Mike Lomax of Bosch Rexroth examines the role of intelligent drive systems in optimising machine configuration and performance.
Machine manufacturers are under pressure to provide shorter development times for highly advanced machines, with less manpower available to them. This can make it difficult to solve complex control problems and challenges OEM’s to respond by seeking to take every possible step to minimise the level of programming required.
The modern drive systems now found in most machinery have sufficient intelligence to perform position movements and velocity control; functions that, even a few years ago, drives would not have been thought capable of, but which are now regarded in the same way as accessing email via a mobile phone – almost a minimum expectation.
These drives can now also be interfaced to a variety of control buses, such as ProfiNet, SERCOS III and DeviceNet. These bus systems are powerful but still leave the machine designer or programmer with the task of undertaking mathematical calculations and programming time-critical events within the central Programmable Logic Controller (PLC) or motion controller. This is no small task when faced with a multi-axis, multi-process machine and, even when using the very latest PLC or motion control systems, frequently remains a highly labour-intensive process.
Thanks to advancements in drives, manufacturers can now allocate functions directly to the drive, by making use of in-built technologies that eradicate the need for an external PLC. The latest generation of servo drives, for example, now includes the IEC61131-3 PLC operating system, which is the industry standard for solving most machine control issues. This represents a major step change from the minimal functionality offered by previous generations of drives.
The implications of this new capability can take time to fully understand, but essentially the drive now has the ability to solve control challenges in real time, whenever and wherever they may occur.
A very simple example of this would be an axis movement which needs to be triggered by a signal input, for instance, from an optical sensing device. The time needed to relay the signal to the central PLC, process the information, perform any calculations needed and then send the command back to the drive can often be too long, causing production errors. Removing the PLC and going direct to the drive eradicates this time lapse, and so optimises cycle time and production consistency.
Another instance could involve the synchronisation of a multiple axis from one part of the machine to another, either via a simple gearbox simulation or a more complex CAM profile.
Staying in sync
Even if the drive system selected contains the appropriate problem-solving tools or function blocks, the challenge of synchronising multiple movements on the machine remains. This task cannot be carried out by most bus systems because they are not deterministic. The solution is for the drives to communicate between themselves, without having to refer back to a central control system. This is where technologies such as SERCOSIII – one of the first deterministic bus systems, which can now be installed directly into the drive – come into their own.
This cross-communication can also function as a command interface from a single drive to one or more slave drives. This allows the master drive to adopt a supervisory role for the other axes.
A variety of function blocks are now available and can be selected depending on the individual application requirements. Their capabilities range from correcting the positions of products on conveyor belts, and the control of winders, to closed-loop register control and even the creation of a complete motion profile for cross-cutters and cross sealers. Other options include implementation of a flying shear process; tension control; and crank kinematics.
Reducing programming time
With such a range of function tools available within the drive, it is vital that selecting the right tool for the job is as straightforward as possible. The goal should be that OEM programmers, with perhaps no previous experience of the particular challenge in front of them, can easily access and use these functions.
The solution lies in the employment of tried and tested PLC function blocks. These function blocks can be used freely in IEC 61131-3 PLCs and can even be incorporated within conventional ladder logic programming.
Creating a machine HMI
Creating an interface between the machine and operator has previously required central PLC involvement along with a bus system, to convey key machine set-up information and variables to the drives. Diagnostics and machine status information is then redirected to the PLC before being displayed on the Human Machine Interface (HMI).
Although modern bus systems can achieve this, there can be a considerable amount of programming effort when all the required information is already contained within the drive system. To address this, many modern drive systems also contain all of the tools needed to create a HMI. In some instances, a central PLC may not even be required as the drives are capable of using a full range of inputs and outputs which would normally be connected to the PLC.
Controlling hydraulic axes
The SERCOS III bus has been designed to accommodate a full range of hydraulic functionality. The algorithms incorporated into the system allow seamless transition between velocity, position and force control of the hydraulic actuator. Drives can also contain electrical systems which automatically adapt the hydraulic flow and pressure, minimising heat generation in the fluid, and consequently wasted energy. Employment of this technology has been shown to deliver energy savings of up to 60 per cent in some applications.
Minimising commissioning time
A key factor which must be kept under control is the time taken to set up and commission the machine. Intelligent drives now offer a number of tools that can reduce start-up time through the optimisation of the axis movements and process synchronisation.
These rapid start-up tools have been designed to enable the drives to be moved at an early stage of the machine commissioning, even before the machine control software is installed on the system. This results in rapid and easy testing of machine mechanics.
Due to the cost of both bespoke stand-alone measuring systems, such as oscilloscopes and transfer function analysers, and the specialist labour necessary to get the best from them, it is rare that this type of equipment is used to test the bandwidth of machine axes.
However, this issue is negated if a drive with a built-in oscilloscope is specified. This type of drive allows users to generate Bode and time domain diagrams, so optimising production output.
Can intelligent drives help with maintenance?
Modern drives can now even play a part in a proactive maintenance regime as they can be supplied with a fully integrated series of software tools capable of performing key predictive maintenance functions, with minimal additional programming effort. This software can be set up to continuously monitor the condition of the machine mechanics and process conditions – from waveform analysis through the analysing the rate of temperature change, as well as checking for backlash, increased friction or process overload. If a fault condition arises, a code is generated and passed to the machine HMI. Furthermore, if a critical issue is identified which required the machine to cease operating immediately, this decision can be made inside the drive, minimising lost production and the risk of machine damage resulting from unsafe operation.
Marrying intelligent drives with computer-based control systems
In some industries, such as aerospace for example, software such as LabVIEW is the standard on machinery, while other industries and applications will each have their own preferred systems. Meanwhile, some OEMs have developed their own PC-based control systems. These invariably contain a great deal of process knowledge which there is a requirement to protect. Many modern intelligent drive systems have been developed using open interface protocols, which are compatible with multiple software platforms easing the process of integrating intelligent drives into the preferred high-level software language system.
Libraries are available for C/C++; C# (.NET), Visual Basic, VBA (Office), LabVIEW G, Objective-C and Java, and can be used with the following development environments: MS Visual Studio; LabVIEW; Eclipse; Xcode; and Wind River Workbench.
This is a vital attribute for OEMs seeking to harness the power of the Internet of Things and provide an opportunity for customers to progress rapidly towards Industry 4.0, where the watchword is connectivity between all participants in the production process, whether human or mechanical.
Whether or not Industry 4.0 is the immediate goal, however, what is undeniable is the enhanced processing power and functionality of intelligent drives which present numerous opportunities to optimise programming, production and maintenance. With the need for central PLC’s now eliminated in many instances, not only is component count reduced but more advanced machines can now be created in much shorter timeframes.
Go to www.boschrexroth.com for more iformation about intelligent drive systems.