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MOTION CONTROL
FEATURE inteLLigent controL
Next-generation electric actuators that support CAN bus (control area network) communication are extending newfound intelligent
controllability to linear motion applications. Anders Karlsson, product line specialist – linear actuators at Thomson, explains
B
ased on ISO standard 11898 for serial data communication, the control area network (CAN) was developed to reduce the cost
and complexity of copper wiring in automotive applications with a simplified electronic bus architecture. To enable designers to take full advantage of this architecture, the Society of Automotive Engineers (SAE) developed the J1939 programming language, advancing it around the needs of automotive, agriculture, construction and other MOH applications. While J1939 was ushering an age of intelligent
automation in the automotive industry, manufacturing industries were seeking ways to leverage the bus architecture for motion control. Industrial applications, however, required higher transmission rates, more bandwidth, and ways to integrate motion control with other applications. Such needs spurred the development of the CANopen language, which uses an open standard platform that enables plug-and-play integration with other standard devices. This provides a smart architecture capable of integrating intelligent motion into other, higher-level, automation schemes.
cAn bus Architecture
CAN bus is a high-level communications protocol that provides a standard messaging structure for communications among network nodes under the control of an electronic control unit (ECU). Every message on an actuator module represents a node that has a standard identifier indicating message priority, data, and control source. This enables plug-and-play interchanges of supporting devices that share the same network and comply with the messaging structure. Figure 1 shows a typical CAN bus network. It
illustrates four actuators with built-in CAN bus- compliant intelligence. Each actuator has two wires, one that connects to an external power source and the other that communicates with the control source. The green box represents sensors or other components that could also be wired to the power source and the communications network without external relays. The orange line represents the two-wire bus that transmits the low voltage of power needed for the system, and the blue line represents the two wires that are used for information exchange. This represents a dramatic improvement over conventional vehicle networks in at least the following ways: • Power is distributed across common wiring, eliminating the need for separate wiring between each device and the power source.
• Switching is embedded in the actuator electronics, eliminating the need for cumbersome external switching and
Figure 1: Typical CAN bus network, illustrating four actuators with built-in CAN bus-compliant intelligence
connectors, etc. All commands are executed in the actuator.
• Information flows to an ECU from each device via the network bus, eliminating the need for independent connections between the devices and the ECU.
• Other equipment that might be integrated into the system connects with the network in the same way, eliminating the need for separate wiring, controls, and additional configuration.
• A typical CAN network supports up to 256 nodes, including multiple actuators or other devices on each node, something that would be all but impossible with a conventional network. The result is an efficient, compact, solution
that provides unprecedented monitoring and advanced control capability. Actuators are programmed to speak the same language as the ECU, allowing communication across a shared bus, whereas conventional electronic architectures require a standalone ECU for each operation. This also enables more complex control strategies, such as deploying the same actuator in multiple applications.
embedded position controL
An actuator with embedded CAN bus can carry position control messages. A 14-bit signal informs the user of the actual actuator stroke position between 0.0mm and a fully extended stroke, the accuracy of which depends on the stroke length and mechanical tolerances of a given model. Accuracy of the signal itself, for example, could be 0.1mm/bit, which could contribute to overall system positional accuracy of ±0.5mm or better depending on tolerances in the gearing, ball nut and screw assembly. Achieving that kind of positional information on
an hydraulic system would be expensive and hard to maintain. Instead of receiving a consistent electronic signal, monitoring the position of hydraulic cylinders requires measuring the amount of fluid that is being pumped through the lines and then using externally mounted encoders
and limit switches to signal a control box when the desired points have been reached. This requires keeping fluid in motion throughout the system at all times, and when pumping stops, system creep affects the position and requires recalibration. It also renders hydraulic actuation much less effective for heavy duty applications requiring consistent, high-precision, position control over longer periods. CAN bus systems, on the other hand, use encoders, limit switches and potentiometers to control position, and these are designed into the system electronics to enable absolute position determination. Absolute position control also enables
consistent, reliable, position memory. Because some machines might be deployed only during peak seasons such as Christmas or back-to- school times for distribution centres, it is sometimes useful to disconnect the battery to prevent it from draining. Without absolute position capability set at the factory, the user will have to recalibrate once they reconnect the battery.
Low-LeveL power switching
Low-level power switching is standard with the CAN bus protocol, allowing operators to program the actuator to extend, retract or stop smoothly using low-level electronic signals rather than a higher-energy electrical current. This improves safety by reducing the hazard of electrical shock and simplifying design by allowing lower-rated control components. Soft start capability also allows the use of lower- rated power supplies and puts less stress on batteries and charging systems in machines. Low-level power switching also enables
controlling standard inrush to be up to three times the full load amperage for up to 150 milliseconds. This would enable direct programmable logic controller (PLC) connections, eliminating the need for expensive relays and the related installation costs. It also could include a sleep mode when the actuator is idle, which extends the battery by reducing energy
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