Configuring the Drive

Double-click on the drive under the I/O. All of the configuration is done under the Drive Manager tab. The first thing you want to do is setup the motor and feedback. If you’ve purchased a Beckhoff motor, this is simple. In the tree, go to Channel A->Configuration->Motor and Feedback. Click the Select motor button on the right:

That will open the Select a Motor dialog box. You will need the motor part # from the motor nameplate, or from the motor ordering information. TwinCAT 3 includes a datasheet for each motor, so it knows all the motor characteristics and feedback characteristics. Select your motor and click OK.

TwinCAT 3 will then display a dialog box titled, Power supply and extra settings for:

This is a “gotcha”. Most people would think this means you should specify what voltage we’re supplying to the drive, which in the United States would be 480V, 3 phase, 60 Hz. However, since I specified a Beckhoff motor and it was designed for Europe, I actually must keep the voltage set to 400V or I get an error (the voltage has to match the motor voltage). This was several TwinCAT 3 versions ago, so I don’t know if anything has changed since then, so it might be a good idea to check with your Beckhoff representative about this. Of course, these are still power supply settings (relating to the incoming power) so if you set the voltage to 400V with 10% tolerance, and connect it to 480V, the drive will fault because the input voltage is too high. Since the drive (and apparently the motor) is actually 480V tolerant, the “correct” way to handle this situation is to select Other settings, and then manually edit the U+rng setting to 30.0% (this is the maximum value allowed). This will allow your voltage to go up to 520V which is an 8% over-voltage allowance based on 480V. This means the drive will actually work from 360V up to 520V. Here are the settings:

Note: in my latest project (spring of 2016) I was able to specify 480V for all the servo motors, so it looks like Beckhoff has updated the software to fix this problem since I wrote it.

Click OK.

Note that if you’re in Canada, the typical industrial voltage is 600V. Unfortunately this means you’ll have to use a down transformer to use these drives. Since there are also many other devices that can run off 480V, it’s typical to step-down to just 480V instead of 400V, and use the settings shown above.

After setting the power supply settings, it will ask if you want to set the scaling and parameters now. If this is your first time adding a motor to this drive, say OK, but if you’ve already modified these parameters and you’re just selecting the motor again (or selecting a new motor), you’ll probably want to say Cancel.

If you click OK, then it will display the Scaling and NC parameters node in the drive manager tree:

Make sure you set the Feed constant (highlighted). Assuming you’re driving a linear axis of some kind (rail or ball screw), this is the distance that the linear axis moves in one revolution of the motor. As soon as you set it, click the Set NC Parameters button. This will set the defaultNC parameters (below).

You need to set reasonable motion limits for the axis (this is done in the Axis node underAxes, on the Parameters tab). Make sure you do this before trying to move your axis. This will depend on your application. For instance, it sets Maximum Velocity to 100% of the maximum motor speed. However, the mechanical limitations of your machine may not be able to tolerate that speed. You have to calculate that maximum linear speed your system can handle, and set this parameter accordingly. Similarly, set reasonable limits for Manual Velocity (a.k.a. “jogging”). The automatically calculated values are typically much higher than you would want during system testing and start-up. Calibration Velocity is used during the homing routines.

Finally, setup the digital I/O on the drive. Not all drives have Digital I/O, but most at least offer inputs for limit switches.

These are important, so let’s take a short detour on the topic of axis limits. An axis can have up to 3 sets of limits:

1. Software Limits 2. Hardware Limit Switches 3. Hard Stops

Software Limits are software-only limits on the axis travel. They are practically free, but they’re also the least useful. First of all, software limits are only useful after the axis has been calibrated (homed), so they won’t protect you during system startup, or during the homing operation. Secondly, software limits depend on the motor’s encoder value, so it won’t catch

certain classes of failures such as a shaft coupler break, a belt break or a tooth skip on a belt. You set your software limits in the Axis node under Axes, on theParameters tab:

Hardware Limit Switches are physical limit switches that sense the axis has moved beyond the expected range of motion.

Ideally these should be mounted to sense the carriage position (after the belt, if you have one, or after the shaft coupler or anything else likely to break). Typically these are normally closed switches, proximity sensors, or through-beam type sensors. The state of the sensor should be “on” when the axis is in the “ok” range, and “off” if it exceeds the limit. This protects you against a cut wire or blocked sensor. They need to be positioned far enough away from the end of travel to allow the axis to decelerate from maximum speed to a stop (or almost to a stop) before hitting the Hard Stop.

The Hard Stop is a physical limit on the axis. Ideally this needs to be able to take the impact of the axis hitting it at full speed, though this is rarely possible on heavier loads. Note that shock absorbers can be installed as well. If you’re wondering why the Hard Stop needs to be able to take an impact at full speed, consider the worst possible scenario: a power outage, or some kind of motor cabling fault while the axis is moving at top speed and near the end of the axis travel.

The drive won’t be able to decelerate the axis, even when it trips the hardware limit switch, so the only option left is a crash. Thankfully this is rare, but it should be designed against if possible.

Your specific choice of axis limits is application-dependent. However, on a typical ball-screw or belt-drive type axis I suggest the default should be normally closed Hardware Limit Switches, placed at a reasonable deceleration distance from a physical Hard Stop capable of stopping the axis in the event of a full speed impact. Given that scenario, I think Software Limits are unnecessary. As always, work with your mechanical designer(s) to find a reasonable solution for your application.

An AX5000 series drive has optional I/O that you can configure for your application, including positive and negative overtravel limit switches. In the drive’s Device Managertab, under the Device->Digital I/O node in the tree, see the Digital I/O settings box. In the AX5000 series, find parameter P-0-0401 and expand it:

Expand Positive limit switch and set the following 3 parameters:

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Configuration and Input number are self-explanatory. For the Limit switch reaction, note that Beckhoff drives categorize events into 3 categories: Class 1 Diagnostic (fault), Class 2 Diagnostic (warning), and Class 3 Diagnostic (status), a.k.a. C1D, C2D, and C3D.

Simply, C1D is a “drive shut down” error, kicks the drive out of OP mode into ErrSafe-OP mode, and requires you to execute a drive reset command to clear the error (from the PLC, this is accomplished with an FB_SoEReset). This seems rather harsh for an overtravel limit fault, which really isn’t an internal drive, motor, or feedback error. I lean towards using a C2D warning, with an E-Stop. Ultimately it’s up to you to choose a reasonable configuration. Does a Hardware Limit Switch fault in your application indicate something internal has malfunctioned?

For selecting E-Stop vs. Axis halt, this refers to programmable error reactions. In theDrive Manager tab, go to the Channel A->Configuration->Error reaction / drive Halt node in the tree:

Here you can set your deceleration rates in the event of an E-Stop or Drive Halt error reaction.