Industrial network communications products contribute to lower cost operating systems
The programmable logic controller (PLC) is used extensively for automation of typically electromechanical processes, such as the control of machinery on factory floor assembly lines and other production equipment. Various industrial networks connect various actuators, electric motors, drives, and other devices to the industrial control network.
Motion control manufacturers have had many demands to develop lower cost technology in recent years without sacrificing operational capability. It is also necessary to bring motion control systems to market earlier than previous product cycles. A major obstacle has been problems with industrial network communication products.
One motion control supplier has responded to this market challenge by creating a family of network communications protocols using three control methods. The new family of industrial networks, shown in Figure 1, displays connectivity to three specific lower-cost industrial networks. This flexible approach allows the user to select the most cost-effective and functional solution of the three industrial network communications available to solve system needs.
Reviewing the options
Three control methods in the network family are compatible with various host controllers. The family is said to meet the lower cost and faster time-to-market by including input/output (I/O) ports, Modbus RTU module networks, and fieldbus networks. An added benefit is that most touchscreens are equipped with standard Modbus communications; visual operation of the network is easier when using a touchscreen. A more in-depth look at these three controls is required.
This device is the simplest control element. Because the I/O control directly turns the I/O signals on and off, it shows the least time lag among all industrial network communications methods. For any equipment fast response time, the I/O control or a pulse-direction input is suitable. Figure 2 illustrates an I/O control diagram. An electric motor can be easily controlled with a simple on/off command using a relay control. The PLC programming is easily accomplished, resulting in a shorter equipment launch time. For simpler equipment motion, the motor can be operated with an external switch directly by a controller thus eliminating the need for a host PLC. This reduces system cost. Not too many motion systems are this simple. Consequently, I/O control is limited to motion control systems with applications that have repeating unidirectional motion profiles and constant application loads.
Modbus RTU control
Modbus is a serial communication protocol primarily used with a PLC for more complicated functions and motion profiles. It is a well-known industrial protocol used with a PLC in a large number of applications such as: bar code reader, displacement, position sensor, load cell, thermal regulator and vision sensors, speed controls, etc. Another recent device, the three-phase inverter, is now compatible with Modbus. To use Modbus controls with a host PLC, a serial communications module (Figure 3) is required.
If a touchscreen is used, most manufacturers install the Modbus communications protocol as standard, allowing easy communication. Modbus has various kinds of connection devices. These products are capable of connecting up to 31 axes to one Modbus master controller. A Modbus controller is capable of controlling both variable speeds and variable loads. Combining I/O and Modbus control (Figure 4) significantly reduces the time-consuming PLC programming tasks. It requires only 10 milliseconds to start up the motor (via the I/O control).
For the inverter control, the Modbus communication protocol uses noise-free, digital-signal commands. A new brushless, permanent magnet (PM), motor-based speed control uses the cost-effective motion control solution with this combination of I/O and Modbus industrial controls (see Figure 5).
Factory automation networks
The highest level of industrial control involves the use of a converter or gateway. There are several leading communication protocols used in factory automation (FA) networks. They include DeviceNet, CC-Link, Mechancontrolink, SSLNet, Modbus, and CANopen. A second generation of network controls led by EtherCAT, EtherNet/IP, Profinet, and CC-Link/IE are quickly gaining popularity (see Figure 6). A network converter or gateway is a protocol converting device that converts any of these protocols into Modbus (AT)/RS485 protocol to control any industrial network communications.
Available industrial network gateways are compatible with EtherCAT, CC-Link, Mechancontrolink II and Mechancontrolink III networks. Motor and drive hardware is compatible with each of the three communication protocols. One gateway interfaces with the motion system. This compatibility feature allows the user to successfully work with equipment variations.
The gateway compatibility with many industrial networks reduces wiring and programming time and saves money. The more motors to be controlled and the more functions that can be controlled, the greater the savings. By using one network controller or gateway with the host PLC, one can expand the number of controllable motors and drives from 31 units via Modbus to 192 units or 12 axes using CC-Link or 256 units or 16 axes connected and controlled. The network gateway eliminates the need for another host master.
The network gateway starts and stops electric motors, sets motion speed and distance, monitors position information, and initiates alarm signals when needed. From the standpoint of the host PLC, motors and drives are seamlessly connected to the fieldbus network. Motor starting and stopping commands are controlled by the I/O control, relieving the need for the PLC control and keeping the PLC programming simple.
Network time delay
Network communications time delay is a function of three factors. They are:
- PLC ladder scan time
- FA network time delay
- RS485 time delay.
By using one gateway with a CC-Link protocol, one motor and one drive will experience an estimated time delay of about 6 milliseconds. Figure 7 computes the total data transfer time delay using a gateway. If 12 units (motors and drives) are simultaneously turned on, the motors will start to move about 30 milliseconds later. Rewriting and updating the data transfer speed and direction over a longer move could cause the transfer rate to balloon to 400 milliseconds in a more complicated motion profile.
Examples can illustrate how the motion system functions. The first example involves a hybrid step motor with an on-board optical or magnetic encoder. Position error can be measured and any speed or position abnormalities controlled. Figure 8 illustrates a motion of a door that hits an obstacle and stops motion. Because it requires an encoder to be connected to a driver to sense zero motion, it is unnecessary to add other devices, such as a counter rotation module, to the host PLC. This approach eliminates the programming labor for the host PLC resulting in a shorter equipment launch time. A lower cost, built-in counter with stored data capability replaces the need for a more expensive position module with the PLC.
Another application that minimizes PLC costs and time to product release is used in conveyor systems and on-shelf stocking systems. They require a function called "sequential positioning operation." This operation uses a motion profile that is repeatable with a simple on/off command that starts the motor's programmed motion by presetting the values such as distance traveled, speed, and operating functions. By repeating various on/off signals, various kinds of motion patterns can be executed. The timing circuitry can be built into the driver, saving PLC programming time and cost.
The use of industrial network communications products contributes to lower cost operating systems. A faster time-to-market also is achievable. The products can control step motors, linear actuators, speed controls, and brushless PM motors in simple and more complicated motion profiles.
- Dan Jones, president of Incremotion Associates, provides consulting services to Oriental Motor; edited by Eric R. Eissler, editor-in-chief, Oil & Gas Engineering, email@example.com.
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