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View Full Version : The worth of microstepping?



stuart576
15-01-2012, 07:24 PM
I have been programming my own microchip to interpret the signals from serial out on mach3 and pulse to the driver. Now this is all done programming wise for full stepping and half stepping (depending on which file I flash the microchip with). I am just wondering whether it's worth programming for microstepping?

I understand how to do it, and can easily incorporate PWM into my existing code, would use 16 microsteps per step just to keep it simple but the reason I ask it if it is worth it is not because of the reduced torque or increased resolution vs. decreased accuracy but because I would only use it for transitioning between steps / half steps. I would do this for the benefits of less mechanical noise and resonance etc, you know the benefits.

Has anyone noticed these benefits or used a similar setup or encountered problems with this? Just to clarify I will be using half steps but transitioning with microsteps.


Any advice?

Thanks

Jonathan
16-01-2012, 02:59 PM
The ideal way to do it is have a look up table of the currents (so duty cycles) for microstepping then when the step frequency is above a certain threshold change the values in the table to the full stepping currents. This is because the motors perform better at higher speed. Sounds like you might already be planning that as you mention 'transitioning'?

Eight micro-steps per step is generally plenty. I didn't bother with 16 but by all means try it as it's easy enough to put the extra values in the code. I did it on a PIC.
What are you using to control the PWM?

On my machine it sounds a lot smoother on 1600 step/rev than less. Whether that actually gets better feed-rates or not I can't tell as I'm limited by the kernel frequency in mach3.

stuart576
18-01-2012, 12:13 PM
I hadn't actually thought about only using it at high speeds, I might just implement it above a threshold as thats all I was worried about, high speeds. I am programming mine with PICs 16F628 although I broke one so I'm using a 16F648A not that it makes a difference. 4

I'm currently using a 4 lead motors, low torque that I had lying around. I've been looking at bigger ones which all seem to have 8 leads. Now I understand that this separates the coils but I'm not sure of the benefits as at the moment with my 4 lead motors I just use two coils and reverse the polarity. Couldn't I just join the two coils on each pole together on the 8 lead motors and use it the same as my 4 lead? I ask this because I have L293 Dual Half-H Bridge drivers by ST and I'm not sure how or whether the 8 lead motors would work with them?

Also I really don't understand why 3-axis controllers are so expensive i.e. CPU5A3 Economy 3 Axis CNC Controller (http://www.zappautomation.co.uk/product_info.php?cPath=15&products_id=202) is 215.00 and from what I can see all a controller is, is a serial interface to a PC, microchips to convert the Mach3 signals to pulses to the drivers and the drivers themselves? Which these parts can be salvaged or bought cheaply. Is it the plug-and-play aspect that makes the price so high?

Gary
18-01-2012, 07:41 PM
Stuart,
The CNC controller that you pointed at does not work with Mach 3 or any of the other software based CNC programs that are available.
It comes with its own CNC user interface software that is licenced to to be used with the controller.
The price includes this software and the hardware.
It is not just a way of getting the clock and direction signals from the software out to some hardware over a USB lead.
The clock and direction signals are generated in the hardware not the software.
Infact most of the processing is done in hardware unlike with mach 3 that takes over the PC.
It is also not limited to the capabilities of a legacy device like the parallel port, and either uses USB or Ethernet.
None of the controllers we sell work with Mach 3, they all come with their own software, that takes time and money to develope, update and add new features.
These CNC controllers are actually very competatively priced.
When you consider how much mach 3 cost and take that cost away from the cost of this controller, do you still think it is expensive?
Mach3 is $175 USD (About 115.00) so now get a good breakout board that has the same amount of I/O as the CPU5? This includes a PWM or 0-10V do control the spindle...
ok, now you are stuck, because to get the same amount of I/O you will need two breakout boards and a second parallel port or a PLC that works over modbus.
I am sorry if i sound a bit abrupt, but there is a lot of difference between mach3 and a CNC controller like the CPU5 or the other controllers we have on our site.



I hadn't actually thought about only using it at high speeds, I might just implement it above a threshold as thats all I was worried about, high speeds. I am programming mine with PICs 16F628 although I broke one so I'm using a 16F648A not that it makes a difference. 4

I'm currently using a 4 lead motors, low torque that I had lying around. I've been looking at bigger ones which all seem to have 8 leads. Now I understand that this separates the coils but I'm not sure of the benefits as at the moment with my 4 lead motors I just use two coils and reverse the polarity. Couldn't I just join the two coils on each pole together on the 8 lead motors and use it the same as my 4 lead? I ask this because I have L293 Dual Half-H Bridge drivers by ST and I'm not sure how or whether the 8 lead motors would work with them?

Also I really don't understand why 3-axis controllers are so expensive i.e. CPU5A3 Economy 3 Axis CNC Controller (http://www.zappautomation.co.uk/product_info.php?cPath=15&products_id=202) is 215.00 and from what I can see all a controller is, is a serial interface to a PC, microchips to convert the Mach3 signals to pulses to the drivers and the drivers themselves? Which these parts can be salvaged or bought cheaply. Is it the plug-and-play aspect that makes the price so high?

Jonathan
18-01-2012, 09:27 PM
Now I understand that this separates the coils but I'm not sure of the benefits as at the moment with my 4 lead motors I just use two coils and reverse the polarity. Couldn't I just join the two coils on each pole together on the 8 lead motors and use it the same as my 4 lead?

The benefit is it allows you to connect the coils in series or parallel, to get either higher current. You're not just limited to what the motor manufacturer chose.


I ask this because I have L293 Dual Half-H Bridge drivers by ST and I'm not sure how or whether the 8 lead motors would work with them?
To make a worthwhile stepper driver you need to make the H-bridge from individual MOSFETs as none of the integrated ones have high enough voltage and or current ratings to be useful on anything but a very small machine. It's still good to use the ready made ones to test the principles.