Beginners: please read this post and this post before posting to the forum.
0 Members and 1 Guest are viewing this topic.
There are two aspects that I'm trying to control, the angle of the shaft (after gearbox) and the speed at which the motor tries to move.
My first thoughts were to create a circuit with a microprocessor at the center.
A PWM output would be used to determine the speed. The following link shows a nice H-bridge driven by PWM: http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.html
I've always had trouble understanding transistor circuits, I can't figure out what the purpose of the 10k resistors are for. Could someone explain to me the purpose?
Of course, which direction used would have to be determined by another circuit. My first thought was to compare an analog output of the microprocessor to the position of a potentiometer.
With everything said, I really don't know if this is a good idea or a bad idea. Can someone give me input and direction.
Quote from: mack33 on July 08, 2009, 05:57:35 AMThere are two aspects that I'm trying to control, the angle of the shaft (after gearbox) and the speed at which the motor tries to move.Is this for continous rotation or limited to eg. 360°?
Quote from: mack33 on July 08, 2009, 05:57:35 AMA PWM output would be used to determine the speed. The following link shows a nice H-bridge driven by PWM: http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt-circuit.htmlNice? Well, for simplicity perhaps, but it's not too efficient, using Darlingtons result in a power loss - perhaps your app. can bear it though, if you've got voltage to spare.
Start with the purpose for the circuit - a specification of what it has to do, how "well" (numbers - speed, force etc.) it has to do it and then the start is said
I've just designed a h-bridge motor driver. In my case it's just an open loop speed controller. But the h-bridge was more complicated to build *to spec* than I expected, and it's probably my 5th h-bridge build. Other times I just built the circuit that I saw online or so. This time I really wanted to do it right.
In most cases, no matter how I tried to *tune* the resistors/caps I ended up having the transistor in the pseudo on/off range (the resistive range) and this would cause the mosfets to get very hot.
My intention is for the output shaft of the gearbox to have limited rotation. The motor shaft may need to rotate at a magnitude greater then the output shaft to create desired torque.
I totally agree on both points. It's simple, so I do understand it. My intentions are to use these on, among other things, autonomous robots. Minimizing power loss is always beneficial. What would you suggest instead of Darlingtons?
Well, I'm going to need a way to devise a process to create the specifications. I can always make guesses but there's no way to know if I'll get something that's appropriate.
I've never used Xspice, I used to use multisim, much more professional, but it no longer work's. I may have to buy another version.
Unfortunately, I don't know what affect the frequency of the pwm signal has on the motor. My understanding is the lower the duty cycle of the pwm, the lower the speed of the motor. I understand that the frequency has to be big enough not to osolate the speed of the motor. I don't know what quidlines dictate what's necessary and what's overkill.
Having said that, my sim is saying that my mosfet driver should work with a 3.3v signal and sink only 5ma, thus it should work as driven by my fpga pins in LVCMOS33 io standard in 24ma mode. However, on my breadboard, no go, the transistor base drives the voltage down to 0.5v. (I cant switch the IOB to LVTTL33 standard because of other signals on the IOB.) The XC9572XL CPLD I am using should drive it fine but I am going to try using a small FET on the low side of the driver, I feel this will work better than the 2n2222.