_oo_oo_oo_oo_oo_oo_oo_oo Gerhild Kirchweger oo_oo_oo_oo_oo_oo_oo_oo_oo
|____ Department of Internal Combustion Engines and Thermodynamics ____|
|__ Graz University of Technology/ Austria ___Tel. ++43 316 873 7212___|
oo_oo_oo_o http://fvkma.tu-graz.ac.at/~gerhild/gerhild.html _oo_oo_oo_oo
Subject: Re: Engineers most in demand are those that are happy in their jobs
From: STEVEN BARTHAKUR
Date: Thu, 26 Sep 1996 10:51:33 -0700
Yan Seiner, PE wrote:
> I spent 8 years working for a wonderful company. I helped it grow from
> a startup to a major regional competitor. Then the owners sold it. I
> left.
Usually, this type of experience with the proper stock options could make one
slightly rich during the cash out period. Afterwards, one can start his own
venture with the capital gain.
> I went to work for another small company. After two years of effort, I
> was no further ahead than when I started. I said HECK WITH IT!! and
> went out on my own.
>
> It's been great so far.
>
> Look to what you want to do, learn it, and do it. Money is just a small
> part of the equation. If you're good at engineering, and don't
> concentrate on some arcane and miniscule portion of our field, you won't
> starve. You won't get rich either, though.
My advice to newcomers is to keep the eye on the money or else one may get
exploited by some business sharks who'll convince the newbee that working for
peanuts is an indication of dedication and prestige.
Apparently, one the reasons why scientists and engineers are the least
compensated among the elite professions is that we tend to lack the business
savvy and focus only on the technical aspects of a job.
-S.B.
Subject: Re: Comma versus decimal point
From: jeffe@blue.seas.upenn.edu (george)
Date: 25 Sep 1996 19:02:54 GMT
this was rhetorical..
:> Why is a 1000 g-m/s *one* Newton, not 1000?
:>
:
:Because, for various reasons (most of them good), the base unit of mass
:in the SI system is the kilogram, not the gram. (They didn't think
:about this when they named the units 200 years ago, though.)
excuse me, when they dreamed up the be all end all most elegant system
forever they *didn't think*?
--
george
george@mech.seas.upenn.edu
Subject: Re: CAN-Tor MECHANICAL DESIGN ENGINEER
From: Pro Tec Technical Services
Date: Thu, 26 Sep 1996 11:34:56 -0700
Pro Tec Technical Services wrote:
>
> ___________________________________________________________________________________
>
> PRO TEC TECHNICAL SERVICES
> _____________________________________________________________________________________
>
> A Global Engineering and Technical Employment Service, headquartered in Toronto, Ontario, Canada
> MECHANICAL DESIGN ENGINEER
The ideal candidate will be a graduate Mechanical Engineer with course
work in machine design and stress analysis, with 3-5 years experience in
manufacturing practices and processes used in the production of large
rotating equipment. The candidate must have working knowledge in the use
of computer based analytical tools including electromagnetic Finite
Element Analysis as well as strong interpersonal skills and ability to
work effectively in a team environment . The candidate must be proficient
in the use of PC based computer software.The position is permanent. The
location is East Toronto. Salary will be very competitive.
Please reply in Confidence to Mike Slimkowich, Pro Tec Technical
Services, 2255 Sheppard Avenue E., Suite
W-414, Willowdale, Ontario, CAN, M2J 4Y1.
E-mail: protec@dis.on.ca Fax: (416) 496-8729
Web: http://www.dis.on.ca/ProTec Telephone: (416)
496-8595
Subject: Re: water level measurement in pressure tank
From: ferguson
Date: 26 Sep 1996 17:50:53 GMT
martin@faceng.anu.edu.au (Martin Stonebridge) wrote:
>In article 488F@sckcen.be, Jan Verstricht writes:
>> To measure the water level in a pressure vessel (vertical cylinder,
>> volume about 30 l), I'm thinking of sensor based on magneto-striction
>> (stroke length of 50 cm to 1 m, still to design).
>> Such sensors are however quite costly (some $3000).
>>
>> Can other principles also be applied (e.g. ultrasonic) in these
>> conditions?
>>
>> Accuracy should be better than 5 mm, temperature is ambient (20 °C).
>>
>> Any suggestions are very welcome,
>>
>> Jan Verstricht.
>
>
>I recently designed a tank level sensor and used a cheap little plastic
>moulded differential pressure transducer to do the job (AuD$50). The
>tank was vented to atmoshere so I measured level by connecting a hose
>from a probe in the base to only one side of the differential
>transducer and left the other side vented to atmoshere. The signal out
>was directly proportional to height. In a pressure vessel it would be
>necessary to connect a hose from the bottom of the tank to one side and
>from the top of the tank to the other side of the pressure transducer.
>this would ensure that the difference is solely due to the head of
>water. You could arrange to keep the transducer in the tank or outside
>the tank as long as the top area and submerged areas are vented
>correctly. One more thing try and keep corrosion out by priming the
>hoses to the transducer with deionised water on the initial
>installation - it will take ages before any sedimentation ever clogs up
>your sensor and at $50 who cares?
>
>- martin
I'd like to know what kind of accuracy you would expect from a $50 differential
pressure transducer! I'm currently working on a level application for an oil tank
farm, and the pressure transducers are worth $1500 each. The accuracy is 0.075% of
range. If there's something better (and cheaper) out there, I'd love to hear about
it.
A further complication is oil temperature compensation. The oil temp varies from 20
degrees C to 70 degrees C on a regular basis. The trick is to come up with a level
reading that corresponds to some reference temp(e.g. 21 degrees C), regardless of
what the oil temperature is.