Precision Fluid Dispensing Using Peristaltic Roller Control

Abstract

A precision fluid dispensing system that can be effectively used for accurate fluid dispensing in peristaltic pumps if roller control is used. A peristaltic pump driven by a primary motor with a first encoder can be combined with pinch valves allowing a dispense tube to be open while a source tube is blocked. Controlled roller movement is used to move the next roller to the same starting position as the previous roller was on the previous dispense, the dispense pinch valve is closed and the source valve is opened. The system uses roller positioning where the each roller is properly positioned so that each dispense cycle starts in the same position. The precision and accuracy is better than 0.3% total. Tubing can be pre-stretched using a second motor and second encoder. Tube wear and stretch during use can be monitored by the second encoder.

Description

BACKGROUND

Field of the Invention

The present invention relates to peristaltic pumps and more particularly to a precision fluid dispensing using peristaltic roller control.

Description of the Prior Art

Peristaltic pumps find use for fluid dispensing in many fields where pump contamination is a concern, aggressive fluids can be used, slurries that can pass through the peristaltic pump or minimizing shear in shear sensitive fluids. A peristaltic pump mostly uses rollers to squeeze, occlude, flexible tubing against a pressure shoe forcing fluid to move between rollers. It is generally assumed in the art that it is very difficult to achieve accurate, repeatable precision dispensing with this type of pump due to roller progression, We have found that this is true except where each cycle can be repeated with the rollers in the same place for the start of a dispense. This is usually one complete revolution of the pump or when a roller is in the same position at the end of a dispense cycle. The cyclic nature of roller progression of peristaltic pumps is shown FIGS. 1A-1B. The results indicate a cyclic nature of the peristaltic pump system where the rollers progress for each dispensing cycle. This is shown in FIGS. 1A-1B. Testing of the three roller system, as shown in FIG. 1A, shows that with each 180 degree index of the pump, there are two groups of weight determinations.

Complete and rotations indicate that the pump repeats very accurately as roller progression has been removed from FIG. 3. Looking at FIG. 1A, it can be seen that the cycle repeats for approximately every 21 cycles. If the rollers are located at any additional fraction of a complete rotation, roller control is needed so that for each dispense the roller can start in the same position. It would be very advantageous to use roller control to achieve accurate dispensing with a peristaltic pump.

SUMMARY OF THE INVENTION

A peristaltic system of the present invention can be effectively used for accurate fluid dispensing in peristaltic pumps if roller control is used. Peristaltic pumps can be combined with pinch valves allowing a dispense tube to be open while a source tube is blocked. When roller movement is used to move the next roller to the same starting position the dispense pinch valve is closed and the source valve is opened. After dispenses and prior to exercising valves, a drip retention is used moving a fluid back from the dispensing tips so when the pinch valve is used the valve actuation safely moves the fluid in the tube. Upon releasing the valve opening the tube to dispensing the fluid moves back to its earlier position. Shown in FIG. 4, is the valve conditions for various connections.

The present invention uses roller positioning where the each roller is properly positioned so that each dispense cycle starts in the same position. The data for this dispensing with the rollers being in the same original position shows that the precision and accuracy is less than 0.3% total whereas most conventional systems only report the precision and accuracy at better than 1.0%. This also depends on the dispensing volume, tubing set and pump/motor being used. FIG. 5 shows roller positioning.

DESCRIPTION OF THE FIGURES

Attention is now directed to several drawings that illustrate features of the present invention:

FIG. 1 shows 3-Roller Pump Cycles.

FIG. 2 shows 4-Roller Pump Cycles.

FIG. 3 shows Half Index Cycles.

FIG. 4 shows a representative Valve Diagram

FIG. 5 shows a Four Roller Pump-Position Control.

FIG. 6 shows a Cole Parmer MiniFlex and Watson-Marlow 313 & 114 Pumps.

FIG. 7 shows a Lexium Motor and Watson-Marlow 114 Pump.

FIG. 8 shows a Lexium Motor Wiring Diagram.

FIG. 9 shows an External Sensor

FIG. 10 shows Portion Dispensing.

