Control electronics power - need for isolation?

Hi All
If running motors (like wheelchair motors) and control electronics from a single battery pack, is it best to use a DC to DC convertor with
transformer isolation to drive the control electronics?
Just using a linear regulator, I can foresee some of the "yuck" coming from the high current components playing havoc with a microcontroller.
Cheers
M
--
Matthew Smith
Kadina Business Consultancy, South Australia
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Matthew Smith wrote:

In anything other than a trivial robot I *always* use two different battery packs: one for the motors and the other for the electronics. -- D. Jay Newman http://enerd.ws/robots /
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Matthew Smith wrote:

Whether or not you elecrically isolate your power system from your computer system is a matter or preference and regulations for your application.
IMHO, if you are careful, and what you are creating is your own hobby, then you can dispense with electrical isolation as it can get expensive.

Most moderate power DC->DC switchers are electrically isolated, but they do something more important. They convert power with great efficiency. A linear power supply is the worst, it burns power that you don't use.
Look at it this way:
Linear regulators use a semiconductor as a psudo-resistor, so 1 amp at 5 volts is five watts, but if you are driving your regulator with 12 volts, you will still be drawing that 1 amp, so you'll be using 12 watts. You will be burning 7 watts of power to produce 5.
A switcher uses a switched driver and an LC filter to product 5 volts. If you drive it with 12 volts, your 5 volts 1A, may only be about 0.42A at 12V.
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mlw wrote: ...

Thanks. I've got half a rail of LT1171s kicking around as well as cores and bobbins, so will knock a 24V->5V supply together for the logic.
I was considering a separate NiMH pack as an alternative, but am going off the idea as this would require another charger AND a switching regulator to keep the voltage stable against changes in NiMH pack voltage.
Cheers
M
--
Matthew Smith
Kadina Business Consultancy, South Australia
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Matthew Smith wrote:

Well, you could use two batteries and use two diodes to allow a charger to charge both at the same time. That way, if the motors kill the drive batteries, the computer can still operate and call for help.
(The only thing about using diode is the ~0.6 volt drop, you'ld need to put the voltage sense on the other end of the power battery diode.)
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Matthew Smith wrote:

Separate supplies provide for a number of simplicities, so it's a trade-off that is fairly situation-specific. If you don't need to do it, don't bother.
There are a number of devices on the typical robot that can cause power supply glitches that affect electronics. Motors are one. Because motors don't usually need regulation, it's often acceptable to use a large battery for those, with maybe a battery-level monitor, but no regulator. A smaller, separate set of batteries can be used for the electronics, and this pack is regulated.
Back EMF from the motor can be coupled into the electronics, so especially for larger motors, it is often desirable to use opto-isolators between the control electronics and the motor bridge. This is effective for both single- and dual-supply designs.
Noisy motors can also be "quieted" by applying disc caps across their terminals. Some designers like to also put additional caps between each terminal and the motor case, assuming a metal case on the motor, or nearby to ground.
VERY noisy motors can induce RFI, which can interfere with electronics, including audio and vision. I recently hacked a tracked-driven robot toy I bought at Big Lots, and found the motors were so noisy that I'd have to use RF shielding to prevent them from lousing up my camera signal, and causing sputtering noises through the sound amplifier. It was easier to use better motors.
There are additional methods that can be used to reduce noise from motors, and these are well-documented in the lit, so I'll leave it at that. (Sometimes even twising the wires to the motors helps a lot. Don't overlook the little things.)
Some electronics pull lots of current when actived, and cause spikes in the power supply. Adding decoupling caps doesn't always solve the problem entirely. Consider separate little regulators for these. It doesn't add much to the complexity of the design to create a small handful of "local power regulators" for several of your sensors and other electronics. Be watchful of some of the Sharp IR sensors, which are known to cause noise in the power supply (affecting other electronics, as well as its own signals). Especially bad are the Polaroid sonar sensors, which pull from 1-2 amps each time they fire.
-- Gordon
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Gordon McComb wrote:

Often times it is good to put an LC filter between the main power supply and the motor aplifier as well as between the amplifier and the motor. The power supply filter will round out the switching spikes of the drivers, and the motor filter will smooth out the power to the motor.

