ShopBot Alpha Control Replacement
Posted: Fri Dec 28, 2018 7:38 pm
An ongoing project for me is to make as easy as possible the changeover from ShopBot's quirky controller to Centroid's Acorn. SB has done some strange things over the years and the connection to the OM Vexta driver is no exception. The OEM version uses an unshielded ribbon cable between the drive and their control board that terminates in an IDC header connector. I've made up some shielded replacement cables with flying leads to allow making connections to an Acorn system much easier.
As you can see in the video linked to below, the motor is capable of much smoother slower movement than with the OEM controller. .5ips (30ipm) is the minimum feedrate unless some specialized trickery is done. I have run feedrates as low as 5/100ths of an inch per second smoothly on ballscrew machines with Acorn Control. Another benefit is that the SB OEM controller has very limited pulse rate. So limited that on the alpha, their flagship machine, they use "electronic trickery" that sets the drive to output 5 step pulses for every one it receives from the controller. Depending on the software version this has been problematic as the "gearshift" would sometimes stick in one mode or another, making a mess out of whatever is on the table.
That gearshift has one other consequence. Resolution is reduced by a factor of 5 when rapids are active. Net result is less than 500 steps per inch resolution, way below even most entry level machines. Doesn't sound like much of a problem at a casual look, but since all positioning moves are rapids, and executed at the much lower resolution count accuracy can suffer. And don't get me started on the rounding errors!
The drive that was tested is an "off the shelf" unit. It does not have SB's weird programming and I have surely not used the "resolution multiplier" for the rapids. Resolution multiplier... sounds like something better is happening, right? Not so much. Resolution multiplier is like most cigarette boats.... someone is compensating for a small something. Pulse stream I would guess.
Anyway, here is the video:
As you can see in the video linked to below, the motor is capable of much smoother slower movement than with the OEM controller. .5ips (30ipm) is the minimum feedrate unless some specialized trickery is done. I have run feedrates as low as 5/100ths of an inch per second smoothly on ballscrew machines with Acorn Control. Another benefit is that the SB OEM controller has very limited pulse rate. So limited that on the alpha, their flagship machine, they use "electronic trickery" that sets the drive to output 5 step pulses for every one it receives from the controller. Depending on the software version this has been problematic as the "gearshift" would sometimes stick in one mode or another, making a mess out of whatever is on the table.
That gearshift has one other consequence. Resolution is reduced by a factor of 5 when rapids are active. Net result is less than 500 steps per inch resolution, way below even most entry level machines. Doesn't sound like much of a problem at a casual look, but since all positioning moves are rapids, and executed at the much lower resolution count accuracy can suffer. And don't get me started on the rounding errors!
The drive that was tested is an "off the shelf" unit. It does not have SB's weird programming and I have surely not used the "resolution multiplier" for the rapids. Resolution multiplier... sounds like something better is happening, right? Not so much. Resolution multiplier is like most cigarette boats.... someone is compensating for a small something. Pulse stream I would guess.
Anyway, here is the video:
Let me step behind you before you hit the button!