January 9, 2006
Principal Embedded Systems Engineer
Logic Product Development
Schmalz Haus LLC
about 3 days ago
Is there any chance that there was a short between any of the four motor outputs? Or, at any time that the BED was powered, were any of the four motor output lines connected or disconnected from the motor?
One thing to keep in mind is that stepper motors (at least with low end drivers) are really made for very fine position accuracy, not speed. Torque drops off with speed, so at some high speed you will get zero torque (which means your motor will stop spinning if you accelerate up to that speed). Normally with a BED at 12V and the simple NEMA17s, I can get up to about 15,000 to 20,000 microsteps/second before the motor has zero torque, which is about 10 revs/sec. Note that you HAVE to accelerate smoothly up to these speeds, you can’t just start from zero and jump right to 20Ks/s. Adjusting your current set pot on the BED can make a huge difference to your max speed and torque, as can adjusting your input voltage. Also note that on a normal Arduino, Accelstepper can’t go much higher than 4Ks/s. With a faster board like a chipKIT Fubarino Mini or Fubarino SD, Accelstepper can easily hit 20Ks/s or more.
The driver chip will get very hot, depending on your input voltage and current setting. At 30V input, you’ll be generating more heat with the driver chip than at 12V for example. I’d try adjusting the current set pot on the BED. Turn it down some, and you will notice the driver chip doesn’t get as hot. You will need to tune the current set pot for the smoothest (but still strong) stepping of your whole setup.
The power supply current you’ll need is not obvious, because of the way chopper drives work. For example, when I run a BED at 12V and am putting 1A through each coil of a motor, I only draw about 800mA from the power supply. This is because the current chopping action of the driver is sort of acting like a switching power supply, trading off a lower voltage to the motor for a higher current. Sort of.
So if you are running four motors through four BEDs at their maximum currents, I’d suggest you get a power supply with at least 6 or 8Amps to be safe.
Both of those boards use the same driver chip, the A4988. Both drivers can (theoretically) deliver 2A (or a bit more) to a stepper motor coil, with lots of heatsinking and forced air cooling. Both can deliver about 1.4A in ambient air with no heatsink or fan (this value depends a lot on what motor you’re running and what power supply voltage you are using - thus the 1.7A figure above. I normally say 1.4A to be safe and conservative.). There is no difference between the two with regards to this spec. The 11876s that you bought will work just fine. The only difference between that board and this one is that we added some pull-up and series resistors to the STEP and DIR pins.
about 7 months ago
topkara - the EBB is able to read analog inputs. However, updating to the latest firmware (e-mail me if you need instructions) will get you much more solid analog reading code. I recently re-wrote the analog sub-system, and it works a lot better now for a lot of people.
It sounds like your motors are compatible - however you won’t be able to get anywhere near the fully rated torque out of them with the BED. That may or may not be a big issue depending on exactly how you’re using them. The 2.8A means 2.8A/phase.
Have you measured the maximum HIGH voltage on the STEP and DIR pins from your PLC? If they are over 5V, then they may have damaged the BED. Your resistors should have prevented that problem, but measuring it (without the BED in place) would be a good double check.
if you disconnect your motor from the BED and then power up the BED, do you still see the same LED pattern?
You may have damaged the board, or it may have been defective. Have you contacted SparkFun?
Cheese - you have good points. The resistors for Vref should be tweaked to give a better range. The notes on the schematic should be changed and amended to better explain how adjusting the pot controls the current. There are a lot of subtleties to this though that can’t really be completely explained on the schematic and are, unfortunately, left up to the user to understand. For example, just because you set the pot at 1.5A doesn’t mean you’re going to get 1.5A/phase. It also depends on the inductance and resistance of the motor coil, as well as the input voltage to the BED. It will never be more than 1.5A if you set it there, but it may be less.
SparkFun has begin using Github to manage design files (which is totally awesome) and I will work on updating the schematic at the very least to better explain. Thanks for the suggestion.
Depending on what you need in CNC, yes it is. It does not natively process G-code, and it only has 2 axis of stepper outputs. However, it does have eight RC servo outputs, and both the Egg-Bot and Water Color Bot products use this board in a 2.5D type CNC configuration where the ½ Z axis is done with the RC servo.
No public wish lists :(
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