Like all marriages, that of fieldbus with IT and/or other fieldbuses is fraught with flare-ups and breakdowns, but ultimately both parties know they have no option but to try and make it work, asserts Boris Sedacca.
From motor control to process calibration in explosive environments, there are almost as many types of fieldbus as there are applications, and some applications will host multiple fieldbus environments without a specific site standard.
The Sellafield nuclear reprocessing plant for example uses Modbus with operator panels for pump control as well as IP67 Profibus nodes. It mixes PLCs from Rockwell Allen Bradley and Siemens, not to mention Schneider Modicon.
The wired connections between devices and groups of devices at the physical layer of all fieldbuses will normally come from specialist terminal suppliers like Phoenix Contact, and the three Ws - Wago, Weidmuller and Wieland.
In fact the Interbus serial bus system developed by Phoenix Contact connects sensors and actuators in accordance with IEC 61158 specifications for industrial computer network protocols used for real-time distributed control.
There is a wide variety of competing fieldbus standards, principally AS-Interface, CAN, EtherCAT, Foundation fieldbus, Interbus, LonWorks, Modbus, Profibus BitBus, CompoNet and SafetyBUS p, let alone underlying industrial automation communication protocols like HART, AS-i, DeviceNet and EtherNet/IP.
“Before we launched the first slice I/O at the Hanover Fair in 1995, all fieldbus products would have been a base and a block of I/O for eight or 16 channels, or maybe four analogue channels, but you could not mix them,” recalls Derek Lane, automation manager at Wago and deputy chairman of the Profibus Group.
“It has always been the automotive sector that has pushed fieldbus and PLC developments, like when General Motors issued its specification for a PLC which led to the development of Modicon and the Modbus protocol, or the CAN chipset specified by Bosch for in-vehicle electrics.
Manufacturing Automation Protocol
“The first time I was involved with Profibus it was using Fieldbus Messaging System (FMS), which is a subset of the Manufacturing Automation Protocol (MAP) conceived by Japanese the automotive industry and adopted by General Motors. Fieldbus only ran at 500 Kbits with FMS and was superseded by Profinet when Profibus decentralised peripherals (DP) came out running at 1.5 Mbits.”
Lane is deputy chairman of Profibus UK, and his employer Wago supports a multiplicity of automation protocols, some of which have now lapsed like Firewire, Apple’s implementation of the IEEE 1394 high speed serial bus protocol for PCs, which has been overtaken by the even faster USB3 protocol.
Before the days of high speed serial communications, PLCs had to be wired in parallel to cope with update times, and before the days of serial or digital communications, analogue 4-20mA current loop communications ruled the roost for measuring process variables like pressure, temperature, flow and pH.
Although not as prominent nowadays as digital communications, current loop is still holding sway in process environments, particularly with the HART protocol, and is still favoured over the voltage analogue method of 0-20V, which cannot detect a broken wire.
Lane continues: “Another problem is where people tie zero volts down to earth because if you get an earth fault, you will actually conduct a signal, which is dangerous.
“At the other extreme, with high speed Ethernet communications originating from the IT environment, some plant and factory managers were initially suspicious that the IT department would take over control of shopfloor networking.
“Some factory people do not understand the fundamentals of Ethernet networks with their MAC addresses, unique IP addresses and subnet masks. The earlier issues of whether Ethernet is deterministic enough no longer apply today because with EtherCAT and now Profinet, you get response times down to 31.25 microseconds.
“On the other hand, there is a danger that some people will think that if they use Ethernet to connect to industrial robots, they can use Category 5 unshielded twisted pair (UTP) cable instead of using shielded twisted pair (STP), and even standard STP cable is not robust enough, which is why Belden cable has an extensive screen which is more robust.
“In the marine environment, companies like GE will use armoured cable for Ethernet, so we supply a converter which takes the terminations of the screen clamp onto an RJ45 socket for a patch socket.”
Wireless mesh networks
One trend that is driving the adoption of industrial Ethernet is the Internet of Things (IoT) as more customers demand connectivity for every device. Instrument manufacturers like Endress & Hauser now connect devices via Ethernet and in some cases via wireless mesh networks.
In the case of 4-20mA analogue measurement, HART users are increasingly converting current loop signals to digital form for Ethernet. Rockwell Automation for example offers analogue cards with HART compatibility to pick up analogue signals and put them in digital form on an EtherNet/IP bus.
“Once converted, these signals can be throughout the system, whether they feed into a real time digital controller or are captured for historical data collection and analytics,” says Mike Loughran, solution architect at Rockwell Automation.
“A couple of years ago, Endress & Hauser launched the first EtherNet/IP-enabled flow meter but most instruments still rely on analogue signalling or on serial fieldbus data like DeviceNet or Profibus. Newer instruments being launched are increasingly using wireless communications to Ethernet.
“This allows customers to be much more flexible with the location of devices without having to run cabling everywhere. The cost of the cabling itself may not be significant, but installing it properly can be very expensive.”
Loughran quotes a Cisco report which identified that there would be $40 trillion worth of opportunities created around the IoT by about 2020, of which 27% will be in manufacturing automation.
He claims over half of the devices in the field will be Ethernet-enabled and will not be automation devices like PLCs or I/O blocks, but items like weighing scales, RFID, bar code readers, cameras, mobile devices.
Loughran adds: “This all fits into the trend to connected enterprises and global manufacturing. Ethernet gives you that connectivity to carry large amounts of data, which the old fieldbuses are not built to do.
“The automotive industry is a major adopter of Ethernet now, because it gives them a single network to link up all their manufacturing and to connect to enterprise IT. Customers can walk into a motor dealership and ask for certain extras on a basic car, with all the order entry data feeding to a particular plant which can be in another part of the world.”
CANbus reads car sensors and accelerometers to adjust damping on wheels
The Volkswagen Golf R outputs an estimated 290 horsepower and features a new 4MOTION all-wheel-drive (AWD)system, the latest version of which uses a fifth-generation coupling from Haldex, a Swedish manufacturer of AWD systems that activates before wheelspin occurs, helping to eliminate nearly all traction losses.
The system uses an advanced control function based on specific driving conditions. When operating under a relatively low load or when coasting, the front wheels are driven and the rear wheels are decoupled, helping to save fuel. However, the rear wheels can be engaged whenever necessary via the Haldex coupling, which is activated by an electro-hydraulic oil pump.
A control unit continually calculates the ideal drive torque for the rear wheels and controls how much the multi-plate clutch should be closed by activating the oil pump. The oil pressure increases the contact pressure at the clutch plates in proportion to the torque desired at the rear axle.
The amount of pressure applied to the clutch plates can be used to continuously vary the amount of torque going between the front and rear wheels. If necessary, nearly 100 per cent of the drive torque can be directed to the rear wheels.
In addition to the Haldex coupling that acts longitudinally, four electronic differential locks (EDS) that are a function of the electronic stability control system act laterally. The system briefly brakes a wheel that is slipping, enabling uninterrupted and stable transfer of drive power to the wheel on the opposite side.
The new Golf R can be ordered with dynamic chassis control (DCC) as an option, which offers three driving modes - comfort, normal and sport – and adaptively controls the damper valves via algorithm.
DCC takes input signals from wheel displacement sensors and accelerometers as well as vehicle information from the Chassis-CAN bus to compute these values and adaptively adjust the optimal damping force.
Moreover, damping forces are selectively applied to the four wheels individually. With the new generation of DCC, it is now possible to independently vary rebound and compression damping while cornering.