Machine Design

DSPs go heavy duty

Digital signal processors, long used in cell phones, now handle much bigger heavy equipment.

Chris Clearman
C2000 Applications
Texas Instruments Inc.
Dallas, Tex.

Heavy-equipment controls, as highlighted in this phantom view, consist of one integrated system automatically regulating the operation of the entire machine. An operator moving a single set of joysticks simultaneously adjusts engine power, transmission gearing, hydraulic power transfer, vehicle motion and direction, and claw angle position and grip.

Once thought suitable only for high-speed number crunching and telecommunications work, DSPs like this one from Texas Instruments now handle areas once reserved for ordinary MCUs.

Manufacturers of heavy mobile equipment have traditionally perceived control systems as a necessary evil rather than as a competitive advantage. Early machines contained multiple control systems with each handling specific subsystems like the engine, transmission, or hydraulics. The subsystem's maker often supplied these controls or the machinery manufacturer developed them as needed.

Heightened demands for performance, operator safety, and tighter environmental regulations have forced equipment makers to upgrade their controls. One facet of modern approaches is more coordination-among various subsystems. Two factors make this coordination possible: better microcontrollers (MCUs) and adoption by subsystem makers of standard connection protocols such as CANbus.

But integrated control systems cost more. Some off-road-equipment manufacturers mitigate the expense by putting high-end control systems they've already developed into relatively low-end models. Of course, there's wasted capacity and unused control features in the low-end equipment.

Control-systems developers came up with a better way to whittle down development costs. They created modular systems that provide mixandmatch building blocks. A control system is tailored specifically for a machine by selecting only the modules necessary for a particular piece of mobile equipment. The key, of course, is to assure sufficient processing capacity exists to handle the requirements for all modules chosen.

"Thinking in the heavy-equipment industry often goes that control-system performance is not critical because of the relatively slow speed at which mechanisms move," said Dan Ricklefs, product portfolio manager for mobile electronics at Sauer-Danfoss, a leading mobile-equipment-controls vendor. "Actually, a tremendous amount of number crunching goes into every machine movement. Ultimately, it's the controller's ability to smoothly turn a valve on and off that determines the rate at which equipment accelerates or decelerates."

Smooth operation requires rapid response to small changes in measured values. The modular controls needed a processor that could quickly process analog as well as digital signals. Digital signal-processors, or DSPs, meet the flexibility and horsepower demands to control complete mobile machines. DSP-based controls are finding homes in a wide range of equipment such as cranes, loaders, and power shovels deployed in industries such as agriculture, construction, forestry, and road building.

A DSP-based control delivers 10 or better processing power than traditional MCUs. This performance reduces the control loop time by an order of magnitude and speeds up a wide range of common tasks. For example, an OEM for a sodharvesting system was able to trim loop time from 20 to less than10 msec. This cut the time needed to process a pallet of sod by over 40%, from 3.66 min to just over 2 min. The net result was a 75% rise in productivity.

An example of the new modular and integrated mobile control systems is the Sauer-Danfoss Plus 1. It incorporates programming and development tools, MCUs, I/O modules, graphical terminals, and joysticks into one integrated environment. Subroutines come prepackaged in the control system for each typical application such as engine antistall, load limiting, or speed control. A library of software control objects lets developers do custom control work. OEM engineers essentially "drag and drop" software modules into the vehicle-management system for a customized control.

Decisions at Sauer-Danfoss that led to the use of DSPs illustrate how the technology excels. Eight and 16-bit MCUs had driven previous generations of controls. The last generation delivered 20 to 40-msec loops — the time needed to complete one cycle of communications among the control system, the equipment under control, and the sensors for closedloop feedback. This loop time became the prime limiting factor as control-system complexity grew.

System engineers did not consider DSPs as viable options — at first. "The common wisdom was that DSPs were for serious number crunching, were too expensive, and didn't have enough input/output capabilities for industrial controls," relates Sauer-Danfoss Hardware-Development Engineer Joe Schottler. "All the major players in our business used microcontrollers. The attitude was that MCUs offered more than enough performance." Further investigation revealed, however, that a DSP system supported all the peripherals needed for industrial uses such as Flash memory, CANbus support, timer channels, and motor-control functions.

The rigorous core selection process and decision to use a DSP was an eye opener for many of the Sauer-Danfoss team. "The first big surprise was that the Texas Instruments TMS320F2810 and F2812 digital signal controllers turned out to be competitive in overall systems price. The cost per part of the DSPs was, of course, higher; but their signal-processing capability reduced the bill of materials to the point that the cost per module was competitive with microcontroller alternatives." As an example, system engineers used software filters in the DSP rather than adding four to 10 hardware filters on each printed-circuit board.

Another issue: MCUs in embedded control systems have a reputation for incorporating lots of Flash memory. System engineers found that the TI DSP's offered more than enough for even the largest integrated control systems.

Use of control objects and a modular software library made it feasible to scale systems to specific needs. Software-customized controls let OEMs build flexibility into their machinery. DSP-based C2000 controllers in the Plus 1 modules formed an integrated core to control transmissions, drivetrains, and valves. The same abilities extend to controlling equipment such as blades, grippers, and buckets as well as hydraulic and electronic steering.

The use of just-one-control architecture minimizes costs. Modules are added and removed to handle specific-machine functions. It's the programming that determines the operating parameters. For example, if the operator of a wheel loader moves the joystick to up wheel speed, the control system determines whether to gun the engine, reduce gear ratios in the hydrostatic transmission, or both.

Control modules handle a wide range of sizes and configurations. As an example, the Plus 1 system comes in three housing sizes and nine configurations. Interface options include CAN buses, camera inputs, USB ports, and RS-232 connections. Design engineers control how much fine-tuning, diagnostics, and servicing technicians may perform with a laptop or PDA. Graphic terminals display selected functions such as oil pressure and machine angles.

Adapting controls to changing regulatory requirements typically involves just reprogramming parameters. A specific example is the handling of the EPA's offroad engine regulations. The regulations are structured as a three-tiered progression with a phase-in period over several years for each tier.

Tier 1 standards phased in from 1996 to 2000. The tighter Tier 2 standards are currently taking affect through 2006. The even more stringent Tier 3 phasein starts next year and continues through 2008, though Tier 3 only applies to engines from 37 to 560 kW. Tiers 1 through 3 use advanced engine design for compliance.

Last year the EPA signed the final rule introducing Tier 4 emission standards starting in 2008 and continuing till 2015. Tier 4 requires a 90% reduction in the emissions of particulate matter and nitrous oxides over today's regulations. Only the use of advanced control technologies coupled with exhaust gas treatment will let engineers hit those goals.

TAGS: Technologies
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