Servo control is about dumb or intelligent choices

In this article we consider the two types of servo control (dumb and intelligent) and consider the factors that determine the machine elements for each.

The availability of intelligent, low-cost servo systems that are easy to use means that servo technology is increasingly being employed on machines which were once the preserve of mechanical variable speed or inverter variable speed drives.

In addition to the performance gains resulting from this development there are wider possibilities for achieving servo motion control using both 'dumb' and 'intelligent' drives.

A 'dumb drive' is one that does not possess the intelligence to process signals and run its own software routines.

On the other hand an intelligent drive can be defined as one that can handle mathematical equations, software routines and a range of I/O.

When used in motion systems, the control of these drives can be achieved using a PLC, a PC or a specialist motion controller, with manual interface requirements taken care of by an HMI.

In this article we consider the two types of control (ie dumb and intelligent) and consider the factors that determine the machine elements for each.

All servo drives can move a load from position A to position B; without motion control this movement is achieved through velocity feedback.

In contrast, a servo drive that is operating with motion control is continuously calculating both velocity and position feedback to achieve far more sophisticated levels of control.

A typical example is a flow-wrap machine for confectionery.

Here the product feed and the wrapper feed must be synchronised, but there is an additional requirement for the logo of the wrapper to be positioned centrally on the product.

The starting point for all motion control systems is single axis control.

In these types of applications the best choice for machine designers is an independent, intelligent drive such as Lenze's Servo PLC.

This unit is cost effective; it can communicate directly with a separate HMI and also with a choice of fieldbuses through convenient plug-in modules.

Although there are limits to the amount of I/O that intelligent drives can handle, the capacity is acceptable for practically all applications.

The processing power within the drive can be used to control other devices, for example valves and guards.

Finally the intelligent servo drive is a good solution for modular machinery where expansion can be achieved simply by adding slaves, which communicate through a system bus to the master drive.

A further argument for using intelligent drives is that programming of motion control tasks need not be difficult.

The use of PLC language IEC61131 is becoming common with the added benefit that programmers are already familiar with the language.

Also drive suppliers sometimes offer dedicated drive software at attractive prices and with preconfigured motion routines.

Although intelligent drives provide the control solution where greater levels of integration and communication are required, there are many other straightforward single axis applications where dumb drives are eminently suitable, not just because of their simplicity, but also their cost.

Market demands mean that many of the lower-power servo drives offered today have limited intelligence, the Panasonic servo range marketed by Lenze being one such example.

This type of drive needs additional hardware to achieve motion control, in the form of a PLC, a PC or a specialist motion controller.

The option of using a PLC to achieve motion control with a dumb drive is a common one and a good choice.

It gives plenty of I/O capacity, but needs a special card for motion control.

Also it must be considered how the PLC will communicate with the HMI, possibly needing an additional fieldbus card.

A further consideration is the accuracy requirement of the drive.

PLCs are relatively slow to process positional data and may not be suitable for an application with a high performance requirement.

The choice of PLC may dictate the choice of fieldbus, whereas PC and motion controllers give greater "gateway" flexibility.

Changing to the option of PC control gives higher performance through higher processing speeds.

Generally the choice will be for an IPC (industrial PC), and although prices have fallen in recent years, these can be expensive.

An additional motion control card will be required, but having a touch screen HMI as part of the IPC can reduce system costs.

One drawback with the HMI is that the front- end software to implement it can be time consuming to generate, so it is more suited to machines in quantity production.

Where the requirement is for high system performance, the option of a separate motion controller should be considered.

For applications requiring synchronization of axes, referencing, or accuracy at high speeds, these controllers offer easy programming, fast response and reasonable costs.

There will be a need to interface with the HMI and the machine master controller, but plug in field-bus cards give high levels of "gateway" flexibility.

Some of the selection decisions change in the case of multi-axis motion control.

In most cases intelligent drives will be an economic solution for two, and sometimes three axes.

The choice between intelligent and dumb drives is not a matter of performance as both can achieve the same level.

Instead it is a matter of costs, and is influenced by the size of the drive, and application factors such as the inter-relationship of the various axis motions.

For example if interpolation is not required, meaning that the dynamic position of one drive is not used in the positional calculations of another drive, then intelligent drives are a possible solution.

Generally in machines that require interpolation, dumb drives with a separate motion controller are the better option.

As the number of axes increases to four and beyond, dumb drives become the economic solution.

This is because a modern multi-axis drive, such as Lenze's new ECS unit, consists of a single power supply connected to individual drives for each axis.

Sharing the power supply in this way provides an economic advantage, in part because a dc bus link can save energy.

As well as the reduced cost of hardware, machine builders benefit from simplicity of filtering, easier cabling and reduced panel space.

As before, the dumb drive will require a controller and the same three options exist.

The PLC is best suited to simple independent motion, without interpolation.

The industrial PC with a motion control card fitted is better for applications requiring a higher specification.

With the HMI being part of the IPC, component count and the need for communications is reduced.

Other benefits are ease of networking into a factory system and general communications.

Where complex motion with interpolation is required, the independent motion controller is worth considering.

Costs are low, but a separate HMI is required and the ability for the motion controller to communicate with it requires a fieldbus card.

In some applications the independent motion controller could be used together with an IPC, the latter being used for networking and communications with the touch-screen HMI, while the motion controller looks after the movement.

Motion controllers are fast operating with process times in the order of 1ms, and this is set to fall to 0.25ms for new designs.

The choice of components that make up a modern servo motion control system is clearly a complex one.

Buyers and specifiers should always liase closely with their suppliers and ideally choose vendors that offer both dumb and intelligent drives for single- and multi-axis machines.

Finally, the vendor should also be able to offer the option of a motion controller and the systems capability to make the overall concept work.

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