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Old Jan 24, 2012, 04:21 PM
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Brushless motor driver circuit

Hi,

I need help in order to modify a brushless ESC for manual operation instead of PWM control.

What I need is to be able to turn an R/C bruhless motor on and run it full throttle forward or full throttle backward with by operating a double throw toggle switch or two push buttons or some other king of logic.

FWD-OFF-REV

Could you tell me where do I need to tap the circuit in order to control the high power circuitry that pulses the motor? What should I look for?

Would it be easier if start from scratch? Any dedicated brushless driver ICs to suggest?

Thank you in advance,
Dimi
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Old Jan 24, 2012, 05:08 PM
"Simplify, then add lightness"
Raleigh,NC
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What you need is called a servo tester. It will drive the proper signal into the brushless ESC. Here is one page that shows one with forward and reverse switches that would be easy to build. http://www.555-timer-circuits.com/servo-tester.html

You can also buy them rather cheaply from www.hobbyking.com, but they will have a pot instead of switches.

OOPS: Should have looked closer at that page before posting the link. The resistor values on the top circuit with the switches are all wrong. The top resistor should be 240k and the resistors with the switches should be 13k and two 9.1k all in series. Would need one normally open switch across one 9.1k and one normally closed switch across the other 9.1k.
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Old Jan 25, 2012, 11:39 AM
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Jeff555,

thank you for your reply!

I have thought about the servo driver circuit too. The problem is that in order for an ESC to arm I need to start with PWM at neutral position first and that makes it complicated.

What I need to do is replace the brushed motor in a linear actuator with a brushless one. The actuator has built-in end switches that stop the motor at the end of it's mechanical travel and will let it run only in the reverse direction when the polarity at the (brushed motor's) terminals is reversed.

Here is a block diagram depicting what I am trying to do:

https://picasaweb.google.com/lh/phot...eat=directlink

The controller circuit's function (the one with the red push button on top-left corner) is described in this thread:

http://forum.allaboutcircuits.com/sh...inear+actuator

It just seems unnecessary complicated to go this PWM route while the only thing I need is full throttle forward and reverse...

Dimitrios
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Old Jan 25, 2012, 12:08 PM
"Simplify, then add lightness"
Raleigh,NC
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I am still not quite clear on what you want to do. Do you want the actuator to continuously cycle from one end to the other, or do you want push buttons to control the direction and just have the limit switches work to stop it at the ends?

Either way, there is no where to "tap the circuit" in a RC brushless ESC. They are all built around a microcontroller which does everything so you either rewrite the code for the microcontroller or you use an external servo tester.

The servo tester seems like it would be the easiest thing to do. If you wanted it to just power up and cycle continuously, you would need something to arm it when power was first applied. Could be a manual initialize button or else a timer that started on power up and applied neutral for 3 or so seconds.
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Old Jan 25, 2012, 02:05 PM
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In the kindest fashion and I hope it brings a smile.
I nicked this from the Pedelecs forum.
Dave
.................................................. .............................


That's the curious thing about engineering. The less you know, the easier it appears to be!

The logical extrapolated conclusion is that if you're a manager you tend to think engineers are overpaid labourers...

Rog.
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Old Jan 25, 2012, 03:51 PM
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Originally Posted by orraman View Post
In the kindest fashion and I hope it brings a smile.
I nicked this from the Pedelecs forum.
Dave
.................................................. .............................


That's the curious thing about engineering. The less you know, the easier it appears to be!

The logical extrapolated conclusion is that if you're a manager you tend to think engineers are overpaid labourers...

Rog.
???
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Old Jan 25, 2012, 03:53 PM
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Originally Posted by jeffs555 View Post
I am still not quite clear on what you want to do. Do you want the actuator to continuously cycle from one end to the other, or do you want push buttons to control the direction and just have the limit switches work to stop it at the ends?
I want to push a button ans have the actuator extend it's arm all the way out then retract and stop as soon as it reaches it's other end. That's what my current circuit does by reversing the polarity of the brushed motor.

Now I want to accomplish the same with a brushless motor instead.

Dimitrios
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Old Jan 25, 2012, 06:16 PM
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Add another MCU that senses the switch position and outputs PWM pulse. You might also want to add some ramping to spin the motor up slowly instead of going neutral->full throttle right away.
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Old Jan 26, 2012, 01:32 AM
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You might also want to add some ramping to spin the motor up slowly instead of going neutral->full throttle right away.
How do I do that?

Dimitrios
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Old Jan 26, 2012, 01:50 AM
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Well uh, use a timer or something and a counter, when you detect hitting one of the end position switches instead of immediately changing pwm duty cycle from 0 to 100% (or whatever), slowly ramp it up in steps of 1% or 5% or wahtever, set some var to be final value, and increment it + set pwm pulse on each timer tick or overflow etc. Just talking generally here, depends on what micro you're using and such.
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Old Jan 26, 2012, 02:19 AM
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Originally Posted by Dimitris76 View Post
It just seems unnecessary complicated to go this PWM route while the only thing I need is full throttle forward and reverse...
As others have pointed out, going straight to full speed is unlikley to work, the motor will just stall I suspect.

You could achieve what you want by using a standard brushless RC controller and provide the servo control pulses with something like a PICAXE, should be no problem ramping up to full speed then.
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Old Jan 26, 2012, 03:08 AM
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You could achieve what you want by using a standard brushless RC controller and provide the servo control pulses with something like a PICAXE, should be no problem ramping up to full speed then.


Look at the picture:
https://picasaweb.google.com/lh/phot...eat=directlink

I was planning to use a standard commercial servo driver circuit (I have this: http://www.hobbyking.com/hobbyking/s...idProduct=8296 ) and replace the potentiometer with a voltage divider and a douple pole solid state relay. The relay will be connected instead of the brushed motor.

What's happens when you hit the throttle very fast on your radio? Does the motor stall?

Dimitrios
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Old Jan 26, 2012, 06:20 PM
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Originally Posted by Dimitris76 View Post

What's happens when you hit the throttle very fast on your radio? Does the motor stall?

Dimitrios
If the throttle is jumped to WOT the ESC will slowly start the motor and after a bit it will go into closed loop mode. After it is in closed loop mode the ESC will ramp up the speed as fast as the motor can drive the load. However before going into closed loop mode the motor torque will be very low and may not start.

If instead you use a sensored motor, it will in closed loop mode from the very start and have full torque from the start.

Sensorless motors are not very good for loads that have high starting torque or loads that change torque fast. Basically they are good for fans and propellers.

The term 'closed loop' can mean different things, the model airplane community uses 'closed loop' for the way model airplane ESC's operate. The rest of the industry would call that mode of operation 'open loop'.
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Old Jan 27, 2012, 05:02 AM
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IMO closed loop is more correct since the MOSFETs keep the circuit closed permanently, except for the polarity switch requered for brushless operation.
Just my 2 cents
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Old Jan 27, 2012, 07:57 AM
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Originally Posted by mjsas View Post
. However before going into closed loop mode the motor torque will be very low and may not start.

If instead you use a sensored motor, it will in closed loop mode from the very start and have full torque from the start.
So in other words a run off the mill brushless-sensorless R/C motor will not deliver full torque at start or during abrupt direction changes.

This problem needs to be addressed for my application.
Do you guys think a "servo slower" between the servo driver and ESC will be up to the task?

Dimitrios
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