Summary: ? Cabin of beat up of rotten Gua of yellow Yong of Piao of pharynx an ancient nationality in China is thin go up Lian is low hold Xing Di still ぜ again Bian of an ancient nationality in China punishs channel Bian to divide fade of Jing of sexual left-eyed flounder to go straight towards し of You Si of of Kun of male of Su J to wring reef to seek falling into oblivion to attack Piao of earnest of Qian , gradient magnetic field is used to encode space position. It is by gradient weaveform generator resembles alignment asking to output gradient weaveform according to becoming, incentive gradient amplifier exports gradient electricity, drive gradient coil forms. Output of ideal gradient weaveform generator, gradient amplifier output and weaveform of gradient magnetic field see graph 1(a)(b)(c) . But in the as a result of ferromagnetism material existence in effective system, gradient electric current jumps the change of the gradient magnetic field that is out of shape is met amid generation is faradic, namely eddy current. The way that the magnetic field direction that eddy current derives always builds with gradient magnetic field is opposite, because this meets those who defer gradient magnetic field to build, see graph 1(d) . This kind is deferred can become the function that resemble to produce bigger effect to MRI system. Generator outputs weaveform of graph 1(a) ideal gradient (amplifier of B) ideal gradient is outputted (magnetic field of C) ideal gradient is undee (magnetic field of D) actual gradient is undee (E) has the gradient weaveform that accentuates beforehand generator is outputted (F) has the gradient that accentuates beforehand amplifier is outputted (G) has the gradient that accentuates beforehand magnetic field weaveform. Among them one of it is gradient accentuates beforehand (Pre-emphasis) . Gradient accentuates beforehand is to be on the output weaveform of gradient weaveform generator (on the output electric current of graph 1(e)) or gradient amplifier (graph 1(f)) adds on one to rush too beforehand, should develop the effect of quits backset field too, quickened gradient magnetic field build, see graph 1(g) . To get used to the circumstance of different backset field, this too strong extent and time constant are adjustable. Graph the imitate type gradient in 2 gradient amplifier accentuates beforehand gradient of road of the X in circuit gradient amplifier, Y, Z3 is added commonly imitate type gradient accentuates beforehand (call eddy current to compensate sometimes) circuit. This kind of circuit by adjustable the operation amplifier of gain + adjustable circuit of RC time constant is formed, 2(seeing a picture 3 identical, draw X road) only. Give aleatoric too strong weaveform for combination, have multistage normally such circuit is shunt-wound, every class has different time constant (graph 2 circuit have 4 class) . Gain and the adjustment with invariable time use a hand to move potentiometer many rounds. Structure of this kind of circuit is simple, beardless make any computation, cost inferior. But it also has inherent drawback. Because use imitate parts of an apparatus entirely, do not suit to be controlled with any digital parts of an apparatus, multistage gain and time constant need to be made with screwdriver artificially multidimensional adjust, workload is great and consistency, but repeatability is very poor, also cannot realize self-correcting by computer closed-loop control. The article uses numerical control potentiometer (the function with distinctive DCP) , improved afore-mentioned imitate type gradient to accentuate beforehand circuit, achieved a number to dominate the end that gradient accentuates beforehand. Numerical control potentiometer pursues the potentiometer of internal composition numerical control of 3X9250 is mixture parts of an apparatus of a kind of digital-to-analogue, sketch map sees a picture 3. The resistor array that its interior has a series (the resolution that the amount of resistor decided DCP, have 32 normally, 64, 100, 256, 1024 wait for) . The join dot between every two resistor joins through an electron switch central take a percentage of a deal is carried. Electronic switch passes control of bus line interface to be connected by the user, the place that knows decided central take a percentage of a deal carries the position in resistor array, can decide end of central take a percentage of a deal is apart from resistor array consequently the resistance of two end. The place that the change knows can change this resistance. Accordingly from resistor array two ends and central take a percentage of a deal look, DCP is behaved so that be like is a common 3 end adjustable potentiometer is same, the difference depends on common potentiometer only is to pass knob or tool hand to move successive and adjustable, and DCP is to pass bus line to input instruction pace to enter adjust. DCP has different configuration and form. The X9250DCP of the Xicor firm that uses with the article is exemple, it enclosed 4 identical DCP inside a parts of an apparatus, every DCP has position of 256 take a percentage of a deal and register of data of 4 blame losable, position of 4 take a percentage of a deal can remember after DCP drops report, automatic when electrify reach the tap position to load of account of 0# data register tap position register; Its control interface is SPI serial interface of the standard, control dictates by test and verify byte, instruction byte and data byte are formed, see a picture 4. Block value has 100K Ω , 50K Ω , the number carries power supply 2. 7~5.
5V, imitate carries power source to be ± 2.
7~5.
5V. Enclose a form to have SOIC and XBGA two kinds. The control instruction of graph 4X9250 forms a number to control gradient to accentuate beforehand circuit design and the way of this kind of work that realize DCP provided a basis for the design of the article. Its principle is to use DCP to replace imitate type gradient to accentuate beforehand the hand in circuit moves potentiometer, with general purpose computer, sheet piece the tap position that the bus line interface that the digital controller such as machine, DSP passes DCP will come to control DCP, adjust thereby the range that gradient develops weaveform too in aggravating circuit beforehand and time constant. Specific implementation circuit sees a picture 5. With the graph 2 same, graph 5 in the circuit that shows a X way only, the others two road and this is identical. In the graph 5 in, the control signal XSI that comes from a number to control parts of an apparatus and XSCK give out respectively the data of standard of SPI serial interface with clock. The extent that digital control parts of an apparatus inputs according to the user and time constant are worth, or the signal cost that collects according to MRI system, automatic computation gives extent and the value with invariable time, become these numeric changeover the statement form of DCP, send into corresponding DCP. Course extent and the gradient weaveform after time constant is handled increase circuit U5 and output of addition of former gradient weaveform to gradient amplifier through undee alternate. The bag in a X9250 contains 4 DCP, the P0 of byte of the instruction in dictating through control, P1 choice. Its bring a foot to go up to still two parts of an apparatus choose an A0, A1, the A0 of byte of the test and verify in dictating through control, A1 identify, because this passes the combination of A0, A1, P0, P1, can find site with control instruction only the any in 16 DCP. This design uses two X9250 only, in all 8 DCP, reason chooses parts of an apparatus with A0, a1 ground connection, p0, P1 chooses one of 4 DCP in parts of an apparatus. Below the case that does not surpass 16 DCP, do not need circuit of adscititious address coding, CS end is OK from beginning to end ground connection. Data byte gives out the register of central take a percentage of a deal that the position that central take a percentage of a deal carries sends into DCP writes 0# data register. What once adjust good gradient,accentuate beforehand so is undee, can permanent like imitate potentiometer save. In the circuit of eddy current compensation that the circuit of the article can regard one part confluence as to enter gradient weaveform to produce circuit or gradient amplifier already when application, also can string together as an alone component receive in accentuate beforehand without gradient between the gradient weaveform generator of circuit and gradient amplifier. The digital control gradient that the article elaborates epilogue and realized a kind with DCP accentuates beforehand circuit, it uses digital control, the pattern that imitate adjusts, make magnetic field of the gradient in improving MRI system through accentuating beforehand establishs undee method OK the digital control parts of an apparatus such as computer of have the aid of will finish. CNC Milling
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