Hydraulics and Pneumatics

Sequential Circuits As with any digital circuit, a sequential circuit can be described in VHDL by the use of gates, Boolean expressions, or behavioral constructs (e.g., process statement s). Although gate level description allows a more detailed description of timing and delays, because of the complexity of clocking, and register and flip-flop controls, sequential circuits are usually described by process statement s. In the next section we will first discuss some basic elements at the gate level and then represent coding styles for more complex sequential circuits. Basic Memory Elements at the Gate Level A clocked D-latch latches its input data when clock is active. The latch structure retains its value until the next active clock cycle. This element is the basis of all static memory elements. Figure 87.19 shows a simple implementation of the D-latch that uses cross-coupled NOR gates, while the corresponding VHDL code is shown in Figure 87.20. Notice the use of buffer for q a...

Writing Testbenches Coding styles presented so far were for coding hardware structures, with the intention of synthesizability. On the other hand, testbenches do not need to have hardware correspondence. The VHDL code of Figure 87.31 which is a 101 Moore detector will be used as the design under test (DUT) in this section. Figure 87.37 shows a testbench developed for the Moore detector of Figure 87.31. The testbench entity which instantiates design under test (DUT), Moore_detector in this example, has no ports. Initial values of DUT inputs are assigned in the signal declarations stated in the declarative part of this architecture. This testbench applies test data to DUT. This testbench uses three processes: one condi- tional signal assignment and two process statements . It should be kept in mind that all the processes run concurrently. The first process waits for 20 ns and then puts 0 into clr_t . It then executes a wait which suspends it forever. Normally a process statement exe...

Pneumatic conveying Pneumatic conveyance in pipelines Pneumatic conveying is the transport of bulk materials through a pipeline by air pressure or vacuum. Materials that can be handled mnge from asbestos with a bulk density of 100 kg/m 3 to crushed stone with a density of 1500 kg/m. The advantages of pneumatic conveying over mechanical conveying include safer working conditions (clean atmos­ phere and reduced fire hazards), greater flexibility, freedom from contamination and the ease in which a change of direction can be achieved. It has to be admitted that the cost of air compression is likely to be higher than that of pure mechanical transport, particularly when the transported material requires a high degree of purity in the air. Fragile materials may not be suitable for pneumatic conveying, but most other materials and some manufactured components lend themselves to this form of transport. The following types of pneumatic conveyors are available: • Vacuum system, simila...

Asynchronous induction motors Main types and operating principles The cage induction motor is by far the most widely used type of electric motor in industry. It is both rugged and reliable; it is also the preferred choice for most variable speed drive applications. Simplicity, low cost, high reliability, fairly high efficiency, coupled with its ease of manufacture, make it widely used in most parts of the world. Figure 6.3 shows the typical arrangement of a squirrel-cage induction motor, which is built with three sets of stator windings arranged around the stator core. The rotor has conductors, which are a cage of copper or aluminium bars and short-circuiting end rings. There are no electrical connections to the rotor. The wound-rotor induction motor, as the name suggests, has insulated copper windings in the rotor similar to those in the stator. The rotor wind­ ings are connected to starting equipment using slip rings and brushes, and therefore this design is substantiall...

Paint spraying There are three basic methods of spray painting: low pressure (the conventional method), high pressure and electrostatic painting. Ail three can be used for cold or hot spraying. They each have their own particular characteristics, and in order to decide which is the appropriate one for a particular application a number of factors have to be taken into account, such as economy in the use of paint, finish and capacity. Table 1rates the various methods for suitability. Compressed air is used in each method, and particular care must be taken to ensure that the air is supplied dry and oil-free, if high quality work is required. Air may also be used for preparation of the surface prior to painting. Cleaning off the rust, old paint etc., is done by blast cleaning using shot, steel sand or some other kind of abrasive particle. Low pressure paint spraying In this technique compressed air is used both for atomizing the paint and for carrying it to the spray gun. The air pres...

Noise from pneumatic equipment The general principles of Noise Reduction in compressors have been outlined in an earlier chapter; this chapter deals specifically with tools. All pneumatic machines and tools tend to produce noise, caused in the main by the discharge of high pressure air from the exhaust port. In addition to the exhaust noise, there maay be mechanical noise, particularly from percussive tools. There is a growing body of legislation limiting the noise from all types of equipment, and this places a duty on both the manufacturer and equipment user. The manufacturer is required to produce a tool which is within the prescribed legal limits, and the employer is obliged to pay attention to the noise exposure of the tool operator by limiting the working time or by providing suitable protection. In considering the noise generated on a building site, it should be remembered that it is the total noise environment that is important when considering the exposure of the opera...