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mlw wrote:

This is not always desirable. You often want the full voltage applied to the motor armatures to retain torque. If you average out the voltage reaching the motor, you also reduce current, and therefore reduce torque. You don't want to simply raise and lower the voltage to the motor if you care about retaining torque; for PWM you want to apply full voltage and current for discrete periods of time.
-- Gordon
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Gordon McComb wrote:

I'm not sure how to go about completely disagreing with you without offending you, but I have to.
Torque is a measurement of force. A constant torque against a constant load means a constant rotational velocity. Constant torque against a varying load means a varying rotational velocity. The object of motion control is to modulate the torque of the motor to control potentially changing velocity against potentially varying loads.
So, to sum up: If the rotational velocity is to remain constant as a load increases, the torque must increase. If the rotational velocity is to remain constant as the load decreases, the torque must decrease.
Similarly If the rotational velocity is to increase against a constant load, the torque must increase. If the rotational velocity is to decrease against a constant load, the torque must decrease.
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mlw wrote:

<snip>
All fine and dandy, but it neglects the actual comments I made (this is becoming a common trait in your responses). You're trying to revise Ohm's Law by insisting you can adequately drive motors under load simply by varying their voltage. Reduce voltage and you reduce current. Reduce current, and you...what happens to torque?
The fact that most PWM circuits do not attempt to "average out" the motor voltage should give you a clue as to the design philosophies behind them.
-- Gordon
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Gordon McComb wrote:

First of all, Ohm's law has nothing to do with motors. Ohm's law dictates the behavior of linear DC circuits with resistance. Motors are a reacive load, and E=I*R does not work. If you apply 6 volts to a motor and measure the current, and then apply 12 volts, you will not see twice the current, the current will curve based on the reactance of the motor and the period that the motor coils stay energized.
Second of all, You WANT to vary the torque. As the load increases you increase the torque to maintain rotational velocity. As the load decreases you reduce the torque to maintain rotational velocity.
Whether you know it or not, A PWM controller modulates the effective torque of the motor. A wider PWM, more torque, and more narrow PWM, less torque. That is is how they work.
If you are not varying the torque of the motor, how are you controlling rotational velocity? Power to the wheels is a fairly simple equasion:
If your robot needs 8 ft/lbs of force to maintain a velocity of 8 RPM.
8ft/lbs = R * 8RPM.
If you increase the resistance on the wheels (going up an incline), say:
8ft/lbs = 1.5R * ?RPM
RPM has to equal 5.3. By maintainling a constant torque, you reduced speed. Now to maintain speed:
?ft/lbs = 1.5R * 8RPM
The torque has to be 12ft/lbs to work.

Just because someone does something a certain way, or even that a majority of people believe something is right is in no way proof that it is. Most people believe that they are safe in a car during a lightning storm because of the rubber tires. Anyone who knows anything about electricity knows it is because of the metal cage that the car provides.
The truth is that it is easier and cheaper not to use an LC circuit, however, you are forced to use low frequency PWM and get servo whine and nice hot motors.
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mlw wrote:

But you're not really espousing true PWM control. You're averaging out the voltage from the PWM, and presenting THAT to the motors. Your approach does not allow the motor to receive full voltage (and therefore maximum current) during the ON time, however short or long that is. Let the inductance of the coils and the mechanical momentum of the motor average out current and torque. Speed, and torque, control are still achieved.
While inductance may involve a complication in voltage/current calculations for motors at varying speeds, coils are also resistive; Ohm's Law works perfectly well in calculating maximum current through a motor. You simply cannot escape the relationships between voltage and current, nor between current and torque.
I'd really like to see some Web references to the design approach you're recommending. Could you provide some links? Surely there must be something out there.
-- Gordon
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Gordon McComb wrote:

What?
Averaging out the power, yes. Voltage at the motor is a factor of back emf, and velocity.

The motor's torque is a product of current.

Speed is a result of torque and load. You control speed by modulating torque based on the load.

