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631  Using Arduino / Project Guidance / Re: turning on 4 1000w flood lights at once with arduino on: May 23, 2012, 10:33:56 am
Fencing wire's a bad choice  -been there and tried it.  Mind you the flash-bang-wallop was quite impressive but it made one hellava mess of the DB board.  All very nicely plated in a mixture of steel and zinc.

Bear in mind that 4kW of lights will demand quite a bit more during the warm up period since the cold resistance is very much less than the hot.

Also trying to switch any form of hot-wire lamp on for only 1/2 second repeatidly is a sure way of keeping the lamp manufacturers in business.

If you only want a short duration, high intensity flash then go for a strobe type device (flash gun) which uses a gas discharge tube.  These can produce very bright light and are designed for continuous flashing.   Although the discharge is over in a matter of milliseconds, the persisence of the human eye makes it look much longer.
632  Using Arduino / General Electronics / Re: battery serie and parallel on: May 23, 2012, 10:24:14 am
Wrong!  The current is determined by the resistance of the load and the voltage delivered to it (if you ignore the internal resistance of the battery)
633  Using Arduino / General Electronics / Re: battery serie and parallel on: May 23, 2012, 03:37:09 am
You don't need any of the diodes.  If you redraw the circuit to its simplest configuration (2 diodes removed as already suggested and the other 2 replaced by solid links) you will find that both batteries are in fact in series with one lamp across one battery and the other across both.   
634  Using Arduino / Project Guidance / Re: Easiest way to control the speed of a motor? on: May 18, 2012, 02:29:17 am
www.4qd.co.uk will astound you with the amount of information they provide on motor control.  As an aside they also have a suitable controller for your application.  Using a 5 volt drive from your arduino it'll give you all you need plus regen braking.   I've no commercial interest in the company, just a very happy customer.
635  Using Arduino / General Electronics / Re: Really cheap Fog machine! on: May 14, 2012, 09:52:18 am
http://www.howcast.com/videos/262687-How-To-Make-a-Fog-Machine-With-a-Household-Iron

Complete guide to building a DIY cheap and nasty machine


Jack
636  Using Arduino / Sensors / Re: 4-20 ma sensor reading on: May 13, 2012, 12:32:12 pm
Post a diagram of what is connected where - that way you will get a definitive answer, rather than guesses as to your problem. 
Most (industrial standard) 4-20 units require at least 18 volts (preferably 24) to drive them properly. 
Even if your problem is a software issue it would be beneficial to see your circuit hook-up diagram.
637  Using Arduino / Motors, Mechanics, and Power / Re: How much weight can my 270oz-in motor lift? on: May 06, 2012, 03:35:22 am
You can negate the effect of arm weight for a rotational system if you instal a balancing weight to the other side of the shaft.  By that means there is an equal and opposite gravitational force acting to assist in pulling up the original weight.  On the basis that you want to lift say 4kg on a 1 metre arm (4kgM) your balance weight can be any mathematical combination that achieves the same torque  eg 8kg on a 0.5M arm or 16kg on a 0.25M arm etc.

You also need to take into account the point at which maximum torque takes place.  Your 4.25kg 1 metre long weight actually requires 2.125kgM of torque when horizontal (the centre of gravity being 1/2-way along the arm). 

Note that weight units under the SI system are actually Newtons and are 9.81 times larger than mass (weight = mass x acceleration due to gravity)   So your 4.25kg mass actually weighs in at 41.7Newtons 

Torque under the SI system is invariably specified as Newton Metres (NM)

Note that this simple balance rule only applies to relatively slow motion since, if speed is involved the effets of dynamic and inertial forces must also be taken into consideration.
638  Using Arduino / Sensors / Re: any sensor that sense human... on: May 05, 2012, 03:40:34 pm
First thing you need to do is define what you mean by "a human", since you cannot sense what you cannot define.  

Then you need to determine what features of your defined human that you want to use as your detection criteria.

For example   :----

Human = biological mass with generally 4 flexing appendices with animation features when live but none when inert.  

Some humans have less than 4 appendices but are still defined as humans.

Live humans operate with a mass temperature of somewhere around 34 degrees centigrade but this may be obscured by their propensity to wear thermal shielding.

Most operate in an erect manner using the lower two appendices,

They vary in height from less than 1 metre to somewhere in excess of 2 metres.

Most emit audible sounds and solid and liquid waste products

Generally considered to function in a rational manner but that is only the opinion of a selected group

etc etc etc

So what features of the human body do you wish to use as your detection criteria ?

Note that a PIR sensor is unlikely to detect the movement of a body operating at ambient temperature - they generally detect the infrared emission of a moving body - which is a pretty good way of detecting a human.   You can get models which are also mass sensitive which will ignore warm bodies less than 20kg (dogs, cats, small children etc)

639  Using Arduino / General Electronics / Re: Coarse and fine adjustment knobs on: May 04, 2012, 05:55:36 am
10k for the coarse with 1 k for the fine.  The 1k is wired as a 2-terminal rheostat (wiper connected to one of the other two terminals) and in series with the supply to the upper terminal of the 10k. The lower terminal of the 10k goes to ground and the variable sense voltage comes from the 10k wiper terminal.  Of course you cannot use such an arrangement to supply "power"; only extremely low currents or as a sense voltage to your regulator system.
640  Using Arduino / Project Guidance / Re: PD of PID in a balancing robot on: April 26, 2012, 05:22:21 am
You have in effect an upside down pendulum since all the weight is above the centre of rotation.

Rather than a piece of wood below the axle (centre of rotation) might I suggest a lead or steel weight such that the robot is almost self-balanced.  Then, once you have your algoritms working you can start reducing the weight.  It's like teaching a child how to walk,  small steps first then as things progress you can get more ambitious.

