H-bridge and PWM for DC motor control

-
Several times when controlling a DC motor with PWM over an H-bridge I asked myself what could be better during the OFF part of the PWM cycle. I reckon three basic choices are possible:

  1. If all the switches are OFF, then the back EMF will flow through the flyback diodes of the bridge.
  2. The same action as above could be achieved if the switches opposite to the PWM-ON cycle where ON during the OFF period. A braking action is caused.
  3. The motor could be shorted by closing the top (S1+S3) or bottom switches (S2+S4) thus shorting motor current and again creating a braking action too.
Options 2 and 3 correspond to the fast and slow decay modes when the H-bridge is used for controlling one coil of a stepper motor. And even there, it is possible to mix and match so both can be used during a percentage of the PWM OFF time. 

But my doubt came from a user of my dcservo project, who alerted me that a motor was burned and he thought it was caused because my code was using option 3 during the PWM OFF time. That got me thinking whether he was right or not.

In different moments I have used interchangeably options 1 and 3, that is to allow the motor to coast or to brake it when the bridge was not energizing the motor. Mostly due to the availability of pins on the microcontroller or the features of the H-bridge used (some may not have a enable pin). 

A bit of searching led to these great articles on H-bridges, where basic operation and the details of Sign/Magnitude drive and Locked Anti-Phase drive were discussed. Each of them using option 2 or 3 but not 1 made me think that my approach was certainly not wrong.

However, it is possible to just let the motor coast during the PWM OFF time if the bridge has an enable pin. The net effect is that this type of drive will reduce power dissipation on the switches (as they will be on less often) and the motor could run freely when disabled. That may be a problem for a speed control of a robot if the speed command is zero and the robot is downhill, as it may roll all the way down. For a position control, this seems less of a problem, but having the drive both positive and negative torque seems like a better deal for a precise control. 

So my next step is to check whether Lock AntiPhase drive can give me better positioning accuracy over Sign/Magnitude or not.

Comments

Post a Comment

Popular posts from this blog

VFD control with Arduino using RS485 link

-
Image
Variable Frequency Drives ( VFD ) allow the control of spindles so speed can accurately be controlled and a detailed acceleration profile for the spindle and reverse rotation can all be handled. In essence a VFD is an three phase inverter for three-phase AC motor. I am using a popular (I mean cheap) Chinese VFD and though the reference manual is not great, I could see there is a built-in RS-485 port. I usually control the start, stop and speed selection using the keyboard on the unit but I thought it will be more useful if I could control everything from the same Arduino is doing our CNC table control. Some cheap RS-485 off eBay and some lines of code later, I can start, stop and change the speed from an Arduino. What a cool thing to have! Some configuration of your VFD are needed before you can use it like that.  You need to set PD163=1 (I am using address 1 in the code). PD=164=1 (for setting serial to 9600bps) and PD165=0 (for using ASCII and 8N1 character for...
Read more

How to get sinusoidal s-curve for a stepper motor

-
Image
After some experimentation, and delving a bit into the math of motion, I realized there can be a significant difference on the motion of a machine by just making small changes. A common and popular approach to smooth one axis of motion is to use a trapezoidal speed pattern, where acceleration and deceleration are uniformly accelerated movements (aka constant acceleration motion) separated by a portion happening at cruising speed. The problem comes with the fact that, while that approach is simple to understand and to code, it does contain sudden changes in the acceleration, that show as spikes on the acceleration derivative, called jerk. These sudden changes in the acceleration translate into undesired vibration in our machines. Most of that can be eliminated by selecting a motion pattern that does not contain sudden changes of acceleration. If we chose a mathematical function for our speed whose second derivative is continuous, then we reduce system vibration quite a lot. One ma...
Read more

Importing OpenSCAD designs into Onshape

-
Image
After using Onshape CAD software for a while I am really fond of it. So one natural thing to do, specially once they have released the Instructor's kit is to start using it in the classroom too. But one thing I was not so sure how to do was to use old designs I made using OpenSCAD. It does not mean I am completely quitting using OpenSCAD but suddenly I can see I can do assemblies in Onshape that will look quite nice for showing and documentation purposes if only I could easily import from OpenSCAD. I am glad to report that the route OpenSCAD --> FreeCAD --> Onshape worked brilliantly. Even more when a few tricks were applied. While some people suggest to import CSG files from OpenSCAD to FreeCAD, I have had more success by directly importing SCAD files into FreeCAD. One trick that worked well for me was to get rid of $fn references, so now circles become real circles (or cylinders) in FreeCAD. If a part keeps some $fn references then your cylinders or circles wi...
Read more