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991  Using Arduino / Motors, Mechanics, and Power / Re: Stepper clarification on: July 18, 2011, 08:38:19 am
Providing your stepper motor is working within its specified torque range then feeding it with "X" step pulses should make it rotate by "X" incremental degrees.  However, you need to be able to establish where it is when you power up.  There are a couple of simple ways, and the simplest of all is to drive the stepper until it triggers a switch of some sort, be it mechanical or optical.  This establishes a reference point from which you can then move the stepper to all other "known" positions.   Your software can periodically re-establish this reference to give confidence.  With a feedback pot you "know" where the stepper is at all times but the resolution of such a system can be no better than one incremental movement so even a feedback pot does not give an exact position, and you may induce hunting as the stepper motor endeavours to establish an exact position.

Servos on the other-hand have a built in feedback control but their power and resolution are feeble compared to the power and capability of a stepper.
992  Using Arduino / Project Guidance / Re: Piezoelectric Power Yield on: July 16, 2011, 12:56:22 pm
This is all getting rather silly !

Wikipedia etc contains lots of useful information on the principles of piezo voltage generation

The voltage generated by a piezo crystal bears no relationship to its cross section.  If you want more volts you must "stack" elements
A piezo element is a "charge" type device in that whilst it is capable of producing voltage, its impedance is so high that it effectively cannot produce current.

Whilst it is only practicable when considered as sensing a varying force, the frequency of this may be extremely low, typically less than 0.5hz

Whilst this all might well be a personal exercise of the grey matter, you have to temper dreaming up conceptual ideas with realism.  If you think about it, an ROV powered by the depth of water that it's sailing in is effectively a perpetual motion machine.

Apart from that, where does the Arduino aspect fit in.
993  Using Arduino / Project Guidance / Re: Piezoelectric Power Yield on: July 16, 2011, 05:21:19 am
Since you're dealing with static pressure (assuming the ROV isn't bobbing up and down at many times per second) the output will be DC and an uneducated guess will be somewhere in the region of milliwatts at best.  The available force will be a direct function of depth (pressure) and cross sectional area of the transducer.  Depth pressure increases by approximately 1 bar per 10 metres increase in depth. 

Yes I know a spark igniter produces thousands of volts but the instantaneous force applied on these is many 10s of newtons.

If it's useful power you require then use a battery (stronger, cheaper, simpler and more reliable)

994  Using Arduino / General Electronics / Re: Choosing a PC based oscilloscope on: July 16, 2011, 03:07:43 am
You need to specify what type/speed, number of channels of signals you want to look at before asking what to buy.
PICOTECH.COM make some pretty good devices in all range of specs and prices.  Even their very cheapest DAQ units offer quite good oscilloscope features.
995  Using Arduino / Motors, Mechanics, and Power / Re: how to make motors spin different ways with arduino on: July 13, 2011, 02:52:32 am
If they are DC motors and you wish them all to operate simultaneously (at the same time) then irrespective of whatever you may need to supply both control and power, it is simply a case of wiring two with normal polarity (motor +ve to power +ve, motor -ve to power -ve) and wiring the other pair with reverse polarity (motor +ve to power -ve, motor -ve to power +ve)
996  Using Arduino / Project Guidance / Re: P_ower supply question on: July 11, 2011, 01:26:40 pm
You might want to ensure that ALL relevant connections are made as close to, and preferably onto the actual motor cycle battery terminals.  The electrical noise introduced from the ignition coil will be buzzing merrily throughout the cable loom so you need to get as close to the battery as possible, where the effect is minimised.  I'd also suggest mounting everything within a diecast metal enclosure which is itself "earthed" to the bike frame.  This will act as a shield to prevent RF affecting the circuit.  Run all power and signal leads within screened cable, with the screening connected to the bike frame.  The use of ridiculously sized capacitors is totally unnecessary providing you take sensible screening precautions.
997  Using Arduino / Motors, Mechanics, and Power / Re: Other ways of controlling water flow on: July 11, 2011, 08:12:45 am
Plaster of Paris ?

Go to your local chemist or pharmacy - they should be able to either supply or source it for you - typically used to set broken limbs.

