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From: Vladimir Vassilevsky on 30 Apr 2010 11:55


Brandon wrote:

> Thanks for the responses. Lots to think about. I have a controls
> background, but most of my work has been on the algorithmic/academic
> end of the spectrum so I don't really know much about technology
> unfortunately.

FYI: engineers don't use democratic words like "big", "little",
"better", "worse", "decent", "reasonable", "pretty". Engineers use
numbers. Before you have the numbers, there is nothing to discuss.

VLV




> I guess I can share a few more specifics about what I'd like to
> accomplish. It's a hand tool for an artist friend, but it has a
> potentially niche-sized market if it's an decent improvement over what
> he's using now, so I'm thinking in terms of production.
>
> Minimum requirements:
> 1) Pretty good frequency control. Any oscillation not noticeable to
> the user.
> 2) Very precise control of one end point of the stroke and reasonable
> control on the other end.
> 3) Little if any noticeable off-axis vibration.




> Really nice things to have that would be probably be necessary for
> commercial success:
> A) Low enough power drain to operate off a small battery (3 cm^3
> max???) for a decent about of time. Sorry, I don't have more
> specifics on that yet.
> B) Production costs for accuator/sensor/electronics of $200 at the
> very most. Probably needs to be less than that unless this turns out
> to be so great that people are willing to pay substantially more than
> the current technology costs.
>
> Other things that would be cool but aren't absolutely necessary:
> a) Shaping of end effector trajectory. Lower velocity on out stroke
> then on return stroke.
> b) Quick adjustment of stroke length.
>
> Force requirements: Pretty small I think. The reciprocating tool is
> very light, on the order of a couple paper clips maybe and it doesn't
> have to apply much force to the material you're working on. Force
> requirements are probably dominated by the friction needed to hold the
> tool in place.
>
> That's all I can think of right now.
>
> (2) and (a) make me want to use a cam. The potential problems I see
> with this are that cutting an arbitrary cam profile on something small
> is not that easy, so (a) may not be that achievable. Expense is also
> an issue. Small motors are pricey and even a 16 line encoder that
> small seems to cost as much as the motor. Easily over $100 each total
> right there which is pushing the limits of what this thing can cost.
> Accuracy and settling time of speed control is not going to be as good
> as I would have hoped for, but it's probably acceptable. Power
> drain? I don't know if a solenoid would be any better, but that voice
> coil has me intrigued. (1), (3), (a), (b), and maybe (B) and (A) are
> what got me thinking about a solenoid, provided that (2) could be
> achieved via some combination of physical mechanism and feedback
> control.
From: Tim Wescott on 30 Apr 2010 12:59
Vladimir Vassilevsky wrote:
>
>
> Brandon wrote:
>
>> Thanks for the responses. Lots to think about. I have a controls
>> background, but most of my work has been on the algorithmic/academic
>> end of the spectrum so I don't really know much about technology
>> unfortunately.
>
> FYI: engineers don't use democratic words like "big", "little",
> "better", "worse", "decent", "reasonable", "pretty". Engineers use
> numbers. Before you have the numbers, there is nothing to discuss.
>
> VLV

I respectfully disagree. In the early stages of a project we most
certainly do -- and it's never long before we realize that we need to
define them in the context of that project, before all go astray.

In the end those smoky words are the 'real' specifications that you are
working towards -- you can hit all the numbers on the nose, but if those
numbers don't look decent reasonable and pretty to the customer, if the
value of the product you engineer isn't better, then your product will
make little money, whoever hired you will thing they could have done
worse than to never start things, and everyone will have big problems.

But yea -- you don't want them hanging around for very long at all
before you pin them down.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Tim Wescott on 30 Apr 2010 13:12
Brandon wrote:

(context restored -- this is USENET)

Brandon wrote:
> I have a project in which I would like to produce a "smooth"
> reciprocating motion with an adjustable stroke length between about 1
> mm and 2 mm and an adjustable frequency between 100 Hz and 150 Hz. I
> think there might be some advantages to using a solenoid for this (as
> opposed to a rotary motor and a mechanical design) but only if I can
> get good feedback control of the stroke and frequency. In a spring-
> mass mass system frequency can be controlled open-loop, but I'm not
> sure how well I could regulate the end and start position of each
> stroke or even what sort of sensor might be appropriate and have a
> fast enough response and a clean enough signal to work at that
> frequency. Wondering if anyone has any good electronic (or
> mechatronic) ideas as to how this might be done.