By the way your comment that fast movements are not reacted to by the motor means you need D factor.  D stands for Differential NOT damping.  Differential has exactly the opposite effect to that of damping - it actually speeds up reaction to rapid errors.  If you want to introduce damping then you need to think about adjusting gain (1/P)  Less gain (larger values of P) = more damping

Can you also identify the sensor in your photograph and which axis of the sensor relates to the rotation about the axle.

A copy of your circuit diagram would also be beneficial.
641  Using Arduino / Project Guidance / Re: PD of PID in a balancing robot on: April 23, 2012, 01:34:03 pm
Can you send a picture of your robot so that we know what we are discussing.

I presume your robot is capable of 2-axis  (X and Y) movement (back-to-front and side-to-side), therefore you must have at least 2 motors to perform correction.  Your controller will also have to run two equation sets, one for each motor, each of which will be a PID control.

Based on the robot being a humanoid shape, may I suggest the following :

Tie a cord via a long spring from the top of the robot's head and pass this over a pulley (vertically above the robot) then route it over a second pulley (clear of the robot) and to the free end of the cord attach a weight sufficient to support the robot upright.  With the robot upright the spring should be slightly extended.

If the robot now shows an inclination to fall the spring will extend sufficiently to prevent a complete fall.

Now, with "D" and "I" turned off,( D = 0  I = infinity) adjust the gain "P" such that the robot is capable of holding itself up.

To test the control give the robot a gentle nudge and it should try to get back to a vertical position.  You will need to perform this for both "X" and "Y" directions
 
It may not be fully upright but there must be a tendency to get there.  If you cannot achieve this then your sensors are supplying insufficient or incorrect data to your controller to achieve an assisted balance.  If you cannot achieve assisted balance, you will never achieve auto-balance.

If you set the gain too high the robot will oscillate.  If too low the robot will fail to hold itself upright.  The optimum level of P is that where it holds itself upright but is just off the point of oscillation.


Once you have P set there will (may) be a slight error and a little bit of "I" should correct this. I'd guess you are probably looking at say I = 5 seconds

Now you need to set some "D" component to compensate for rapid movement so you might want around D = 0.05 seconds

You will of course have to adjust the values to suit the application and please be aware that adjusting any one of the PID factors will/ may require adjustment of the other two to maintain stability.

Lots of tweaking but hopefully a guide to a way ahead.
642  Using Arduino / Project Guidance / Re: PD of PID in a balancing robot on: April 22, 2012, 01:40:18 pm
Why are you not using any I term (integral)

Basically the three terms of PID are :

P  Proportional which produces a correction which is proportional to the deviation from set-point
I   Intergral which produces a correction based on how long the measured variable has deviated from the set-point
D  Differential which produces a correction based upon the speed with which the deviation is going away from the set-point

So for a balancing system you will need a small integral (I) to correct any tendency to "lean"
You will need a fair amount of differential (D) to compensate for fast movement (the more your robot leans the faster it will tend to fall)
You will need a fair amount to proportional (P) so that correction is capable of reacting fast enough.

Ideally you should be looking at a balance stability of <5 degrees (not the 30 being mentioned) so the non-linear COS function (not SIN) is minimised.  When you think about it, the human body finds it awkward to walk along a 5 degree slope but extremely difficult to do likewise along a 30 degree slope.  So expecting a robot to do what a human finds difficult is really making things hard for yourself.
643  Using Arduino / Project Guidance / Re: detect 240v or Hall sensor or light sensor? on: April 21, 2012, 06:39:48 am
Train inhabitants to turn light off when not in use - cheap, safe and ups your claim to be the alpha dog
644  Using Arduino / Sensors / Re: pressure sensor reading on: April 19, 2012, 04:30:12 pm
With a transmitter span of -500 to +500 pascal (1000 units) you are endeavouring to operate around a set point of -10 which is only 1% of span.  Presumably your desired control range is somewhere around 0 to -20 pascal.   Since you are operating over such a narrowband of the transmitter span you appear to be attempting the to achieve the impossible.  In effect every 1 pascal represents 1 point of the 1024 input resolution, so at best you are looking ast a nominal range of only 20 points in the 1024 available.   Rescaling the signal or using an amplifier with lots of offset will increase the number of points available but it will not increase accuracy and may introduce lots of noise.  It is a common misconception that you can increase accuracy by rescaling.  You can never increase the accuracy of a measured variable by rescaling since accuracy is defined (controlled) by the primary transmitter and the portion of span being used. All that rescaling does is give you more numbers to look at.

I'd suggest you find a sensor with a range better suited to the application, say -20 to +20 pascal.  You need a +range to cover the eventuality of the exhaust fan being inoperable.
645  Using Arduino / Project Guidance / Re: Finding Items in a building with lots of rooms on: April 18, 2012, 04:34:31 pm
Looking at it as a logic problem, you have several demands that must be addressed

1) each item must be uniquely identified (bar code, RF identification, frequency etc)
2) every possible location must be uniquely identified ( ditto)
3) when an item enters a location, the location must be capable of identifying it's stock update
4) if any new (extra) items enter a location a stock update must be identified
5) if any items are removed from locations a stock update must take place

Hence every item is identified as being in particular locations and hence any item can be immediately located by reading from the item/location set

I bel;ieve Amazon use such a system for stock location as no items have dedicated locations, they are simply placed on "random" shelves but each and every time items are placed, their location is "logged" so that they can be readily recovered.  I suppose you could call it "fuzzy filing"

All of the above requires communication, be it written, wired or wireless  -  none of which comes cheap - apart from a paper system  Technnoligy ain't all it's cracked up to be !!!
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