998  Using Arduino / Motors, Mechanics, and Power / Re: Other ways of controlling water flow on: July 10, 2011, 03:24:25 am
Why not simply visit your local junk yard and strip some valves out of scrap washing machines, dishwashers etc

They will typically be mains AC operated so you'll also need a bunch of interfacing devices, whether they be relays, SSRs or triacs.
999  Using Arduino / Motors, Mechanics, and Power / Re: Capability of Lead Acid Battery on: July 06, 2011, 12:51:45 pm
"burglar alarm" type batteries are generally SLA type (Sealed Lead Acid) so by definition are filled with an electrolyte (jelly ?) during manufacture, which bears no relationship to when the retail outlet actually sells them.

Wet lead acid are of course a totally different matter, but they are not the topic of discussion.
1000  Using Arduino / Motors, Mechanics, and Power / Re: Capability of Lead Acid Battery on: July 06, 2011, 03:16:45 am
You should be very concerned about how old the battery was when you bought it.  Somewhere on the case will be stamped a date code, probably 4 numbers (month -year).  If it's more than 2 years old then I'd return it for a refund irrespective of whether it's charged or not.  Normal consumers stores are not set up to periodically refresh lead acid batteries so it's probably never been charged since the manufacture gave it an initial charge.

If you really want to test its capacity then you can do a rough and ready test by placing a resistor across it and monitoring/logging the voltage every so often until you start to see a rapid drop off (at around 11 volts)   You need of course to ensure the battery is fully charged before you start the discharge test.  On the basis of 7AH nominal capacity and using C20 as a conservative guess then you need a resistor that will consume 7/20 amps.   R=V/I  so R= 12*20/7  =  approx 34ohms.  33 ohms is a preferred value so you need one of these rated at 7 watts minimum.

Simply strap this resistor across the battery and be prepared to monitor the voltage at 1/2 hour intervals for the next 20 hours !!    True, the current will drop as the voltage decreases but it is a method of approximation only.  You can then produce a nice graph of volts versus time at your nominal 0.35 amps and from that estimate total ampere-hours.

An automatic datalogger and a constant current device helps greatly but, as above, it can be done manually, albeit it takes a bit of time and effort.
1001  Using Arduino / General Electronics / Re: Need some help!! on: July 05, 2011, 08:52:13 am
led plus resistor across 5 volt supply must glow unless :-

a) no 5 volts
b) resistor defective
c) led defective
d) led connected wrong way round
e) you just dreamt it
f) you just went blind
1002  Using Arduino / General Electronics / Re: Controlling 12V @ 12A with an arduino. on: July 03, 2011, 01:20:07 pm
If you care to read my earlier posting you will see I was complimenting the relay design as being short-circuit proof.  In fact I even went to the bother of giving you a mention in relation to the original emitter follower circuit.   My concern was regarding a home-made FET system to perform change-over switching by an individual not yet experienced in the problems of FET circuit design.
1003  Using Arduino / Motors, Mechanics, and Power / Re: what is this part called?(shematic from playground) on: July 03, 2011, 03:59:31 am
and that's why they are often referred to as "flywheel" diodes
1004  Using Arduino / General Electronics / Re: Controlling 12V @ 12A with an arduino. on: July 03, 2011, 03:53:58 am
You have a problem here.  Your original design has a reversing facility and a simple low side N channel will not provide that. You are looking to switch both + and - lines and there is now a real danger of a DIY type FET build shortening out your power supply.   If you want to go FET and you are not knowledgeable in such matters you might be better simply buying an H bridge which will provide both forward and reverse and will not short circuit the power supply.
1005  Using Arduino / Project Guidance / Re: Arduino Relays for Failsafes on: July 02, 2011, 09:15:51 am
Just out of interest, how do you propose to handle the eventuality of your failure detector system failing.  Laugh if you want but these sort of problems are typical of what Emergency Shutdown Systems designers have to handle.   The simplest way of doing it is by having multiple processors running the application in parallel then voting on a 2-out-of-3 basis.   As far as your application is concerned, whatever it may be, a simple watch-dog output is all you need - that's what it's designed for.
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