Search

Digital transmission of cab signalling and train control data to moving traing using railway track circuits may be achieved with FSK modulation of an audio frequency carrier signal. By transmitting coded data, information concerning train location and system status may be carried in addition to safety commands such as target speeds. The distributed nature of railway control systems using track circuits for train detection and track-train data transmission requires the use of coding schemes that tolerate sudden discontinuities at track circuit boundaries in addition to being able to operate in a high noise environment. With audio frequency jointless track circuits, severe constraints are imposed on the transmission frequencies by the characteristics of the electrical separation joint, resulting in low data rates. The paper describes several coding schemes that may be used to overcome the resulting synchronization and error-control problems.

The refractivity of atmospheric water vapor at frequencies less than 1 THz is expressed as the sum of (1) the empirical radio refractivity, (2) a simple approximation to the line-by-line summation of dispersion contributions by resonances above 1 THz, and (3) a line-by-line summation over resonances below 1 THz. This simple approximation, which is valid in the temperature range -40 degrees C to 40 degrees C, reduces labor and computer time required to calculate dispersion. A similar approximation for absorption predicted by three different line shapes is also given. Measured and calculated radio refractivity data are compared, revealing discrepancies in temperature dependence. The three different dispersion line shapes are then compared and shown to be in good agreement with both. >

Application of Kotelchuck and Scheraga's theory suggests that many immunoglobulins contain extensive regions of α-helix, which contrasts with previous experimental findings using optical rotatory dispersion methods. In human light chains the K II and K III subgroups are predicted as containing about 24 per cent α-helix, while the K I and λ subgroups on the average have only 12 per cent helix. The heavy chains, particularly of subgroup II, are predicted as havng up to 41 per cent helix. In both light and heavy chains, the helical regions occur in three groupings, the most characteristic of which is the middle one occurring in the semi-conservative part of the V region. The presence of a tryptophan residue in this helical section suggests experiments which could test the validity of these predictions.

1. 1. The major, carbohydrate-free macropeptides derived from the A and B genetic variants of κ-casein have been purified. 2. 2. The amino acid compositions of these purified components confirm the data of previous workers and are consistent with the replacement of the two amino acids, aspartic acid and threonine in the A variant by alanine and isoleucine in the B variant. 3. 3. Tryptic fingerprinting has located both amino acid replacements within a very large fragment comprising the C-terminus of κ-casein.

1. 1. The insoluble material formed by the action of rennin (EC3·4·4·3) on χ-casein (and its derivatives) consists of two discrete components, or para-derivatives. 2. 2. Variation of the pH, temperature and composition of the reaction mixture fails to produce an alteration in the relative amounts of these two products. They are evidently formed independently of one another and represent primary products of rennin action. 3. 3. O-(Diethylaminoethyl)-cellulose chromatography of S-carboxymethyl-χ-caseins obtained from individual cows establishes the existence of two types of component in each genetic variant series which give rise separately to the major and minor para-derivatives. This result is consistent with an attack by rennin at only one position in each χ-casein molecule.

1. 1.|In the absence of secondary effects, the insoluble product of rennin (EC 3.4.4.3) action on S -carboxymethyl- κ -casein (SCM-κ-casein) consists of two components, para-SCM-κ-caseins I and II. The C-terminal sequences of the separated species are identical, and this very strongly suggests that the rennin-sensitive bond in the precursor molecule (κ-casein I or II) which gives rise to each is the same. 2. 2.|By treatment with concentrated urea over extended periods of time it is possible to bring about a serial conversion of the para-SCM-κ-casein components to ones carrying a higher net negative charge. This is most probably caused by reaction of the protein species with cyanate derived from urea. Thus, it is not necessary to invoke an attack by rennin at more than one site in each κ-casein molecule to explain observations by other workers of changes in the relative amounts of para-κ-caseins I and II as well as the appearance of a third and more negatively charged species.

An electromagnetic speed sensor has been developed for use in automatic-train control on London Transport railways. The primary sensing element is a composite ferromagnetic-conductive rotor which introduces amplitude modulation, with frequency proportional to train speed, into a fixed audiofrequency carrier. A number of discrete rotational speeds are then defined by demodulation and low-pass filtering. An override system, operating by reducing the amplitude of a further AC signal with increasing train speed, provides integrity at low speeds, and a magnetic voltage reference unit maintains the power supply within predetermined safe limits. Fail-safe design principles are applied extensively on both component and system levels.

