On Sat, Oct 17, 2020 at 8:45 PM Nicklas SB Karlsson <nk@nksb.online> wrote:
> > > The theory is that if the PWM signal is high the H-Bridge is switched to > say turn the motor clockwise. If the PWM signal is low > >> the H-Bridge is switch to turn the motor counter clockwise. Therefore > with 50% the motor is first asked to turn one way and then > >> the other. That results in the motor essentially being locked in place. The above is not the way it works. No one would design anything like that. But let's say you did. What would happen? The motor would vibrate and slowly rotate and have close to zero torque There are two cases where PWM is used (1) as a control signal and (2) as a control pin on an h-bridge Case 1 is closest to the abve description but it is implemented differently. The PWM signal is used to generate an analog voltage. Usually, the duty cycle is used to determine how long a capacitor is charged. But always the result is the PWN is converted to a voltage. Then there s a variable resistor that is used to encode the servo position and it creates a voltage based on position. The controller inside the servo tries to make the voltages the same. In no case is the mother directly driven by the PWM. For servo control, the PWM is used as a kind of quasi-digital signal. Case 2 If we are talking about Polulu motor drivers then the chips all have control pins to place the motor in forward or reverse or brake or freewheel modes. You have to read the data sheet every chip uses a slightly different convention. Then there is another Pin called PWM It is bet to think of this as controlling if the power supply is connected or not. How it is connected depends on some other pins. You can think of PWM is controlling the power supply voltage, but not the direction or if the supply is even connected to the motor. How to et this up in LCNC: I see two different ways to go. (1) use some kind of single-board computer like the "Blue Pill" (STM32) or an Arduino or "whatever" as a servo controller. It would drive the motor to a speed as commanded by LCNC. The motor encoders would connect to the single board computer and the single-board computer would run a PID loop to control velocity. How does LCNC communicate with the small computer? ANy why you like, PWM (case 1) or and analog voltage or a command sent over a serial link such as i2c, SPI, USB, CAN bus, Ethernet, RS232,.... I've implemented this using commands over USB many times. The "blue pill" is the cheapest and by far the best value and the Arduino is the easiest to get working. The advantage is that because the PID loop is running on a microcontroller it can be very fast and very stable. You can also put the homing logic on the microcontroller (2) the other way is a HAL module. It could communicate with the Polulu drive over a parallel port. You need a pin on the port for every pin on the driver. A PID loo is run in HAL that controls both the direction pins and the PWM duty cycle. The advantage here is that you don't need a single-board computer. Using this method PWM duty cycle controls motor torque but not motor direction. -- Chris Albertson Redondo Beach, California _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
