Hi guys. I'm new to DIYDRONES!
Building a Quad and had some hangups with regards to Brushless Motor control. I've never really used an ESC to control a Motor before. I've never built a Quad before either, so forgive me if this sounds stupid.
I have included a design that I thought might work to control the Motors myself. It just uses a basic NPN FET and Pulse Width Modulation to supply power to the Motor.
Vcc will depend on the Motors I end up choosing. (I'm liking the Tiger MT2814 at the moment! But still shopping around) ... It is connected to a Motor terminal, while the other Motor terminal is hooked up to the Collector of my Transistor
GND is Ground. It is connected to the Emitter of my Transistor
PWM Pulse is a 500 Hz signal that will be outputted from my Arduino Uno R3. It is connected to the Base of my Transistor.
When the PWM signal is HIGH, circuit is closed, the Motor is ON. When the PWM signal is LOW, circuit is open, the Motor is OFF.
By playing with the Duty Cycle of my PWM Signal ... I should be able to get any change the 'Average' RPM of my Motor.
Diode is there to protect the Transistor from the Motor (Unlikely but can't hurt to be safe) and Resistor is there to pull down the 5V of my PWM Signal (coming from my Arduino UNO). I might not need the Resistor if I can find a Transistor with higher Base Voltage rating.
What does an ESC do differently? I've never used one before. This is how I was taught in School to 'fake' Analog inputs from Digital inputs.
What would an ESC do for me that this circuit doesn't do?
Thanks! I really appreciate your input!
If only it was that simple. Unfortunately your circuit will not be able to drive a brushless motor. The term "brushless DC motor" is a bit of a misnomer - these devices are in fact synchronous 3-phase AC motors with permanent magnet excitation. There's no DC involved at all.
ESCs provide a 3-phase drive from DC supply, and perform synchronisation and speed control. They are quite complex devices and relatively inexpensive for all that they do.
Upon closer examination of these Motors you are indeed correct. I completely missed the 3 'hot' connectors.
Oh I'm not looking to cheap out, I just thought I understood the concept and so I tried to get creative. I really wanted to understand what is all going on behind the scenes.
So pretty much the ESC is acting like an inverter then?
Yes, although you won't hear speed controllers referred to as inverters. Although a BLDC motor will follow a 3-phase AC signal at low rpm (open loop), once a load is applied and/or the speed increases, maintaining synchronisation requires a knowledge of rotor position. Back EMF is the most commonly used technique to estimate rotor position, probably because it requires minimal additional hardware.
Thanks! I think I'm getting closer to having a better understanding, I've read several articles on the internet but they just didn't click.
I know Back EMF varies with the load on the Motor; so for a constant load (e.g. Propeller) ... Let's say we're moving at a constant RPM (e.g. 3000 RPM) ... The Back EMF should be constant as long as the 'average' Voltage being applied to the Motor is constant right?
I'm just not seeing why Rotor position is important to know here. I guess my question is ... Why is this is closed loop control system and not an open loop control system.
Wow, you guys explain better than the articles I've been searching. Thank you!
Lastly, how exactly is the Back EMF being measured? All 3 leads are used to 'Pulse' the phases of the Motor ... Wouldn't the ESC require some feedback connection to know the Rotor's position?
All the ESCs I have seen have 3 terminals (one for each phase) ... How do I hook it up so that the Back EMF is monitored?
Or is all this done automatically?
Alrighty, that does make sense.
Now, I'm still a little confused on how the ESC 'controls' the Motor speed. I've done some reading and to help you better understand my question I made a sketch in Visio.
I would like to know ... When the Phases operate as they do below, is the Motor in Full Throttle (100%)?
How then does the diagram change for say 75% Throttle? I'm assuming that the ESC itself is always powered, and that for 75% duty cycle the ESC just uses PWM to power the individual phases for the appropriate time?