« PreviousContinue »
United States Patent  [ii] Patent Number: 4,812,724
Langer et al.  Date of Patent: Mar. 14, 1989
U.S. Patent Mar. 14,1989 Sheet 1 of 2 4,812,724
 INJECTOR CONTROL
 Inventors: Alois A. Langer, Forest Hills; Albert W. Rinne, Bethel Park, both of Pa.
 Assignee: Liebel-Flarsheim Corporation, Cincinnati, Ohio
 Appl. No.: 670,525
 Filed: Not. 13, 1984
 Int. Cl.« G05B 11/28
 U.S. a 318/599; 318/468;
 Field of Search 128/655, DIG. 1;
604/67, 65, 66, 155; 318/376, 599, 609, 610, 611, 615, 603, 327, 341, 318, 335, 302, 481, 563, 565, 564, 434, 466-467, 468; 361/168.1, 194, 186, 203; 307/442, 116, 123
 References Cited
U.S. PATENT DOCUMENTS
3,553,535 1/1971 Weber 361/194
3,596,111 7/1971 Harr 361/194
3,603,857 9/1971 Crane 318/302
3,611,092 10/1971 Wilmunder 318/302
3,623,474 11/1971 Heilman 128/655
3,628,101 12/1971 Dietz 361/194
3,631,847 1/1972 Hobbs 128/655
3,701,345 10/1972 Heilman 128/655
3,721,866 3/1973 Mcintosh 361/194
3,731,679 5/1973 Wilhelmson 128/DIG. 1
3,736,930 6/1973 Georgi 604/67
3,864,608 2/1975 Normile 361/194 X
4,006,736 2/1977 Kxanys 128/655
4,230,977 10/1980 Nelson 318/302
4,311,949 4/1982 Pelkmann 318/341
4,328,800 5/1982 Marx 604/67 X
4,422,619 12/1983 Griffiths 318/563 X
4,435,173 3/1984 Siposs 604/155
4,449,082 5/1984 Webster 318/327
4,475,073 10/1984 Hawkins 318/609
4,475,666 10/1984 Bilbrey 604/155 X
4,477,753 10/1984 Rated 318/563
4,491,905 1/1985 Arakawa 318/563
4,501,531 2/1985 Bilstad 604/67 X
4,529,401 7/1985 Leslie 604/67
4,697,221 9/1987 Pasquarella 361/194
Primary Examiner—Bentsu Ro
Attorney, Agent, or Firm—Wood, Herron & Evans
A control system for an angiographic injector which precisely regulates the speed of a motor used to expel contrast media from a syringe. The control system is centered around a microprocessor which receives commands from the operator via a keyboard, and then regulates the speed of the motor by supplying drive command pulses to a circuit which integrates the difference in frequency between the command pulses and another pulse train whose frequency is proportional to actual motor speed. The control system also includes a velocity loop which uses the motor back EMF to supply an analog voltage also proportional to motor speed to a difference amplifier in a conventional velocity loop. The system is a dual loop controller where the second loop provides approximate speed control, while the first loop provides a "velocity correction" voltage required to bring the system to zero steady state speed error. Motor speed pulses are provided by an incremental encoder, the pulses from which are counted by the microprocessor to limit the volume of fluid injected. Various safety systems guard the injectors performance and help prevent over volume injections. These systems are part of the velocity control and are also disclosed.
6 Claims, 2 Drawing Sheets
BACKGROUND OF THE INVENTION
This application relates to the general field of angiography; which is the technique of studying the vascular system by means of X-rays while injecting a "contrast media," typically an iodine based fluid, into the blood vessel or organ to be studied. In order for this process to be maximally successful, the flow from the syringe must be precisely controlled and this control has been the subject of several patent applications. These systems have all exhibited various problems, most of which are eliminated by the system disclosed here. In U.S. Pat. Nos. 3,623,474 and 3,631,847 issued to Heilman et al and 15 Hobbs respectively, a signal directly proportional to flow rate is fed back to produce a flow error signal used to control motor speed. The disadvantages of straight velocity control systems are discussed in Heilman's subsequent U.S. Pat. No. 3,701,345 and include mainly 20 the problems in measuring motor velocity over a wide range. For example, the velocity feedback system originally disclosed included a tachometer which produced an analog feedback voltage. This feedback scheme is later described to drift and be subject to noise at low 25 speeds. To solve this problem, in U.S. Pat. No. 3,701,345 a position feedback system is described. In such a system, a potentiometer is used to sense the position of the plunger. The position signal is compared to a command signal and the difference drives the motor. 30 If the command is a ramp of constant slope, then the motor will move at nearly constant velocity. The position signal from the potentiometer is also used to limit the volume of fluid ejected from the syringe by comparing it to a known position command signal which repre- 35 sents the position at which the injector should stop.
