Electronic cam compensation of pressure change of servo controlled pumps

Abstract: A pump system designed to minimise pressure changes at pump changeover by sampling pump pressure characteristics for each pump cycle, calculating a compensating motion profile and applying the profile to the motor which drives the pump. This control can be used with any pump which has the following characteristics: positive displacement, repeating cycle characteristics, rotary motor drive and an output pressure cycle curve which never falls to zero.

Description: BACKGROUND OF THE INVENTION Various types of pumps have been used for transfer and circulation of fluids for many years. In many cases the desirable pump design is a piston pump however one of the less desirable aspects of such piston pumps has been that such pumps are prone to output pulsation which requires either compensation or the willingness to live with such pulsation. One such attempt at reducing pulsation is shown in US Patent 5,145,339. While such a construction is a substantial advance over other prior art designs, some pulsation does remain. Further prior disclosures are as follows:- EP 0334994 discloses a reciprocating type fluid delivery pump designed so as to compensate for pulsations occurring due to the compressibility of fluid. WO 9732128 relates to a pump system comprising two piston pump units, at least two cylinders each with a piston and control unit for controlling the movement of the pistons in each cylinder so as to deliver a pulse-reduced flow in response to measurement data indicative of pressure changes in the system on the pressure side. US 4137011 provides for a flow control system comprising a motor driven pump for delivering a carrier to a column under pressure wherein pulsations and flow changes are removed so as to provide a generally constant flow during operation of the system by continually draining the pressure reference level applied to the pump. US 4810168 discloses a low pulsation pump device wherein a pulse control means determines the location of a high speed region in each period and on the basis of pressure information, reduces pulsation gradually. EP 0050296 provides for an improved pulsation-free reciprocating volumetric pump wherein a circuit controls a driving motor for controlling rotational rate. Another circuit is used for detecting a pressure of the combined volume discharged by two plungers. SUMMARY OF THE INVENTION It is an object of this invention to provide a piston type pump which is substantially free of pulsation or at least to reduce such pulsation and yet which retains the desirable aspects of a piston pump. Other pumps which also enjoy at least some pulsation include gear pumps and lobe pumps. This invention is applicable to all such pumps in order to decrease pulsation. In accordance with a first aspect, the invention provides a pump system, including a pump, a rotary motor, means for sampling the pressure curve for each pump cycle; means which calculate a compensating pressure curve from said sampling means; and control means for the motor for applying said compensating pressure curve; characterised by further comprising means which calculate the phase lag between a control input to the motor control means and the pressure sampled by the sampling means, and compensating for said lag. As preferred the pump is a multi-cylinder reciprocating pump, preferably a dual piston pump. The system according to the invention can be used with any pump which has the following characteristics: positive displacement, repeating cycle characteristics, rotary motor drive and an output pressure cycle curve which never falls to zero. In a further aspect, the invention provides a method for controlling a pump having a rotary motor and a motor control means, comprising: sampling the pressure curve for each pump cycle; calculating a compensating pressure curve from said sampling means; controlling said motor by applying said compensating pressure curve; and characterised by calculating the phase lag between a control input to the motor control means and the sampled pressure and compensating for said lag. The construction and operation of the control of this invention is designed to minimise pressure changes at pump changeover by sampling pump pressure characteristics for each pump cycle, calculating a compensating motion profile and applying the profile to the motor which drives the pump. In fact, this control can be used with any pump which has the following characteristics: positive displacement, repeating cycle characteristics, rotary motor drive and an output pressure cycle curve which never falls to zero. This control system is thus able to minimise the fluctuations in pressure at pump changeover. Additionally, it has the ability to adaptively modify motion profiles to compensate for condition changes such as rate changes, material changes (viscosity, etc.). It also has the ability to diagnose pump performance, deterioration and failure. Previous attempts to create pulseless output have used mechanical methods such as the aforementioned US Patent 5,145,339. While attempts have been made to compensate for pressure changes by electronically