﻿Speed mode is ideal for accurately controlling things like the speed of a
spindle on a CNC or a conveyor that has to travel at exactly the right speed.
In this video, we are going to get the speed modes up and running quickly
using mostly default settings. To switch to speed mode - with the
Servo motor disabled – we just go to parameter 1.001 and change the mode digit to a 2 or a 4.
We’ll start with mode 2 and once we understand how that works, we’ll take a look at mode 4.
There are three ways you can control the speed of the motor.
First, you can use a +/-10 volt analog input to control the speed.
That voltage swing is normally provided by the PLC or sensor, you are connected to.
In this demo, we’ll use a 5K potentiometer across a zero to 5 Volt supply to keep things simple
and so I can show you how to scale the analog input. You can also filter
it to minimize noise and help smooth the transition on fast analog voltage changes.
The second way to control the speed is by selecting one of three preset speeds.
The third way is to simply fill these guys in via coms.
Check out the communications videos to learn how to do that.
You can apply an S-Curve to all of these so when you switch from one speed to the other you get
a nice smooth transition which prevents a sudden speed change from jerking your machine around.
The analog input or one of the three presets are selected, using two digital inputs. We’re going to
use digital inputs 9 and 10 in this example wired as normally open switches with an external 24-volt
supply. We need to tell the drive that’s what they are being used for in parameters 2.036 and 37.
Remember that the two rightmost digits are the function code
and the leftmost digit tells the drive we are using these as normally open switches.
A zero in that first digit would tell the drive we are using a normally closed switch.
I’m not showing the wiring here to save space, but I also still have the servo enable, alarm reset,
estop and overtravel limits wired like they were in the quick start video. Those are all defaults
so there are no parameters to change for those. There is a lowpass filter here to help reduce
noise and one final catch-all speed limit. Parameter 1.055 limits the max speed of the
motor which by default is the rated speed of the connected motor. So no matter what stupid thing
you do back here the motor will never be over driven. We aren’t going to use it in our demo,
but I did want to make sure you were aware it was available because you might want to
set it to your max process speed to protect your system, for example.
That’s what we'll do in this video. We call this the command section where we tell the
drive what we want it to do. There is a whole other section that given this command stuff
then tries to maintain the speed of the drive no matter what. That’s where the gains, and inertias,
and tuning and resonance filtering kinds of things are taken care of. That’s the Speed Control portion.
Given this diagram, enabling speed mode is easy. Here is a list of all the parameters from the
previous diagram. I went ahead and did the Factory Reset and power cycled the drive – because you
always want to power cycle the drive after a Factory Reset to ensure any residual settings
get cleared out. The blue parameters are factory defaults, so I then entered the
remaining 6 parameters. This is important: you should also power cycle the drive whenever you
change the mode so you are ensured of a clean start … AND when you change I/O functions. I
assume that’s because the I/O’s are probably configured on power-up. Regardless, it’s a good
idea to do the Digital I/Os last so one power cycle covers anything you might have missed.
The speeds are in rpm with one implied decimal place. So, this is really 120 rpm.
This servo is capable of doing thousands of rpms. I’m intentionally keeping the
speeds low here to make it easier for you to see them change in the video.
And while we are not going to mess with it in this video, I did want to make you aware that you
can enable torque limits and specify a different amount of torque for each of the four channels.
We’ll leave them at the blue defaults for this video, which is 100% of rated torque.
You can also enable torque limiting via a digital input and you can monitor when torque limit is
active with a digital output. Well, let’s try it.
I have these two selector switches connected to the digital inputs so you can see what is
going on. Normally these would be connected to your controller. Let’s enable the servo motor
and I’ll change the display to show the shaft speed in rpm.
The two-speed selector digital inputs are off right now so we’re looking at the analog speed
control. If I rotate the potentiometer, yep the speed changes. If I rotate all the way to 5 volts
we see roughly 1500 rpm. Why? Because parameter 1.040 specifies the max rpm
to be 3000 rpm at 10 volts. So the 5 volts we are giving it is at 1500 rpm.
If I want 5 volts to give me 3000 rpm, I need to tell the drive 10 volts should give me 6000 rpm.
So I’ll go to parameter 1.040 and change it to 6000 rpm, select that, and sure enough, we get 3000
rpm at 5 volts. That’s all there is to scaling. If I select speed 1, we get the 6 rpm,
speed 2 we get the 60 rpm and speed 3 we get the 120 rpm we asked for. Perfect
There is one problem. Suppose you want to use this analog channel as your zero speed
so you can have zero, low mid, and high-speed settings, for example. Well, trying to get an
analog input exactly zero is almost impossible. I don’t know if you can see this, but when I rotate
my potentiometer to zero, the shaft is still turning and it’s actually turning backward.
And that will drift over time and temperature and someone could even bump the potentiometer.
So is there a way to GUARANTEE this will be absolutely zero? Yep. Remember we said there
are two-speed modes? We are currently using mode 2 which is the standard speed mode. If we set that
to a 4 we get something called Speed Zero mode. It’s the EXACT same thing as we have been doing,
EXCEPT, it breaks this connection, and forces this to a true zero. Of course, scaling and
filtering doesn’t do anything when the speed is zero so those aren’t necessary in this mode.
Again, just set this to a 4 and you have Speed Zero mode. So I’ll disable the motor. Change
parameter 1.001 to a 4 and the display reminds us to do a power cycle. Remember – we always need
to power cycle the drive after a mode change. Now if I enable the motor, the speed potentiometer
does nothing. But, the other three digitally selected speeds are still working. Perfect.
The S-Curve option lets you specify how smooth the transition between speed changes will be.
Parameter 1.034 sets the acceleration time, parameter1.035 sets the deceleration time
and parameter 1.036 sets the smoothing time at each transition. These are independent, so you
can have a zero transition time and still set the acceleration and deceleration times to create just
a regular trapezoid if you want to. Notice that this affects the analog speed transitions too.
And finally, you can remove the high frequency noise using this lowpass filter.
You may ask, what is the difference between this filter
and this one? This one is a straight low pass filter which means it rounds off the
sharp corners. This one is a moving filter or averaging filter, so it smooths everything.
In general, you will want to use the low pass filter if the commanded speed is
coming from a position command (a point to point move). Use the Moving filter if the
commanded speed setting is only for the speed control, that will give you better smoothing.
They both create delays and make your system less responsive so always keep
any filtering you add to a minimum. A zero for either of these turns the filtering off.
That ought to be more than enough to get you started with Speed Mode.
And while Speed Mode is simple in concept, there is a lot of stuff going on under the hood that’s
required to optimize your system. The good news is, Auto-Tune does an amazing job of taking care
of all of this, so you really never have to worry about it. Could you tune it manually yourself?
Sure, but if you watch this Auto-Tune video, you’ll quickly realize that most of the time
there’s absolutely no point in doing it manually yourself.
Auto-Tune takes full advantage of the drive’s 3.1KHz bandwidth
to produce optimal results for you. Meanwhile, click here to learn more
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