YIKES!!! Coils are NOT resistive!!!!! There may be parasitic resistance because copper wire is not a perfect conductor, but resistance is *not* a good part of the motor.

When the motor is stalled and you have saturated the coil, yup, your nice copper wire and carbon contacts will act like a resistor. All coils do. That's why motors burn up, because rather than convert emf into rotaional force (torque), people like to use them like resistors.

As I have said, torque is directly relational to current. Let me correct this, torque is directly related to current on a non-stalled motor. Hell, if the thing is stalled, it will act like a resistor, but it isn't supposed to. Motors are supposed to produce rotational force, not heat.
A motor amplifier controls the power applied to the motor, NOT VOLTAGE. Voltage is a product of back emf and motor speed. Motor speed is a product of current (thus torque) and load.

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Gordon McComb wrote:

This just caught my eye, and I didn't notice it before. I have *never* talked about "voltage" to the motor. I have always talked about power.
The torque of a motor is proportional to the motor current, but the speed is related to back emf, and thus the applied voltage.
Under a constant load and torque, there is a direct relationship between voltage and current. Under varying load, the motor current must be adjusted to keep the speed constant.
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mlw wrote:

Okay, let's go with this. Here's an excerpt of one of your comments: "and the motor filter will smooth out the power to the motor."
You did use the term "power," but what does it mean? I've always strictly considered "power" an output measurement, not an input, though I relize the term is often colloquially used to indicate a voltage source of some type. What electrical term are you meaning when you say "power"?

Don't you mean that back emf is related to speed?

In robotics, the *typical* adjustment is to alter speed, independent of load, in order to simply move more slowly, or to make realtime course changes in response to odometer readings. We can simplify the discussion by limiting ourselves to that aspect only for the time being.
What I'm talking about is "starving" the motor of the benefits of a full inrush of current to its windings by averaging out its voltage BEFORE that voltage reaches the motor terminals. The construction of the motor itself amply averages out voltage and current (and therefore torque) when given a pure PWM signal. Robotics is a high-torque application, where speed variation may need to be 50% or more. Is this equally achievable -- while maintaining efficiency -- using a servo control with what is basically to a DAC voltage output?
Again, traditional PWM discussions are throughout the Internet. Please provide some links that demonstrate your approach, either with a discussion of the math, or with practical experiments. Thanks.
-- Gordon
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Gordon McComb wrote:

Power is "power." The ability to do work. In electrical terms "watts."

Sigh.
Current to the motor is directly related to the torque the motor produces. The speed that a motor spins at is directly related to torque and load.
Now, assume you are applying a constant current source to a motor under a constant load. The motor will accelerate until the torque is in equlibrium with the load and velocity:
tourque = resistence * velocity.
As long as the torque is greater than "resistance * velocity," velocity will increase (acceleration). Once they are in equilibrium, there is no more acceleration. Right?
Now, during the acceleration, the voltage measured at the motor will increase proportionatly with the acceleration.

No, speed is a result of torque and load. We control torque in the presence of variable load to maintain or adjust speed.

That makes no sense.

Yes and no. A PWM signal, if it's frequency is too low will affect the motors rotational velocity and make it too hard to control. If the frequency is too high, the motors inductance will end up filtering the signal.

A PWM amplifyer is not a DAC. A PWM system is more like a "power" source, lile a current source that reacts to load. This is *not* a voltage source.
However, yes, a filtered PWM output will work better and more efficiently than a raw PWM at the motor typically would.

I feel like someone trying to defend darwinism to creationsists. There are *many* text books on the nature of electrical motors and control. I can not cover all the theory in a few internet posts, and I can't spend a few days looking for articles published on the internet.
I would suggest reading up on switch-mode power supplies for a solid understanding of PWM amplifier theory. (Directly relatable to PWM motor amplifiers.)
Howowitz and Hill, "The Art of Electronics," has a brief explanation of a "step down" switching power supply. You'll notice that this resembles one PWM corner of an H-Bridge. All the math still applies.
As for motors, they are harder because typical electronics books tend to ignore the dynamics of them. A good treatment is in "Basic Robotics Concepts" by John M. Holland, but you may have trouble finding it.
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