robotic wheel chair help

dear sir

               i have almost completed the code. I need u to check it once. if u provide me your email id I will send the code and the circuit diagrams so that u can check it.

The ESP article is OK as far as it goes but it's really only concerned with simple mains power supplies. It has a tiny section at the end about switching supplies but it's way out of date.

The wheelchair motors are likely to be driven from lead acid batteries (although the OP doesn't actually say this) which have a very low source impedance and no ripple issues. The capacitors across the motor drive are needed to ensure that the AC impedance of the supply at PWM frequenices is low. The steady state voltage drop will be function of battery and the wires and no (reasonable) amount of capacitor will make any difference.

Let's look at that 1000uF per amp rule:

A reasonable target would be that the ripple at PWM frequency should be less than 250mV for lowish PWM frequencies (<50kHz).  The peak ripple voltage will be approx. current/(C *2 * f) so for 1A, f = 1/(c * 2 * 0.25) = 2000Hz

The ESR of the capacitor should be < 50mR (for 50mV drop at 1A)

At higher frequencies the ESR demand is unchanged but the capacitor can be smaller. So if the H bridge PWM frequency is 50kHz then 40uF per motor amp is OK. (The ESR demand is unchanged by frequency)

For a 10A motor 470uF will do.

In practice it won't be easy to get 470uF rated at 25V and 10A ripple with an ESR of less than 5mR - the best listed in Farnell has an ESR of 62mR and costs about £0.7

The best 10000 uF capacitor listed is £17 each and still only 24mR ESR.

So for this very hypothetical design 10 x 470uF at 25V, carefully chosen for low ESR would be much better than 10,000 uF in one big lump.

In fact the 10,000 uF would only be better at quite low PWM frequencies.

So the point of all this ?

You need to take all the factors of the application into account and think hard about the characteristics of the parts you can actually buy. Without knowing more about the OPs power source and motor control system it isn't possible to give good advice.

MK

Michael

Your explanation is great and shows the difference between low frequency (50Hz) type supplies and High frequency supplies that require the ESR to be taken into account.

I've always used the ballpack figures of 1000uF/Amp for 50hz type supplies.

I've fixed a few switch modes in monitors, but not designed any.

So thanks for the insight and figures.

What I find most interesting is what should be a project for learning and teaching upcoming engineers, is instead a 'lets get on the internet to see who can give us the answers'.

This approach doesn't give me confidence in our upcoming engineers, since they seem to rely on someone else telling them the answers.

All the theory and answers are out there, you just need to do some looking and reading.

The end result will be far better even if you don't accomplish the goal .... you will have learned.

If you get stuck along the way, discuss the problem and where you're stuck, and someone will assist or hint at the direction you should take.

Mark

Victor, you might want to read this article... http://sound.westhost.com/power-supplies.htm

While this is a very good article, and is easy to follow, it is related to designing Power Supplies for Audio applications.

It touches on SMS but doesn't really give the sort of advice that Michael has.

allowed us to reach a thumb-rule we apply successfully to our drives.

Funnily enough, this same rule of thumb (1000uF/amp) was around 35 years ago when I started electronics.

Do you have some further info that you could share, or perhaps some performance v economics figures that support why you use it.?

Do your drives have to meet any EMC regulations, and does the 'rule of thumb' apply then.?

Thanks

Mark

how can i control the speed of dc motors using the same joystick...pls help me...

Hi Prajwal,

This is supposed to be your project not ours. If you keep asking questions and expecting answers, you will never learn how to be an engineer. That is why you are training, isn't it?

I'm sorry to post such a negative comment and I don't mean any offence but you need to do your own research. You have been told how analogue joysticks work and you've been told about motor drivers. You need to work out the bit in the middle!

The internet is a wonderful resource of code and hardware examples: use this help to get a value from your joystick. Find a motor driver with the type of input you understand - it might be analogue, digital or PWM. Then find out how to convert your joystick value to the type of input required.

We all want you to become a good engineer and that requires hard work on your part - no shortcuts I'm afraid.

Best wishes,

Alan

Thank you for that explanation, Michael. This should reduce the cost of driving servomotors substantially...

You're welcome to send me the code, Prajwal. I'll comment in your code and send it back to you. But I hope you're keeping Alan Wood's advice in mind...

Send mail to rajiv.tctech AT gmail.com

The left right movement of the joystick can be used to steer the wheelchair by differentially driving the wheels. The Y-axis of the joystick, or say the forward and back motion of the joystick can be used to vary speed and direction - pulling the joystick back from the centre-point could make the motors reverse direction. But in all this, do remember to keep a deadband around the centre-point of the joystick, wherein the values do not do anything. Otherwise you might land up with a wheelchair that keeps creeping even without any control input...

ofcourse sir I learnt a lot with my project... thank you all...