Noob Question about voltage requirements

[Sylvestre]

Hey guys,

This may sound like a real novice question.  If the axon II's maximum voltage is ten volts how would you power something that requires more voltage such as a 24 volt motor or 5 LEDS that require 3.3 volts each?  Sorry to waster your guys' time, but I never learned this and I'm new to robotics. If someone could help me that'd be great.

Thanks

[Sylvestre]

Also what would I have to do to run two 24 volt batteries with the axon II

[DangerousElectrician]

Put another voltage regulator in front of the axon to regulate the voltage from 24V to 10V, then feed the 10V into the axon. If you are very skilled with a soldering iron, you could replace the voltage regulators on the axon with ones that can handle higher input voltages.

To control a 24 volt motor you'll need relays or transistors (read this http://www.kpsec.freeuk.com/trancirc.htm). If you lack the electrical knowledge to build a motor driver, consider buying a motor driver (here's one http://www.sparkfun.com/products/9668).

Controlling the LEDs will require a transistor or relay. The LEDs can be powered off the 24V directly but will require a current limiting resistor (read this http://www.kpsec.freeuk.com/components/led.htm)

[Sylvestre]

Ok so by putting a voltage regulator on my circuit am I just losing power?  Would I be better off getting two smaller batteries?  Would the motors still get full power from the batteries with voltage regulator or do I lose some of the power from the voltage regulator?  Last question, is it possible that motor driver for that motor driver to control two 12 volt motors?

[corrado33]

Ok so by putting a voltage regulator on my circuit am I just losing power?

A normal voltage regulator converts energy into heat, so yes, you're losing power.  (I may or may not have said that right.)  A switching regulator is much more efficient. 

[Sylvestre]

What is a battery you guys would recommend for the axon mote?  Hawker?

[Soeren]

Hi,

What is a VERY POWERFUL battery you guys would recommend for the axon mote?  Hawker?

Asking such just reveals that you care more for the *My balls are bigger than yours* kinda thing, rather than careful selecting the battery that is the best fit for what you need.
A too large battery mans you're dragging dead weight and as such, that you are not running it optimally.

Further, we don't use subjective terms in engineering, since we all have different ideas of what eg. "VERY POWERFUL" means.
Consider instead your actual needs (load and runtime) and select a battery to cover that with some margin.
Forget the power drawn by the controller board - the major part of the load are taken by the motor(s) and other actuators you might use.

[Sylvestre]

No actually I was just looking for a battery people may have used in the past that they found successful.  Not the saying you said above.  Excuse my misuse of words.

[Sylvestre]

Also the motors will require a separate power source

[Soeren]

Hi,

No actually I was just looking for a battery people may have used in the past that they found successful.  Not the saying you said above.  Excuse my misuse of words.

OK then, what are your loads, besides the controller?
24V Motor current?
1 or 2 motors?
The 5 LEDs are they supposed to be on/off concurrently?
Do you have other loads to consider?  (If so, please list voltage/current for those).

And for how long do you want it to run between recharges?

If you answer all of the above, the best suited battery can be found.

[Sylvestre]

If so Im going to use two 24 volt 450 watt motors.  I want it to go 9 mph and I want it run 2 hours before I have to recharge.



I'm going to connect a gps, search light, rangefinder, 3 servos, and a infrared camera to the axon mote.  I'm not sure about the specs on these.  I'm just looking for the battery/batteries i need for the motors.

[waltr]

Quick calculation:

450W / 24V = 18.75Amps per motor, 37.5Amps for both motors.
37.5Amps times 2 hours = 75 Amp-Hour battery capacity as the minimum requirement.

Look in the above "Robot Tutorials" for a more detailed explanation.

[Sylvestre]

I will be able to control those with this motor driver then, right?

http://dimensionengineering.com/Sabertooth2X25.htm

When it says 6-24 volt nominal I assume thats input from the battery, but will I lose power if my motors require 24 volt each?  That probably doesn't matter because it meets the amp requirements, right?

[Soeren]

Hi,

I will be able to control those with this motor driver then, right?
[Snip link to Sabertooth 2X25.]

Yes, 450W at 24V is (450/24=) 18.75A and the driver can handle 25A - you'd need to use the current limiters though, as your motors will try to grab a good deal over 25A at startup and reversal.

When buying a battery, add around 20% to the capacity requirement, as the capacity decreases somewhat over time - a 90Ah battery would cover that.

And get a golf cart/Marine batteriy - they can handle deep discharge, which  a truck battery cannot.
(Prepare to pay through your nose though).

When it says 6-24 volt nominal I assume thats input from the battery, but will I lose power if my motors require 24 volt each?  That probably doesn't matter because it meets the amp requirements, right?