>> snip <<

This sort of question always boils down to questions like how much money
you have to spend, whether you're using custom electronics, what sort of
production volumes you're contemplating, how precise it needs to be, how
long it can take to achieve precision, what sort of environment it needs
to work in, how heavy it can be, etc.

_The_ indicated position sensor for this would be a short-stroke LVDT.
It may be hard to get the accuracy you want from it at 100Hz without

>> snip <<

(end restored context)

> Thanks for the responses. Lots to think about. I have a controls
> background, but most of my work has been on the algorithmic/academic
> end of the spectrum so I don't really know much about technology
> unfortunately.
>
> I guess I can share a few more specifics about what I'd like to
> accomplish. It's a hand tool for an artist friend, but it has a
> potentially niche-sized market if it's an decent improvement over what
> he's using now, so I'm thinking in terms of production.
>
> Minimum requirements:
> 1) Pretty good frequency control. Any oscillation not noticeable to
> the user.
> 2) Very precise control of one end point of the stroke and reasonable
> control on the other end.
> 3) Little if any noticeable off-axis vibration.
>
> Really nice things to have that would be probably be necessary for
> commercial success:
> A) Low enough power drain to operate off a small battery (3 cm^3
> max???) for a decent about of time. Sorry, I don't have more
> specifics on that yet.
> B) Production costs for accuator/sensor/electronics of $200 at the
> very most. Probably needs to be less than that unless this turns out
> to be so great that people are willing to pay substantially more than
> the current technology costs.
>
> Other things that would be cool but aren't absolutely necessary:
> a) Shaping of end effector trajectory. Lower velocity on out stroke
> then on return stroke.
> b) Quick adjustment of stroke length.
>
> Force requirements: Pretty small I think. The reciprocating tool is
> very light, on the order of a couple paper clips maybe and it doesn't
> have to apply much force to the material you're working on. Force
> requirements are probably dominated by the friction needed to hold the
> tool in place.
>
> That's all I can think of right now.
>
> (2) and (a) make me want to use a cam. The potential problems I see
> with this are that cutting an arbitrary cam profile on something small
> is not that easy, so (a) may not be that achievable. Expense is also
> an issue. Small motors are pricey and even a 16 line encoder that
> small seems to cost as much as the motor. Easily over $100 each total
> right there which is pushing the limits of what this thing can cost.
> Accuracy and settling time of speed control is not going to be as good
> as I would have hoped for, but it's probably acceptable. Power
> drain? I don't know if a solenoid would be any better, but that voice
> coil has me intrigued. (1), (3), (a), (b), and maybe (B) and (A) are
> what got me thinking about a solenoid, provided that (2) could be
> achieved via some combination of physical mechanism and feedback
> control.

Swiss precision motors are pricey; if you can make a Mabuchi or a
Johnson motor work reliably they'll come pretty cheap. I think that
such a motor, properly used, would last for a good long time. For a
motor/cam assembly all you need is a once-around sensor on the motor --
there are a number of cost-effective ways to do this; the two that leap
to mind are a magnet and a hall-effect sensor, or a slotted disk and an
LED/phototransistor pair.

A speaker coil is probably more efficient by far than a solenoid, and
will probably be lighter as well. Better yet, it's something that you
can fabricate yourself with a bit of wire and some easily obtained
magnets. A motor/cam is going to be a power hog if you really just need
to overcome inertia -- but a speaker coil's power requirements go up
significantly with load, so if you have to actually _push_ on anything
or maintain rigidity, that motor/cam may start looking good.

Of course, if you want to change the motion profile easily, the speaker
coil beats the pants off of the motor/cam.

If this is going to be a hand-held gizmo, then an LED/phototransistor
position sensing arrangement will have no temperature range troubles --
and such a position sensor is something you can probably womp up fairly
cheaply.