closing the velocity loop or maintaining a constant torque load at the motor, these methods are reactionary and thus have a tendency to overcompensate and be delayed due to the high inertia of the system. This is particularly true since the pressure changes tend to be relatively quick pulses especially as pumps reach higher flow levels and higher speeds. To reduce overcompensation, gains may be lowered but then the pulsation will be reduced and not eliminated. Continually sampling the output pressure curve of the pump and calculating a true compensating motion profile addresses both of these problems. Continuous sampling by the control can compensate for changing conditions and also diagnose pump degradation and failure. By modifying the motion profile of the pump simultaneously with the pressure change, overcompensation of the pressure output is eliminated. Also, by adjusting phase, the motion profile can compensate for mechanical lags in the system. A BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the invention will appear more fully from the following description made in conjunction with the accompanying drawings wherein like reference characters refer to the same or similar parts throughout the several views. Figure 1 is a schematic of a preferred embodiment of a pump system according to the invention. Figure 2 is a graph of actual response and calculated compensating response. Figure 3 is a graph of a singular compensating profile. DESCRIPTION OF THE PREFERRED EMBODIMENT Illustrated in Fig. 1 is a system 10 consisting of a low pulse dual piston pump 12 (a preferred form of a multi-cylinder reciprocating pump, which term includes gear pumps and lobe pumps) driven by a servo motor 14. Of course, other pumps and motors may be utilised. The absolute position of the pump 12 is determined by a proximity sensor 16 tracking a singular position of the pump for each pump cycle and then an encoder (17) determining the absolute position of the servo motor coupled to the pump. A pump sensor 18 at the output of the pump 12 monitors the instantaneous pressure. A computer 20 records the pressure output of the pump 12 correlated with the absolute position of the pump 12. By analysing at the pressure curve for single or multiple cycles of the pump, a pressure curve versus position can be determined as shown in Figure 2. Thus, compensating profile (also shown in Figure 2) can be calculated for the motor which when applied to a motor drive 22 results in a pulseless output. In addition, computer 20 is operative to determine the phase lag between the control signals applied to motor drive 22, and the resulting pressure variation sensed by sensor 16, and to compute an appropriate compensation. This analysis is compensation can be repeatedly applied to continuously tune the system. By continuously monitoring the pressure, any condition that is out of the normal range of pump characteristics can be realised and an appropriate alarm supplied indicating a fault. In addition, continually growing compensation may well be evidence of deterioration and an alarm can be sounded at the appropriate time. It is significant that a single phase advance may be a characteristic of a pump. This can be determined by observing the response delay of the output to a pressure spike input which can be easily ascertained during running. For example the output may lag an input by X degrees of motor/pump rotation. It appears that a singular compensating profile may be applicable to most pressure drops with its amplitude and length determined by the pressure drop amplitude, area and length. This would significantly reduce the calculations needed for the cam to compensate in real time. Figure 3 might correspond to such a compensating profile.

Claims: A pump system, including a pump (12), a rotary motor (14), means (16,17) for sampling the pressure curve for each pump cycle; means (20) which calculate a compensating pressure curve from said sampling means; and control means (22) for the motor for applying said compensating pressure curve, characterised by , further comprising means (20) which calculate the phase lag between a control input to the motor control means (22) and the pressure sampled by the sampling means, and compensating for said lag. A system according to claim 1, further comprising means (17) for determining the rotary position of said motor. A system according to claim 1 or 2, wherein the pump is a multi-cylinder reciprocating pump, preferably a dual piston pump. A method for controlling a pump (12) having a rotary motor (14) and a motor control means (22), comprising: sampling the pressure curve for each pump cycle; calculating a compensating pressure curve from said sampling means; controlling said motor by applying said compensating pressure curve; and characterised by calculating the phase lag between a control input to the motor control means (22) and the sampled pressure and compensating for said lag.

Inventors: HANDZEL JAMES J, SCHAFFER ERIC J, WERNER NEAL A, HANDZEL, JAMES J., SCHAFFER, ERIC J., WERNER, NEAL A.
Applicants: GRACO INC, GRACO INC.
Application year: 1997
Main classification: F04B11
IPC codes: F04B49, F04C15, F04B11