Best way to regulate voltage supply that with hi current draw?
rwgast_logicdesign
Posts: 1,464
Ok so let me put this in context first. Ive been looking at buying a parallax sting ray bot, the motors are 7.2v. Now the motor control chip I use needs 8v, so thats not a big deal really as far as over volting the motors.
But heres the issue say Im running a 12v battery to prevent the the h bridge from browning out, among other reasons what would be the best way to go about regulating a straight 8v to the h bridge. Id also like to regulate the voltage to make sure the motors always have 8v available to them. Anyways the sting ray motors stall at 4.5 amps, so i would need to set up regulation capable of 15 amps, I did a quick search for LDOs that can handle 15 amps and didnt find any, plus I dont think an LDO is the best way to go in this situation. I personally would rather use a switching regulator, but im not even sure if thats the best way. I know theres systems out there that regulate there voltage down for motors im just not sure exactly what the best approach is.
But heres the issue say Im running a 12v battery to prevent the the h bridge from browning out, among other reasons what would be the best way to go about regulating a straight 8v to the h bridge. Id also like to regulate the voltage to make sure the motors always have 8v available to them. Anyways the sting ray motors stall at 4.5 amps, so i would need to set up regulation capable of 15 amps, I did a quick search for LDOs that can handle 15 amps and didnt find any, plus I dont think an LDO is the best way to go in this situation. I personally would rather use a switching regulator, but im not even sure if thats the best way. I know theres systems out there that regulate there voltage down for motors im just not sure exactly what the best approach is.
Comments
I suspect you don't want to regulate 15A gown to 8V. Very wasteful of power.
Duane J
-Phil
http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00002345.pdf
with a 22v battery.
The motors would be what ever is in the parallax stingray kit, i havent been able to find much info besides they are 7.2v nominal.
@Phil
I thought about doing it that way but then you would not get the full range of your PWM signal.
Seriously, it doesn't matter. By selecting a current and using the monitored current as feedback to keep it constant, you will be varying the duty cycle dynamically. In the process, you will have very fine control over the current regulation -- finer than the nominal precision of the PWM itself.
OTOH, if you don't want to monitor the current, you can still make use of the PWM's full precision simply by leaving an extra gap between pulses.
-Phil
Now obviously im missing something here...
Of course pure torque control like this isn't actually very useful for many purposes, speed control is much more natural (with current/torque
limiting). PWM roughly controls speed - but for current limiting a current-feedback loop needs to apply when the current is high.
Basically voltage defines the speed and the load defines the current.
Duane J
What exactly is LDO?
Sorry for my not understanding this, BUT:
Motor control is not very easy to understand.
I've built 2 motor controllers
One of them uses the 2N3055 power transistors from Radio Shack.
This controller did work for a time, but the drive current needed to drive the 2N3055 transistors was more than the
1/2 watt base control resistors could handle. The 3 Volt drop occuring at the transistors caused them to run very hot.
I'm planning on revisiting that failure. It should work even if it's not very efficient.
The 2nd controller used IRF510 MosFets from Radio shack.
This should have been very simple parallel connected MosFets (4 of them)
That project did not work out at all, I don't know why.
I tried a few other things also that did not work out, but I ended up using a strategy of
Diode voltage drops to regulate my speed to my 7.2Volt drill motor.
My Next endever will more than likely use a Parallax HB-25 motor controller, BUT
It appears that the Max voltage supplied ot the HB-25 is 16 voltsDC.
I'm needing to connect to 18V DeWalt battery.
Concerning motor control,
I would guess that the duty cycle of the PWM would control the Voltage applied to the motor.
The current drawn by the motor is a result of the motor design.
SO:
As long as the Average voltage applied to the motor through the PWM does not exceed the design spec of the motor
The motor should work, whatever motor you choose.
Am I incorrect about this?
Just my two cents worth.
The 2N3055 is not a good choice for switching circuits. It is more suited to linear circuits.
Fets are a better choice for PWM motor control but you have to select a suitable one and provide the proper drive voltage and current to get the best results from them.
You should be looking at a controller that is rated for a 24V input for that.
True up to a point, but it also depends on the motor. Higher than rated instantaneous voltages can cause arcing and high frequency PWM can cause inductive heating if the motor is not designed for those conditions. While I might run a motor rated for 7.2V from a 12V supply with PWM to control the current I would not want to go much beyond that.
This reduces heat dissipation a lot. For instance http://www.coolcomponents.co.uk/catalog/attopilot-voltage-current-sense-breakout-p-433.html
Incidentally I think such sensors generate less noise than hall-effect sensors, but don't have as good isolation performance.
Volts across the dropping resistor is not related to volts on the motor. So V=IR is 10*.1 which is 1V. Then you can work out watts in the dropping resistor two ways = W=IV which is 10 x 1 = 10W, or IIR which is 10x10x0.1 which is also 10W and that provides a double check that the number is correct.
You can use lower value sense resistors if you like. 1V won't need any amplification and you might be able to send that into a propeller or something. Or if you don't want to waste 10W, you can use a lower value resistor. Or even a bit of wire, and an op amp amplifier. I've even used the wire going to the motor as the current sense resistor.
There are other options like Mark_T says.
Then you need what is called a differential amplifier. This is an op amp with four resistors and it measures the difference between voltages. So the input does not care that it is 21.9V and 22V. All it cares is there is difference of 0.1V. And by changing the resistors you can amplify the signal as well. So you can get a gain of 100 if you like. So with one op amp and four resistors you can have a 0-1V output, and you can sense the current either on the high side wire or the low side wire.
The other little challenge is calibration. But you can easily do that with a known current (eg 1 amp as it makes all the calculations easier). Put 1A through your motor wires, start with a gain of 100 on the op amp, then adjust the gain resistors till you get the volts you want (eg 0-3V if you want to feed into a propeller).
If you use the motor wires as the current sense resistors you don't waste any power as the wires were going to be there anyway.
There is a good solution in the CA3140 op amp which can swing rail to rail. I saw this used first in the current sense of a power supply, and I recall in the magazine some discussion in the article about how the rail to rail limitation of many op amps can be a problem. The current sense for power supply is the same problem as sensing current for a motor. It won't matter if you use the high side or the low side wire as the CA3140 supply can go up to 36V.