The SureServo 2 system natively has almost 17 million pulses per revolution – that’s
24 bits of resolution. And unlike many systems out there where if they do have 24 bits of
resolution, it’s only for a few select high-end expensive models. That’s a key difference
- every Sure Servo 2 System has 24 bits of resolution.
But, since your general-purpose PLC typically can send only around 1 million pulses per
second it will take almost 17 seconds just to rotate the shaft ONCE!
Which is why we use electronic gearing – it allows you to adjust the servo system, so
it can be more responsive to your controller. The drive uses parameters 1.044 and 1.045
to do that. If you leave parameter 1.044 at the default – which is that 24-bit resolution
number - then whatever you put in parameter 1.045 will be the number of pulses your controller
needs to send to get one revolution. The Sure Servo 2 system refers to that as Pulse User
Units, or PUU. For example, The SureServo 2 system defaults
to 100,000 pulse user units which means it should only take 100,000 pulses to rotate
the shaft 1 revolution instead of 16 million pulses.
So I set up a BRX PLC with a high-speed differential output module to send 100,000 pulses at 100,000
pulses per second to the sure servo 2 drive. So we would expect to see one revolution in
about 1 second. I say about one second because it takes time to accelerate and decelerate.
Yep, that was one revolution in about 1 second. Perfect. I’ll send the same pulses but in
the other direction and we are back where we started.
If I change the gear ratio and double the pulses per revolution – oops! The drive
is telling me I can’t change the gear ratio while the motor is enabled, so I’ll disable
the motor, double the gear ratio again and re-enable the motor. Now the 100,000 pulses
sent by the controller only rotates the shaft halfway. Exactly what we expect. And I’ll
send it back again. So with electronic gearing, you have control
over how many pulses your controller needs to send to get one revolution of the motor
shaft. But, this isn’t like a stepper. In a stepper
system when you change the pulses per revolution, you also change the resolution and accuracy
to be that many steps. In the SureServo 2 system the servo motor’s encoder ALWAYS
use 24 bits of resolution. The drive automatically converts your controller pulses using the
electronic gear ratio to get back to the native 24 bits of resolution of the motor and encoder.
Which means, you don’t lose any resolution no matter what you choose for the geared down
controller pulses per revolution. And remember – EVERY SureServo 2 system has the 24 bits
of resolution. And while having that incredible 24-bit resolution
is great – it’s important to understand that it’s the SureServo 2’s 3.1KHz bandwidth
that allows the current loop to give you a fast and efficient system response. So the
3.1KHz bandwidth coupled with the 24-bit resolution is what makes the SureServo 2 System so responsive
and accurate regardless of the gear ratio. One of the cool things about the SureServo
2 system is you actually have 4 different gear ratios to choose from. You just configure
any two digital inputs to select one of four gear ratios you specify in these parameters.
One word of caution, remember a minute ago the drive wouldn’t let me change the gear
ratio while the servo was enabled? That’s because the new gear ratio takes effect immediately
which could potentially damage your system if you enter a wrong number, right?
These guys don’t have that protection. The SureServo 2 system assumes that you have
tested these numbers and they will work in your system regardless, so it will let you
change these gear ratios on the fly while the servo motor is running. But I’m still
going to caution you against changing gear ratios on the fly unless you REALLY know what
you are doing and have thoroughly tested everything and know it is going to work. It’s a powerful
feature, but you will need to be extra careful in using it while the system is running. If
you get it wrong, you could really mess things up.
Of course, it’s an awesome feature if you need to just quickly switch process speeds
between runs while the motor isn’t enabled. And that’s really all there is to electronic
gearing for pulse train mode. Electronic gearing also affects the position
register mode. For example. I used the SureServo 2 Pro software
to set up the motor to move back and forth between two positions, 1 half rotation each
way. There’s a separate video showing you how to set up position register mode using
the free Sure Servo2 Pro software that you can download from the AutomationDirect.com
web site. If I disable the servo, then cut the gear
ratio in half, and re-enable the servo, and get the path running again, we see it is now
rotating a full rotation for each move. Why? Because we told each move to go this many
controller or PUU pulses. And since we cut the gear ratio in half, the same number of
pulses go twice as far. And yes, position register mode can also use
the 4 different preset gearings. And, all the same cautions apply, so please be careful
if you choose to change gearing while the motor is running.
That should be enough to get you going with electronic gearing in a SureServo 2 system.
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