It will be fine. Not because of the current, but the small loss there'll be at any given current is a smaller percentage at a high voltage. Assume you loose half a volt at a given current. at 6V, that would be (0.5*100/6=) 8.3%, while st 24V it will just be (0.5*100/24=) 2.1%

With such hungry motors, the rest of the loads will be close to invisible in the greater picture, but the 450W will only be when given 100% PWM (which, strictly speaking isn't PWM at all). At any lower speeds, consumption will be less.

Use a switch mode converter to go from 24V to eg. 6.0V to 6.5V and regulate the rest of the way down to 5.0V with a LDO linear regulator, then your losses will be small, while still getting a very clean 5V line.
A linear regulation from 24V to 5V means an efficiency of 20%, while a switcher can give you from 80% to 95%, as the average current draw will be the load current divided by the conversion rate.

[Sylvestre]

What I had in mind is that I will be hooking up the motor driver to an axon II.  I'll have two separate batteries.  One for the Axon and one for the motors.  I'm not sure if I quite understood the last paragraph correctly.  I wouldn't need a switch mode converter or linear regulation if I was using a separate battery for the axon, would I?  Also it says in the description of the sabertooth that in can handle 50 amps for a few seconds (startup and reversal).  I'm using scooter motors.

[Sylvestre]

If someone could verify that I am doing everything correctly, I'd really appreciate it.

[corrado33]

To answer one of your questions, no, you would not need to do the switching regulator and LDO if you had two separate batteries.  With that said, I think two separate batteries complicates things.  

That's just my opinion.  You could very well use two batteries.  (But then you have to charge two batteries, check the voltage of two batteries..., buy two batteries...)

Basically what soeren was saying in his last paragraph is that you would use a switching regulator (which is good at dropping voltages way down without losing a lot of energy as heat), to regulate the 24V battery to about 6volts.  Then you would use a LDO (low drop out) linear regulator to regulate it to 5V because (I'm making assumptions here) it gives you a cleaner DC signal than the switching regulator would.  AND, it's easy for a linear regulator to regulate such a small voltage drop so not that much heat is lost, giving you good efficiency.  A low drop out linear regulator simply means that it can work with a very small input/output voltage difference.  So you can provide it with a lower voltage source than a normal linear regulator.

I can't help you with the sabertooth questions as I have no experience with it.  

[Sylvestre]

Ok so I would be able to run both the axon and the motor driver at the same time.  I may need some help wiring this up.  Thanks a lot though 

[Soeren]

Hi,

[...] I wouldn't need a switch mode converter or linear regulation if I was using a separate battery for the axon, would I?

With a separate battery you'd still need a voltage regulator, preferably an LDO regulator.
If you use eg. a 4 cell NiMH battery, they'll be at up to 5.8V when freshly charged and will discharge to somewhere between 3.6V and 4.4V (depending on discharge current).

So, the battery goes from too high a voltage, to a too low voltage (if you use any standard 5V components, which usually needs at least 4.5V, you cannot discharge it fully, i.e. a shorter runtime than the nominal capacity suggests).
A 6 cell battery is the best in that respect, going from ~8.7V down to 5.4V - 7.2V and an LDO will keep the logic happy, by providing a steady 5V throughout the discharge curve.

[Sylvestre]

Ok the recommended voltage for an axon is 6 volts. Which regulator do I buy? Would any of these from this website work? http://www.dimensionengineering.com/

[Soeren]

Hi,

Ok the recommended voltage for an axon is 6 volts. Which regulator do I buy?

None.
I didn't realize that it was for running an Axon, which has got an LDO installed. I am not aware of which make and model the LDO is, but typically, LDO's require less than 1V (some less than a tenth of that). Any 5V LDO can stand at least 10V input and most do better, so just connect a 6 cell battery to the input terminal of the Axon and you will be able to use most, if not all of its available charge (some is lost as heat in the LDO).

You can find the datasheet for the LDO if you google its name and it will tell you the drop out voltage as well as the maximum input voltage.

Would any of these from this website work? http://www.dimensionengineering.com/

If you were to use a single 12V battery, their switchers would be a way to switch down to 6..7V for feeding the LDO (although it can be done much cheaper), but when using a separate battery, they're only needed if you want to make do with fewer battery cells (as it can switch up as well as down).

[Sylvestre]

Ok I didn't quite understand that last paragraph.    Can you possibly explain what the circuitry will look like just for the battery, motor driver, and axon.  Basically how I'm going to connect these parts.

[Gertlex]

In the last paragraph, Soeren meant switching regulators ("switchers" apparently).  They're basically the same, functionally as regular voltage regulators, but they're much more efficient (aka less heat produced, less energy drained from the battery)

I've attached a picture   The wires from axon don't necessarily go to the right pins on the motor driver but whatever.  Having not used PWM with the axon, I don't know where the PWM signal comes from offhand, either... but hopefully it helps

[Sylvestre]

Thanks for that schematic!  I'm not sure which switching regulator I should get though.  Any suggestions?  Could I use this adjustable regulator? http://www.dimensionengineering.com/DE-SWADJ3.htm

[Soeren]

Hi,

Ok I didn't quite understand that last paragraph. 

