General
Axis commissioning particularly requires knowledge and use of the safety functions of the NC, application of the necessary safety precautions, and the observance of a particular sequence of commissioning steps.
| |
Risk of injury due to movement of axes! The commissioning results in a movement of axes.
|
| |
Incorrect axis position during initial commissioning Without referencing / calibrating the axis position, the displayed axis position may deviate from the actual axis position.
|
General safety precautions
Make sure that you know about the NC safety functions, and make use of them.
Handling the emergency stop case is one of the most important and safety-relevant features of a machine. For this reason it is usual for an additional hardware safety circuit to be built on top of all the logical (logical axis enables) and software enables (PLC: controller and feed enables). Extensive statutory regulations exist governing the reaction and the implementation in hardware of the emergency stop system. Here we shall only make reference to these.
Within the limits prescribed by the statutory regulations, there remains often a degree of free choice as to how, for instance, NC axes are stopped the in the event of an emergency stop. In some cases the mechanical properties of the machine mean that it is not always possible to bring axes that are moving to an abrupt halt (e.g. to bring an analog interface abruptly to 0.0 Volts). It is therefore quite common for NC axes to be slowed following special braking ramps in an emergency stop situation, after which they are then electrically and mechanically locked.
One of the first procedures for commissioning a machine is to test the emergency stop circuit. We of course begin with safe, standard situations (the axes are stationary under emergency stop conditions, although special attention must be paid here to axes that move vertically). The testing then moves on to more complicated cases (axes moving slowly, then axes moving rapidly).
It is possible, in order to test and commission rapidly, to set the logical axis enables (controller and feed enables) and the velocity override of the axes from the TwinCAT System Manager "Axis online" for the particular axis. This is, however, only possible if the axis interface is not involved in cyclical exchange with the PLC (mapping between the NC and PLC tasks). In that case, the single write of information from the System Manager would immediately be overwritten again by the cyclic data exchange. It must, however, be pointed out that this procedure can be very dangerous, because the usual safety monitoring and manipulation facilities of the PLC are disabled. This property can be used without risk on an axis test stand with endless axes. It is extremely inadvisable to use this facility on a real machine.
System Manager: Axis: Online: Set
Essential preconditions
The description below covers all the steps of commissioning an axis. Several types of axis and a range of different situations are covered. In each step, all the settings which must be made are mentioned, even when they are identical with the settings of the previous step. In practice, you will only actually carry out of some of the partial steps described for an axis.
If you already have experience of axis commissioning under TwinCAT, it will not always be necessary to carry out every step. In that case, you should use this description as a memory aid and for assistance in problem cases.
Essential preconditions:
Before you can begin the commissioning itself, a number of preparations must be made.
- Check their completeness and correctness of all the electrical connections.
- All parts of the axis (encoder, drive, controller, PLC interface) must be of appropriate types, and must be provided with the correct resources (PLC variables, I/O hardware in the fieldbus etc.).
- The NC architecture must not only be created, but must also be written into the registry, and TwinCAT must be started with it.
Recommended safety precautions
The following safety precautions should be taken if at all possible.
- Ensure that no one can enter or reach into the machine: the machine can behave unpredictably during commissioning.
- Isolate those parts of the equipment on which you are currently working. Anything that is not required at any particular time should be securely brought to a halt - it can only be a source of interference or distraction.
- Inform all personnel in the area that there is an increased danger of accident.
- Make sure that you will not be disturbed: nobody should be nearby without good reason.
Sequence
It is necessary to observe a particular sequence, because the individual steps of axis commissioning are logically built one on another. The sequence, however, does depend on the particular axis composition, i.e. on the combination of the encoder, controller and drive types.
Stepper motor drives
This section only concerns stepper motors.
Masks are used to send drive control signals to a stepper motor. These masks are packages of eight bits that correspond to the motor's control signals. The signals are sent out in one phase, and switch the associated motor coils on. A particular mask is output when the motor is stationary, and has the purpose of switching on the holding current. The masks depend on the motor drive unit in use and on the wiring of the control cabinet, which means that a universally valid version cannot be given.
Find the bit combinations from the documentation provided by the drive manufacturer or by the electrical constructors, and enter them on the drive's stepper motor tab. If, during later commissioning steps, the axis behaves incorrectly, these masks a should be checked first.
The mask is entered in the form of a decimal number. In order to convert a bit pattern into the value that must be entered, the weighting must be determined for each bit that is set.
Bit | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
Weight | 1 | 2 | 4 | 8 | 16 | 32 | 64 | 128 |
Example: In the bit pattern 00101011, bits 0, 1, 3 and 5 are set to "1". The associated weights are 1, 2, 4 and 32, so the total to be entered is 39.