The safe operation of large power transistors in motor-drive applications often requires the use of snubber networks. The design of these networks depends upon the type of device being used and upon the circuit configuration. Some operating experience with a monolithic Darlington device is reported. The importance of controlling the switching losses in these devices by the use of snubber capacitors is indicated. It is shown that these capacitors produce side effects in a variable-voltage, variable-frequency inverter which can lead to overcurrent conditions in the transistors. These currents can be controlled by the use of snubber chokes. But the chokes, which are usually included primarily to limit reverse recovery currents in the free-wheel diodes, must be designed according to a different criterion to achieve this. It is suggested that `thermal feedback? should be provided for power transistors in order that they may operate at a safe and stable temperature.

Stepping motors, as digital machines, are commonly used in electromechanical systems since they can easily be interfaced to and controlled by computers. Electrical and mechanical interaction in multiple-motor systems, however, can lead to severe control problems, and in intelligent systems such as programmable XYtables, rotor position feedback from each machine is necessary to implement effective control algorithms.This paper describes new methods of detecting rotor position in stepping motors with chopper drives. By monitoring the motor phase current waveforms and detecting the chopping current rise-times, motor phase excitation angle may be varied continuously over a step cycle, and performance may be maximized.Two industrial applications are described: accurate trajectory control in an XY table, where close position coordination between two motors is necessary; and velocity control in a two-motor system where mechanical interaction creates a variable inertia load for one motor.

A closed-loop current controller using power MOSFETs has been constructed for two-quadrant chopper control of a DC disc armature motor intended for use in an electric cycle. Theoretical analysis shows that lower energy requirements and higher efficiency are obtained using constant current, rather than voltage, control, and that approximately 30% of the energy expended in accelerating to maximum speed would be available for regeneration. The practical difficulties of using MOSFETs for motor control are discussed, and experimental results are presented which agree well with the theory.

This paper describes the circuits and algorithms used in a microprocessor-controlled AC/DC thyristor convertor to implement a wide-bandwidth closed-loop current source. The convertor consists of two three-phase bridge circuits for four-quadrant operation, with direct digital control using a M6809 microprocessor. The timing pulse generator is linked to the supply frequency via a phase locked loop, making the system insensitive to supply frequency variation. The use of proportional-plus-integral (PI) control algorithms leads to fast responses to step changes in reference input. Results from constant-gain PI controllers are compared in the paper with theoretical responses based on approximate z-transform analysis to obtain optimum parameters for various modes of operation. In addition, results are presented from a simple variable-gain adaptive controller which adjusts the controller parameters depending on the reference change and operating point.

A general method is described for calculating the current drawn by an inverter from its DC source. It has special relevance to pulse-width modulated inverters and allows the ripple current rating of the filter capacitor to be specified with some precision. The method is for an inverter of any number of phases and uses switching functions derived from the harmonic coefficients of the inverter waveform. Experimental results from single- and 3-phase inverters demonstrate the validity of the method. Some generalised results are presented for a 3-phase inverter for the special case where the output current can be assumed to be the fundamental component only. Variation of the DC link current harmonics with power factor of the load and depth of modulation under these conditions is presented and discussed.

The paper describes the design of a fail-safe sensor used for the detection of the speed of a moving railway vehicle. The sensor is required for use in automatically controlled trains that run at one of a number of predetermined speeds, selected with reference to safety criteria, and governed by conditions ahead of the train. The device uses an axle-coupled permanent-magnet a.c. generator that, at the predetermined speed, saturates a transformer with low-frequency current, choking the passage of an audio-frequency signal from a constant-voltage source, and hence cutting off the output. Positive feedback is employed to sharpen the cut-off slope. A computer model based on a sinusoidal analytical method, superimposing the low and audio-frequency signals, predicts the possibility of in stability and hysteresis occuring in the output characteristic under certain conditions. Computed and test results are found to be in good agreement, and a series of circuit-design criteria are derived.

A theory is developed for the ambipolar diffusion, drift, distortion, and production of ionization inhomogeneities within weakly ionized plasma in the presence of the geomagnetic field and motions of the neutral gas. The theory developed includes spatially varying velocity fields of the neutral gas, with a special emphasis on investigating the length scale restrictions necessary for the plasma to be treated as spatially homogeneous. Several effects of the magnetic field on ionization inhomogeneities are discussed. These include the anisotropy of ambipolar diffusion, the distortion and drift due to a gravitational field, the production of irregularities due to the turbulent acceleration field, the drift relative to the neutral gas, and the distortion of ionization inhomogeneities due to the effects of a uniform wind. Pertinent equations are derived for a multispecies plasma containing positive or negative ions, and for the case of electrons and one species of positive ions.