Jack Johnson is an electrohydraulic specialist, fluid power engineering consultant, and president of IDAS Engineering Inc., Milwaukee. Contact him at firstname.lastname@example.org, phone (414) 236-5350, or visit www.idaseng.com.
Large positional errors can result when a programmable logic controller (PLC) selects the deceleration point. The error is caused by the scan-time delay in the digital controller, and it is directly proportional to the speed at the instant that the deceleration decision is made. To reduce this error, many open-loop motion controllers decelerate the system to a creep speed to approach the target position slowly. Thus, positioning error is made at creep speed instead of at maximum speed, so the ultimate position will be acquired more accurately.
Most motion-control applications are of a critical nature — they must meet accuracy, bandwidth, or some other performance demand. The most sensible and expedient way to design such systems is to use performance requirements as the design goals at the very outset of the design process. The techniques are analytical in nature, so they require mathematical descriptions of all elements of the system. Only then can synthesis and simulation methods be applied to direct the design process toward the end result without undue trial-and-error techniques.
To understand the function of and need for integral control, you must understand the shortcomings and limitations of the proportional electrohydraulic positioning servomechanism. A simplified, combined cutaway and block diagram is shown in Figure 1.
A histogram generated as a “gedankenexperiment” for a wheel-mounted front-end loader, described in Part 1 of this article, July’s “Optimize Mobile-Equipment Control Through Statistical Analysis,” was based upon reasonable estimates of a real work process. The operating scenario proposed a relatively long distance between the load pickup pile and the load dumping point.
Stationary machinery within automated, industrial manufacturing and fabrication processes typically operates in very repetitive, measured, predictable cycles. In these environments, total lost energy over the course of a given time period, say, a day, can be measured or calculated rather easily due to regular, predictable motion cycles.
Fluid power technology emphasizes the use of efficiencies as key figures of merit for many products across multiple marketplace segments. Such reasoning is sound, especially with the push to reduce energy consumption. However, efficiency is too simplistic a measure, and dare say, tends to be rather abused.
To demonstrate the characteristics of a motion control system, we will examine test results of a valve-controlled cylinder in a closed-loop, positional servomechanism, represented below. Otherwise known as a torque cell, the mechanism was designed for special electrohydraulic motion-control training programs.
Analogies exist between hydraulic flow and electrical flow, and the molecules of fluid in a hydraulic circuit behave much like the electrons in an electrical circuit. Let’s examine analogies between pressure and voltage and between ground and the hydraulic reservoir.
System design normally calls for a specific load to be overcome and propelled at some required velocity. This is called the design pointor design target. A further reality is that most machines are required to operate at an essentially unlimited number of operating conditions as the actuator accelerates, decelerates, and stops. When the system is designed, the design point must accommodate the absolute worst-case operating point expected over the entire lifetime of the machine.
System design requires that components supply pressure adjusted so that the operating envelope encompasses the worst case force-speed operating point. An infinite number of combinations exists that will accomplish this, so some other strategies must be applied to reduce the number of possibilities.
The hydromechanical resonant frequency (HRF) of a valve-cylinder circuit is an interesting concept and an important value to know. If a cylinder is stroking, and its control valve suddenly shifts to block flow, the cylinder and its load will vibrate, usually with considerable noise and sometimes with considerable violence. This is HRF in action. The noise arises from the resonance that exists when the kinetic energy of the load mass and the potential energy stored in the hydraulic fluid’s compressibility are exchanged.
The term bus is extremely general in the electrical engineer’s parlance. It is a synonym for wires or conductors. The wires that carry power around your factory, office, or house are a power bus. The wires that carry your telephone conversation are a communication bus. Most of the peripheral devices connected to a PC communicate with the processor through an internal, digital, parallel data bus. That bus has about 50 different conductors. A data bus provides a universal means of two-way communication among machines.