You can go about it in a couple of ways:
1) Use only a 12V battery and then a switch mode regulator for getting the 5V
2) Use  two batteries12V and something lower. The lower voltage battery could bnefit from a buck/boost switchmode regulator, as it can generate a stable +5V from both lower and higher voltages, so the battery can be drained properly between recharges.
3) Check in the datasheet of the Axon voltage regulator if it is contend with up to 15V or higher - if so, you can run the Axon from the 12V battery, although you'd need a larger heatsink as the heat to dissipate is: (Ubatt-5V)/I [W]
Where Ubatt is simply the battery voltage, the 5V is what the regulator outputs and I is the current
drawn.

Can you possibly explain what the circuitry will look like just for the battery, motor driver, and axon.  Basically how I'm going to connect these parts.

Are you going for a single battery or the "one for the motors and one for the logic/sensors"?

Basically, the 5V side is easily fed from the 12V battery, in the same way that you would with 2 batteries - only here you derive the low voltage from the 12V rather than another battery.

If you don't need all that much current, this is another (very cheap) option if you want to switch down and you can easily change the output voltage to eg. ~6V.

[Sylvestre]

Thanks for that.  I decided I'm going to use a separate battery for the motors and a separate battery for the electronics to avoid brownouts.  I'm going to need a lot of current from the battery because I will be powering wireless camera that gives me a live video feed.  That will powered by the battery that corresponds to the electronics.  Also I will be running many sensors/devices off of the axon II (ie gps, range finder, search lights, servo) so i will be needing a lot of current.

[Soeren]

Hi,

I decided I'm going to use a separate battery for the motors and a separate battery for the electronics to avoid brownouts.

OK, then we only need to talk about the low voltage.

I'm going to need a lot of current from the battery because I will be powering wireless camera that gives me a live video feed.  That will powered by the battery that corresponds to the electronics.  Also I will be running many sensors/devices off of the axon II (ie gps, range finder, search lights, servo) so i will be needing a lot of current.

It's hard to say how much current you need without knowing exactly what you are going to load the battery with, but if it is over (or just near) the limit of the onboard voltage regulator, you'll need a separate regulator for some of the extra equipment.
Since it would be a little harder to change the regulator on the Axon, should it give up it's mojo, perhaps just load it at half what it can handle and draw the rest from an external regulator.

An external regulator for this is easy to make - the regulator IC, a few caps and perhaps a couple of resistors (if using a variable regulator). And as with all the rest... Fuses are paramount. Mount one for each tap from the batteries, as close to the battery terminal as possible.

[Sylvestre]

Once I figure out the which parts (gps, servo, etc) I am going to use, I'll post the specifications (current/voltage required) on here of each one and we'll figure it out from there.

[Gertlex]

Once I figure out the which parts (gps, servo, etc) I am going to use, I'll post the specifications (current/voltage required) on here of each one and we'll figure it out from there.

Sounds good.  As a friendly tip, when you post that list, also post your guess as to how you'll split the power requirements.  That way we can spy any incorrect concepts you might have picked up (we all learn/infer incorrect ideas )

[Sylvestre]

Ok here is the list:

(2) 24 volt motors-
22 amps each (separate battery, 55 ah)

(1) Xtend modem-
Supply voltage: 2.8 - 5.5VDC regulated
Frequency band: 902 - 928MHz
Serial data interface: 3V - 5V CMOS UART - No configuration required
Transmit current: 730 mA (@ 5V, 1W TX Power Output)
Receive current: 80 mA (@ 5V, 1W TX Power Output)


(1) Axon II
(http://www.societyofrobots.com/axon2/axon2_datasheet.shtml)

(2) LEDS
3.3 volts @ 25 ma each

(1) GPS
3.3V @ 41mA

(3) Hitec HS-645MG servos
-Operating Voltage- 4.8-6 volts
-Idle Current- 8.8 ma
-Running Current- 350 ma
-
(1) Sabertooth 2x25 motor driver  (WILL BE POWERED BY A SEPARATE BATTERY THAN THE ELECTRONICS TO AVOID BROWNOUTS)
Specs: 25A continuous, 50A peak per channel.
6-24V nominal, 30V absolute maximum

The following items will not be connected to the Axon II, but will run off the same battery.

(1) Cantek CTTN501R24 Outdoor IP67 Bullet Camera with 24 IR LEDs
-Requires 12 vdc, doesn't say current requirements (http://www.123securityproducts.com/tn501r24.html)

(1) 2.4 Ghz transmitter and receiver set (Comes with a wall transformer to plug into your home outlet, but I will just cut the transformer from the wires and then hook the wires up to the battery). No specs.
http://www.123-cctv.com/seccams/trans.html

Also I want my robot to run for 1 hour- 1 hour & 1/2 so I was thinking I'd probably need a 55 ah battery for the motors.  I'm not sure how large the battery should be for the electronics.