By R. B. Walters (auth.)
Force and movement keep an eye on platforms of various levels of class have formed the lives of all participants residing in industrialized nations around the world, and including verbal exchange know-how are principally answerable for the excessive regular ofliving normal in lots of groups. The brains of nearly all of present keep watch over platforms are digital, within the form of desktops, microprocessors or programmable common sense controllers (PLC), the nerves are supplied through sensors, usually electromech anical transducers, and the muscle includes the force approach, typically both electrical, pneumatic or hydraulic. the criteria governing the alternative of the main compatible force are the character of the appliance, the functionality specification, dimension, weight, environ psychological and protection constraints, with greater strength degrees favouring hydraulic drives. earlier adventure, in particular within the laptop software area, has in actual fact proven that, within the face of festival from electrical drives, it truly is tricky to make a resounding case for hydraulic drives on the backside finish of the ability diversity, particularly at fractional horsepower point. one more, and often overriding think about the alternative of force is the familiarity of the method fashion designer with a specific self-discipline, that could inhibit the choice of the optimal and least expensive resolution for a given program. one of many targets of this e-book is to aid engineer conquer his usual reluctance to use the other than electrical drives.
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Extra info for Hydraulic and Electric-Hydraulic Control Systems
Fig. 8 Reversing velocity profile open loop control system with velocity deviation. introduces a cumulative displacement error to an open loop system. The only practical way to eliminate these errors and to achieve accurate and repeatable position control is to close the loop by utilizing an output-driven position feedback element. An alternative approach which is suitable for systems where absolute positioning accuracy is less critical is to reduce positional overshoots by changing the velocity profile and to eliminate cumulative errors by introducing output-activated limit switches.
Maximum value for the closed loop amplitude ratio j80 /B;I would normally be around 1·4 or 3 dB. An alternative technique of system analysis is the pole-zero approach developed by Evans as the root-locus method for the investigation of closed loop systems.
2. There are, however, exceptions to this rule, where pressure control devices are used to control motion, and flow control devices to control force. The main justification for closing the loop in force control systems is the presence of frictional forces such as seal friction in cylinders. An open loop pressure-controlled system cannot differentiate between external and frictional forces, whereas in a closed loop system the external force can be identified and fed back by means of a load cell.