Even this improved control system has its limitations, however. Since position is fed back, the system only guarantees that the average velocity between two positions is such that the plunger is in the right place at the 40 end of a certain time interval. This means that velocity can increase and decrease around this average value as long as the average stays the same. From a control systems point of view, there is an extra integrator in the feedback loop and system stability is compromised. The 45 implications to performance in angiography is that a position control system is not as resistant to factors which might cause velocity to briefly change. Such a factor which is quite significant, is imperfections in the ballscrew mechanism, which is typically used to con- 50 vert the rotary motion of the motor to the linear motion required to move the plunger. The present injector is of significantly greater power than previous machines by almost a factor of two. The force on the ballscrew is doubled and a larger ballscrew has to be used. This 55 means that the possibility for velocity variations due to ballscrew effects is increased. The inventive control system addresses this problem by controlling velocity directly and incorporates a velocity correction feedback path to maintain velocity control with great accu- 60 racy.
Angiographic injectors have typically employed a mechanical means for stopping the plunger in case of failure. Given the increased power of this system, such mechanical means would have to be extremely rugged, 65 making the injector head very large and unwieldy. However, given the power of microprocessors for performing performance tests at extremely high speed, it is
possible to design an injector with microprocessor control and eliminate any mechanical means for limiting plunger travel. Given the fact that many users do not use mechanical limits, the fact that they are subject to great stress and may in fact fail themselves and the impossibility to set them for each volume limit in a multi-bolus injection, the use of an "electronic stop" can in fact result in enhanced system reliability.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for improved injection rate control in an angiographic injector, and in particular, a method yielding great system stability and being resistant to factors which may excite modes of short-term speed variations. Since a microprocessor is being used in the injector to enhance flexibility for injection parameter entry by the operator and to provide a means for sophisticated "electronic stop" volume limiting backup mechanisms, a further object of the invention is to provide a control system which is easily commanded by a microprocessor.
Microprocessors by their nature cannot directly control in an analog fashion such as has been the custom in the past. To be part of a system in an analog fashion with great precision requires a precise digital to analog converter, a very expensive and potentially unreliable component. Microprocessors are best suited to supplying some sort of pulse perhaps with the pulse interval representing the variable to be controlled. It is, therefore, an object of this invention to provide a velocity control system which is commanded by a computer's natural output, namely, a pulse train.
Computers have one potential drawback, however, and that is they are subject to "crashing." This means in general that the processor fails to execute its program in a controlled manner and, in fact, may be running useless code. A "crashed" computer may do unpredictable things, and it is an object of this invention to provide circuits which cause the motor not to run in case of microprocessor failure.
Preparing a patient for angiography is a significant procedure. A catheter is introduced into a peripheral artery or vein or other vascular element so the process is invasive. Should the injector fail, all the patient preparation might be in vain, so therefore it is an additional object of the invention to provide a self test feature so that it can be determined whether or not the machine is operational before it is asked to perform an injection. This self test feature is activated when the machine is turned on and is very thorough and extensive.
Basically, the present invention is an improvement over the prior art by employing dual loop control. One of the loops is very precise and is under direct control of the microcomputer. Plunger position is tracked by two independent measuring circuits, an incremental encoder and a potentiometer. Since volume is such a critical parameter, these are not arranged as a primary circuit and backup circuit but rather are constantly being simultaneously monitored by the computer. Both are measures of injected volume and must agree with each other in order for the injection to continue. Thus, both volume measuring circuits must be operational or else the machine stops. The incremental encoder is based on a wheel attached to the motor shaft and gives, say, 32 pulses per revolution of the motor. These pulses are counted by the computer and are used to limit the injec