You'd have to play with this to find out about power drain, but 25cc is
enough volume for a lithium-polymer battery to hold a significant amount
of energy.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Grant on 30 Apr 2010 18:16
On Fri, 30 Apr 2010 09:59:29 -0700, Tim Wescott <tim(a)seemywebsite.now> wrote:

>Vladimir Vassilevsky wrote:
>>
>>
>> Brandon wrote:
>>
>>> Thanks for the responses. Lots to think about. I have a controls
>>> background, but most of my work has been on the algorithmic/academic
>>> end of the spectrum so I don't really know much about technology
>>> unfortunately.
>>
>> FYI: engineers don't use democratic words like "big", "little",
>> "better", "worse", "decent", "reasonable", "pretty". Engineers use
>> numbers. Before you have the numbers, there is nothing to discuss.
>>
>> VLV
>
>I respectfully disagree. In the early stages of a project we most
>certainly do -- and it's never long before we realize that we need to
>define them in the context of that project, before all go astray.

Good engineering is being able to guesstimate ballpark numbers then
verify once requirements are known. Often the customer has no real
idea what's involved, but they usually know what they want? Unless
you happen to work in a field where the customers also understand the
technology they ask for.
>
>In the end those smoky words are the 'real' specifications that you are
>working towards -- you can hit all the numbers on the nose, but if those
>numbers don't look decent reasonable and pretty to the customer, if the
>value of the product you engineer isn't better, then your product will
>make little money, whoever hired you will thing they could have done
>worse than to never start things, and everyone will have big problems.
>
>But yea -- you don't want them hanging around for very long at all
>before you pin them down.

Isn't the design process just that? Pin down the numbers and make some
thing real from some loose specification. Otherwise the design task may
as well be handled by a technician, as no engineering involved.

But then, lotsa people like to call themselves an engineer these days.

Grant.
--
http://bugs.id.au/
From: Grant on 30 Apr 2010 18:23
On Fri, 30 Apr 2010 08:38:18 -0700 (PDT), Brandon <brandon.joseph.moore(a)gmail.com> wrote:

>Thanks for the responses. Lots to think about. I have a controls
>background, but most of my work has been on the algorithmic/academic
>end of the spectrum so I don't really know much about technology
>unfortunately.
>
>I guess I can share a few more specifics about what I'd like to
>accomplish. It's a hand tool for an artist friend, but it has a
>potentially niche-sized market if it's an decent improvement over what
>he's using now, so I'm thinking in terms of production.
>
>Minimum requirements:
>1) Pretty good frequency control. Any oscillation not noticeable to
>the user.
that's waveform control to coil -- I still think spring loaded slug in
solenoid is practical here, like the old dot matrix pin drivers?
>2) Very precise control of one end point of the stroke and reasonable
>control on the other end.

Optical sensor for limit points? Move the sensor assembly for fine
adjustment.
>3) Little if any noticeable off-axis vibration.
Easy with solenoid
>
>Really nice things to have that would be probably be necessary for
>commercial success:
>A) Low enough power drain to operate off a small battery (3 cm^3
>max???) for a decent about of time. Sorry, I don't have more
>specifics on that yet.

Low power if run at resonance, but that conflicts with variable frequency.

>B) Production costs for accuator/sensor/electronics of $200 at the
>very most. Probably needs to be less than that unless this turns out
>to be so great that people are willing to pay substantially more than
>the current technology costs.
>
>Other things that would be cool but aren't absolutely necessary:
>a) Shaping of end effector trajectory. Lower velocity on out stroke
>then on return stroke.

waveform shaping?
>b) Quick adjustment of stroke length.
adjustable optic sensor position?
>
>Force requirements: Pretty small I think. The reciprocating tool is
>very light, on the order of a couple paper clips maybe and it doesn't
>have to apply much force to the material you're working on. Force
>requirements are probably dominated by the friction needed to hold the
>tool in place.

Like a tattoo gun? Look at old dot matrix print head designs.

Grant.
--
http://bugs.id.au/
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