More on DC motor "stepper emulation"

-
The code I developed a while ago for using a DC motor (or a BLDC) to replace a stepper has got quite an interest among youmagine users. Over time I realized that some of the quick and dirty things I did were preventing the easy exploration of system parameters for users. Using different PID parameters required a recompile and of the sketch.

I recently received a Printrbot Simple from Brook Drum with the goal of testing with that printer my closed-loop solution.  As usual, simple things (in theory) become tougher that initially expected. First of all, it took forever for the printer to clear through customs and while Printrbot provided and shipped the printer for free, the taxman wanted its cut no matter what, and it took more than too weeks for that to be worked out.

We settled on the Simple because it had more room for the motors to fit in though carriages might be heavier than the Play.



At the time I did not know the CAD of both models was available on Youmagine, so I only had a vague idea of the potential problems of fitting the DC motors I was planning to use on each model.

Once I downloaded the Simple CAD model and imported into Onshape I was able to redesign one of the plates so it will fit the DC motor instead of the stepper.
That part will help attaching the motor to Y-axis of the Simple but there was no may I can make that while I keep Z-axis threaded rod in place. They do that with the stepper because it has a long shaft but that is not the case with the DC motor I am using. Ok, time to think about it. Maybe I will use some fishing line and a couple of pulleys to have a crane-like Z-axis. For now, let's focus on the drive. While the original part was made of metal, I though I will save some time just 3D printing that part. On hindsight that was not a wise move as the plastic part does not help motor heat to spread to the rest of the metal body, so heat becomes a problem. 

I knew that the axis was heavy so the first test was to determine whether or not the motor could handle the load. I learned that 12V might be a bit on the low torque side but a few more volts in the supply voltage will make the same tiny motor to get much more authority. Same driver electronic could do with a higher voltage so no big deal.

Next problem was to replace the axis belt as unfortunately my motor came with MXL pulley. I managed to get an acceptable setup, but due to my pulley lacking a belt guide I later needed to add a big washer to one of the bearings so the belt could not derail on fast moves.

But once everything was up and running I realized how inconvenient was to need a recompile every time I wanted to test a new set of PID parameters. So what I did was to include some additional serial commands so I could set parameters on-the-fly using the serial port. 

And then, I realized that once you have set the right set of values, it will be great if you can save them so they are not lost when you power cycle the controller. EEPROM was the logical choice and after a bit of fiddling with the EEPROM library (as I have an old iMac I cannot update due to a SMART harddisk error, which forces me to still using an old Arduino IDE) I get the thing working nicely.

Not only you can select P, I and D values, but you can also check current encoder value, output value and target location, command manual moves or set a sequence of random moves, plus you can store current parameters into Arduino EEPROM so next time you power it on it will remember them. 

While all previous code was just uploaded to youmagine, this time I created a github repository so hopefully that will make collaborators life easier.

Meanwhile I still need to figure out how to get another DC motor to X-axis.


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 format)
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

Stepper motor step signal timing calculation

-
Image
While we use stepper motors in many devices, the timing of the step signals is not always well understood. Most of us can see how a fixed speed is a bad idea unless the speed of motion is really slow, but it is fixed speed what is really to generate with simple code. You just can use the Arduino Blink example to send a fixed frequency of pulses to a stepper motor driver. If too slow, just reduce the delay value. But for most real-life applications variable speed controls will perform better. These systems will speed up the motor to a given maximum speed and it will slow it down to stop at the desired step count. But understanding the math behind this process is sometimes not obvious. Let us start with the simple case of the so-called trapezoidal speed motion profile. Here motion takes place in three phases: acceleration, constant speed (cruising) and deceleration. But the point is how can our code create such a behavior. Stepper motor drivers usually accept two signals to control
Read more