4442
« on: May 28, 2018, 07:57:38 AM »
Hi,
the spec, BEMF/1000RPM=39V, seems pretty plain. This is known as the Back Electro Motive Force constant, hence the acronym BEMF.
If the motor were driven by another motor at 1000rpm there would be 39V at the terminals. This is an important figure of merit for a servo.
Doing the calculation with a figure of 7V per 1000 rpm, which I suspect is in fact the tacho feedback device, then the shaft power of 800W could only be sustained
for a short period of time. With a figure of 39V per 1000 rpm that output could be sustained continuously. This is a more realistic and more probable than my previous
reasoning. I think it wise to consider the existing servos to be in the region of 800W continuous.
That has implications for your rebuild. You would ideally replace them with something of the same power but that is going to get expensive quickly. If you were doing
this for commercial purposes it would still be pretty cheap but if you have to convince your wife that you really really need it she might not believe you! You could use smaller
servos and have either a gear or belt reduction, that would increase the torque to that necessary to overcome friction and inertia but be a little slower. That would probably be
a good compromise for a hobby machine.
As I've pointed out the medium inertia 750W DMM servos and drives are about $412 an axis. They could be relied upon to be a direct replacement for your existing servos.
You could save a bit by getting 400W servo and drives, $385 per axis. You'd probably need a belt reduction of 2;1 to match your existing servos, but that would be very doable.
If you choose to retain the two X and Y servos there are some choices to make. Given that the Gailil is only two axis its no good as a controller. If you chose a HiCon board from
Vital Systems AND paid the premium to get the analogue features enabled the you could drive your two servos and still have the capacity to add another or even more than
one for the Z axis. A HiCon with activations is about $1200-$1300. Note this would allow you to use the two servos AND your original drives.
If you replaced the drives with Geckos or something similar then you could use a much cheaper controller like an ESS or a UC300. Either controller would allow you to feed the two
Geckos with step/direction pulses and still have plenty of capacity left over for the Z axis. You'd still need a servo and drive for the Z axis, say $400 and two Geckos say another $300
for the pair, for about $700 total.
I think you'd be very impressed with AC servos but despite them becoming ever more affordable they are still somewhat of a premium. If you were to try to buy NEW DC
servos, then I suspect they would be even more expensive.
I was given a 100W DC servo off a customers plasma table, this table is $150,000 or more and the whole business of thirty or more people rely on its output to stay
working. It has a faulty line driver in the encoder. I can probably jury-rig a new line driver, this servo is now only a spare. But no, we want a new encoder, I found the
OEM and priced getting a new encoder made to suit, $585USD, prepaid, one month leadtime plus freight to New Zealand, and we want a markup....Funny that they didn't
seem that keen once the numbers were put in front of them. I could get TWO NEW AC servos and matching drives with more power than they could get one encoder!
Even more, the business is about 4 doors down from the local distributor of Omron/Yaskawa. I could walk in get what I need off the shelf and Yaskawa is serious good quality
but they cant get their head around AC servos......they insist that servos have to be DC, how else can they be closed loop they ask. I've given up trying to explain to them how they
work....When the table craps out we'll see how amenable to alternate solutions when they're out of action!
Sorry about my rant. The point is that DC servos are rapidly going obsolete.
Craig
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