Team,
I recently was given 5 UDN2998W Dual Full-bridge Motor Drivers made by Allegro Microsystems. I am building up the pieces to make a smaller tabletop cnc machine and would like to use these for the stepper motor drivers. What are your thoughts on these drivers for such an application?
Thanks in advance.
Josh
Full-bridge Motor Driver
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Please be positive and constructive with your questions and comments.
- cstratton
- Posts: 294
- Joined: Wed Sep 29, 2010 3:52 pm
Re: Full-bridge Motor Driver
It really depends on what size motors you have and what kind of performance you need to get from them. Those chips seem to be rated about 2 amps peak, which would be okay for smaller/slower tabletop machine but not up to high performance ones.
They are also limited to 50volts. A stepper motor's winding has lots of inductance, and at high step rates that resists the drive. So it's normally to run stepper coils using many, many times the rated voltage in order to "get the current started" and then start chopping (PWMing) the H bridge to maintain the current at the desired value. 50v (or even 35 as on some other driver chips) is okay for smaller machines at slow to moderate speeds, but not higher performance ones will often use 90v or more. The lower the resistance of your stepper coils the less on an issue this will be - modern motors tend to be better in that regard than older ones.
Essentially, you have some IC H bridges that can let you get started with cheap small-medium NEMA23 motors or even larger NEMA23 motors if run below their full capability (but probably not larger NEMA34's unless it's use a weak motor in a large case). That's a lot better than no CNC at all. Later on, you might need to replace some or all of the drives with higher performance bridges made from discrete MOSFETs - but that can be later once/if you find the performance of these dis-satisfactory.
Just to clarify - a wimply driver will not produce enough torque to move your machine without losing steps, or will do so only at low speeds but not at higher step rates where the inductive effects even further limit the current. A motion planner that smoothly ramps speed up and down will help get more out of your motors and drives, so that's pretty standard.
They are also limited to 50volts. A stepper motor's winding has lots of inductance, and at high step rates that resists the drive. So it's normally to run stepper coils using many, many times the rated voltage in order to "get the current started" and then start chopping (PWMing) the H bridge to maintain the current at the desired value. 50v (or even 35 as on some other driver chips) is okay for smaller machines at slow to moderate speeds, but not higher performance ones will often use 90v or more. The lower the resistance of your stepper coils the less on an issue this will be - modern motors tend to be better in that regard than older ones.
Essentially, you have some IC H bridges that can let you get started with cheap small-medium NEMA23 motors or even larger NEMA23 motors if run below their full capability (but probably not larger NEMA34's unless it's use a weak motor in a large case). That's a lot better than no CNC at all. Later on, you might need to replace some or all of the drives with higher performance bridges made from discrete MOSFETs - but that can be later once/if you find the performance of these dis-satisfactory.
Just to clarify - a wimply driver will not produce enough torque to move your machine without losing steps, or will do so only at low speeds but not at higher step rates where the inductive effects even further limit the current. A motion planner that smoothly ramps speed up and down will help get more out of your motors and drives, so that's pretty standard.
- jperson
- Posts: 6
- Joined: Thu Oct 27, 2011 2:54 pm
Re: Full-bridge Motor Driver
Thank you so much for that response, as it was well above and beyond. The motors I have are NEMA23 and after looking them up the amp and volts pairing looked alright to me with this H bridge. I should have been more thorough in my original post because after some thought my real personal dilemma came after looking up their replacement cost (around $55 each) and the fact that this would be a learning project. I didn't want to waste the IC's on a learning experience that is a real stretch from my current level when I could simply sell them on the bay and use that money to buy a PCB kit that I would be much more confident in having a working device in the end.
Thanks again for you help. I think I will indeed be using them and learning from the process. It's only money right?...now time to go reread the data sheet.
Thanks again for you help. I think I will indeed be using them and learning from the process. It's only money right?...now time to go reread the data sheet.
Please be positive and constructive with your questions and comments.