Alpha Control board with PRS steppers

[khaos]

Can an alpha control board discover missed steps from the 7.2 steppers that come with PRS? Or are the motors themselves the reporting unit?

My end game desire is to be able to handle missed steps. Not really looking for a faster cut.

[angus_hines]

Turn off you screen saver.....LOL

[Brady Watson]

No. The ShopBot electronics do not handle positional feedback of the steppers. This is handled internally on an Alpha-motor equipped machine, by the motor driver itself and the stepper motor's internal encoder. PRS Standard (4G) machines are open loop, unless of course, you decided to hook up another motor driver to the board that handled positional feedback internally, like an Alpha.

Regardless of which setup you have, I would focus on why you are losing steps. If you are running a 7.2:1 Alpha, and you are losing steps, something is not right. Explaining what you are doing, cutting, speeds, what you want to do etc - goes a long way towards getting your questions answered accurately.

-B

[richards]

All stepper motors will loose steps if they are pushed too hard, including the Alpha motors. Depending on the settings on the Alpha drivers, the Alpha motors can have about 1/2 second of non-movement before they report an error and fault. Depending on the speed of the cut, that 1/2 second delay can lead to serious problems. At 8-ips (MDF, particle board, plywood), the Alpha could be four inches out of sync before it faults.

By slowing things down so that the motor can do its job without missing steps, you will get optimum performance.

[khaos]

Sorry guys I have been out of pocket (out of State Funeral)

Thanks for all the responses.

The question came about when I Jogged across the table and the jog was apparently too fast at 9ips ... This seemed weird as it had already ramped up and moving smoothly. I don't have a lot of hours on the machine (less than 200). I cleaned the rack & pinions off completely and re-greased evenly (and dust free). I have lost steps cutting at 6ips. I am not sure what the deal is as I cant reproduce the loss dependably. It does not seem to occur on the same area of the table its also not common.

I was curious about 'how' the steps were monitored. Motor to driver closed loop is the answer.


And how difficult it would be to get the benefit of that feature. Seems that at a minimum new drivers and matching motors is the answer. Assuming that my existing board could even handle it.

Not a likely mod on my part. primarily because of the bucks. :o

[richards]

Joe,

If you're handy with electronics, you can easily build a 'pseudo' missing step detector for any stepper motor. You will have to add an encoder to the motor. http://usdigital.com sells the E6 size for about $80 each. Some steppers have the shaft extending out the rear of the motor for easy connection to an encoder. With other motors you would have to attach the encoder in some other way.

With a $2 microcontroller (like the AT89C2051) you program the microcontroller to count the pulses going to the stepper motor. You subtract the encoder pulses coming from the encoder (assuming 1:1, otherwise you use a ratio). When the difference in the counts exceeds a preset number, the microcontroller sends a fault signal. Those that want to get a little more exotic could add an LED display to show the difference in steps/encoder pulses. Total cost per axis is about $100. A PK296A2A-SG7.2 motor with rear shaft costs about $260 each. A Gecko G203v costs about $150. The encoder w/controller costs about $100. Add in $30 for 1/4th of a 35VDC power supply (four motors on the machine). Cost per motor is about $550. Compare that to the AS98AAE-T7.2 Alpha 7.2:1 motor that lists for a little over $1,300 each from Oriental Motor.

Save $800 per motor and still have feed-back.

[khaos]

Great info! Thanks Mike.