Machine Control Networks

Ethernet networks are finding their way into advanced production equipment like CNC machines. For a part, they thank their popularity to the flexibility they offer when it comes to designing and configuring the machine.

A typical modern CNC machine contains a respectable number of components. Thanks to the networked topology a lot of trouble is taken out of the initial setup. Components are all connected in the same manner, with standard connectors and cabling; and once connected, the controller can automatically detect the functions each of these components makes available. As communication between components is standardized, moving, adding, changing or removing some of them during the design stage is fairly straightforward.

Because Ethernet networks offer low latency and high transfer rates, endpoint devices can be kept rather simple. There is no need to provide them with lots of logic when raw data can be sent over the network to the central controller and processed there. In the following we're going to have a look at how this can benefit the machine's servo drives. This is how a servo drive is typically connected, supplying 3 phases to the motor and receiving feedback of the motor's position from an encoder.

A servo drive like the one above needs to perform a number of complicated calculations in order to guaranty proper functioning. It needs to keep track of position, speed and acceleration of the motor based on the feedback of the encoder, after which it calculates the compensation for the error between desired and actual values. Then the switching algorithm calculates the sequence of pulses that are to be sent to the transistors.

An alternative would be to send the information from the encoder over the network towards the controller, and let it do the necessary calculations. The central controller could be an off-the-shelf computer, with processing power way cheaper that that on a typical servo drive. Once computations are complete, the controller can send the required control signals back to the servo drive.

With a setup like the one above, the servo drive module could be greatly simplified on the side of control circuitry; all the power components would of course stay the same. In below design the IGBT controller sets the pace for each of the transistors in order to generate the desired output to the motor. It would need to be connected to the Ethernet network in order to receive commands from the machine's central controller.  

Electrical Symbols

To be able to read a machine plan, one needs to know what all those symbols stand for. The following is a short explanation of the most frequently used.

This is a normally-open contact. If you push it, the circuit closes and it passes potential and current. When not pushed, nothing passes.

Contrary to the one above, this is a normally-closed contact. The circuit opens when pushed, otherwise it is closed.

Switching a relay can cause voltage peaks in the circuit. To avoid the latter having a detrimental influence on the circuit, a relay with a diode is used, this to channel superfluous current.

Often a relay with R/C circuit is used to avoid arcing when switching the relay. The voltage peak that could causes the arc is handles by the R/C circuit.

This is a push-button, it lets current pass when pushed and bounces back to its open position when force is no longer applied.

This illuminated push-button lights up when pushed. On some of the less well-designed types your finger sadly covers the light.

The circuit of a key selector can only be closed or opened by use of the correct key. It is for example used to put a machine in programming mode.

A flow sensor closes when a flow of some liquid or gas is detected. It is used when flow needs to be detected but not measured.

This inductive sensor closes its circuit when a metal object comes close enough.

A pressure switch closes depending on pressure. The part on the left of the symbol represents the membrane typically used in these devices.

When coming in a strong enough magnetic field, this magnetic sensor closes its circuit.

A limit switch closes a mechanical contact when approaching the end of a trajectory.

An emergency button. When pushed it opens the circuit and keeps it open. Mostly these buttons need to be turned to close the circuit again.

This is a differential circuit breaker. The circuit opens when the current going to the lines is different, this as a safety measure.

Another safety measure is the thermal magnetic circuit breaker. This device quickly opens when too much current is flowing through the circuit.

A three phase AC motor. The three phases are connected to the left, the neutral conductor is the one leaving at the right side of the symbol.

This is a 3-phase electrical AC motor like the previous, but it contains two windings instead of one. This allows it to run at two different speeds.

A three phase motor like the ones above sometimes come with a mechanical break. It mechanically holds the motor shaft in position.

This is an AC/DC converter, as the name says, is converts alternating current into direct current.

This symbol represents a transformer, which derives its name from the fact that it transforms between potentials.

An electrically controlled, or actuated valve.

This is a connector, which is like a plug. A wall socket could for example be represented like this, as could a microphone connector.

An grounded outlet, like the ones at home.

An electric lock, typically seen on the doors of CNC machines to avoid the operator from opening them while the program is still running.

A horn makes noise when actuated. A horn of a car is a good example of this, several production machines however have horns to.

A light bulb.

Signalization lights, which indicate the current status of the machine. Green might refer to normal operation, orange might signal an operator intervention to be necessary, and red usually signals an error.

EN9100 Quality Management System

The standard EN 9100 defines the requirements for implementing a quality management system in companies that design, develop, produce, install or service aerospace products. As a quality management system it is based on EN ISO 9001, and among other things derives its thinking about a company in terms of processes from the latter.

Company-wide implementation of a quality management system according to EN 9100 requires, first and foremost, rethinking and describing how the company works in terms of abstract entities called processes. As a matter of definition one could say that a process takes something, no matter what, and transforms it into something new. When making soup, for example, the cooked vegetables would be the inputs, the act of mixing itself would be the process, and the resulting soup would be the output of the process. Business processes should be described in a similar manner; they take something as their input, perform some transformation/work on them, and as a result deliver some kind of outputs. The sales process for example starts from a list of possible customers as it's input, contacts, meets and convinces these customers, and generates signed deals as its output.

Aspects of a company's working can be described at different levels of detail. At the highest level of abstraction a company may be thought of as taking customer orders as it's inputs, processing these in a number of steps, and delivering finished goods as its outputs. When thinking of how the company functions in a little more detail, a number of top-level processes will become apparent. In a typical industrial company one might find the following processes:

After having defined a sensible number of top-level processes, a next step analyses how these processes communicate with each other and with the outside world. Processes are said to communicate by means of interfaces, at which they take the outputs of another process as their inputs, or generate outputs to be used by other processes. The production process will for example take its inputs from the purchasing process, under the form of raw materials, from the planning process in terms of priorities and machine occupation, from the design department for machine instructions, and so on. At its turn the production process might feed its outputs to the delivery department, more then likely these will however pass by quality control and quality assurance departments first.

Once the company has been described in terms of top-level processes and interfaces between them, a next step in implementing the quality management system consists in judging if a process is functioning well or not, with other words, evaluating its performance. How well a production process function could for example be measured by the number of parts it produces in a week, or the benefit that's generated from selling them, one could however also bring this number back to what is generated by each person or machine. This performance analysis of a process is an essential part of the EN 9100 quality management system, and it is one of the main reasons why the norm requires the generation of an abstract company description that allows such performance measurement.

Apart from requiring an abstract process description to be created, the EN 9100 standard furthermore enforces a number of other requirements onto the different processes. Independently of the kind of products or services a company offers, or to which standards of quality it performs those, each certified company needs to uphold good practices in terms of:

All companies considered to have a quality management system respecting EN 9100 can be found in the OASIS database (Online Aerospace Supplier Information System).