FIG. 11 shows Watson-Marlow 114 Rollers

FIG. 12 shows a pre-stretch system

Several drawings and illustrations have been presented to aid in understanding the present invention. The scope of the present invention is not limited to what is shown in these figures.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Peristaltic pumps can accurately dispense fluids if roller control is used to position each roller to the same starting position for each dispense. A motor is used to move the rotor containing the rollers. In order to index rollers to the next dispensing position, it is necessary to close off the tube used to dispense fluid and open a source tube to the pump so that a roller position can take place without fluid dispensing. A roller can be moved in either direction once the source tube is activated, and the dispense tube closed. The present invention uses the forward motor direction so the each roller is moved in the direction of an output of the system. This puts the tubing in the same direction as a dispense move, and not the drip retention direction.

The present invention is designed so that upon each dispense, the number of counts from a stepper motor or an encoder is known. External sensors such as proximity or optical sensors can also be used. A typical stepper motor is made by Lexium. This Lexium stepper motor has 51,200 micro steps per revolution which can be used in determining roller positions. Using a 4-roller pump, the number of steps between each roller is 51,200/4 or 12,800 steps. Knowing the number of rollers, or by sensing each roller in the system, the dispense count after subtracting full revolutions can be used to move the next roller into position.

Steps between roller repeat positions based on a Lexium motor with 51,200 steps per revolution.

• • 2 Roller Pumps—51,200/2=25,600 Steps between rollers
  • 3 Roller Pumps—51,200/3=17,066.667
  • 4 Roller Pumps—51,200/4=12,800
  • 6 Roller Pumps—51,200/6=8,533.33333
  • 8 Roller Pumps—51,200/8=6,400
  • 10 Roller Pumps—51,200/10=5,120
  • 16 Roller Pumps—51,200/16=3,200

If a 4-roller pump is used, the progression can be up to 12,800 steps for each dispense. If 12,800 steps, or and even multiple that is being used for each dispense, then there is no roller correction. Otherwise, for example, assume that the total dispense amount is 288,800 steps for a 4-roller pump. Knowing this value, the next roller can be advanced to the same roller position as in the previous cycle by moving 12,800 micro-steps. Thus, 288,800/12,800=22.5625 roller moves. Discarding the full roller moves, the fraction is 0.5625 or 7200 steps beyond the roller start position. With 12,800 steps between rollers—7,200 is equal to 5,600 steps to bring the next roller into the same start position. If the number of rollers does not divide into the 51,200 possible motor micro-steps evenly, then another calculation can be made. Assume that the pump has three rollers, or 17,066.667 steps between each roller. The calculation is made with 17,066 steps for the first two dispense positions, and then the last move is for 17,068 steps making the step count of 17,066+17,066+17,068=51,200 or one complete motor revolution.

While the preferred motor is a stepper motor, any type of motor may be used including a linear motor or solenoid or other type of actuator.

The pump system also needs to have valve control so that dispense and source valves can be actuated. The use of Lexium motors, the smart motors have input and output built into the motors and can be programmed to operate the pump system correctly. The following sequence is used for accurate dispensing:

1. The pump system is primed allowing all the tubes to fill with the dispensing fluid. First the dispensing leg of tubing is primed, and then the source tubing. This also allows the tube to “wear in” if a new tube is being used.

2. The “Z” pulse of the encoder is found, and the motor is stopped. Each motor has a different “Z” pulse as the motor shafts are not registered to a known position.

3. A step offset from this position is used for the starting point of each dispense. Usually this is at the 10:00 position for the roller, but may be different based on pump type.

4. Several dispenses are made before a weight determination is made. After these dispenses, a single dispense is made and the weight determined. Specific gravity must be known for each fluid. This is repeated several times, and an average is determined where by the corresponding steps can be assigned.

5. The valve(s) are set without excitation with the dispensing valve open and the source valve closed.

6. A drip retention is selected where the fluid is reversed resulting in a movement of the fluid back from the end of the output nozzle. This drip retention is also automatically entered into the dispense calculation.

7. The first dispense has the valves in the not excited values, and at the end of the drip retention, each of the valves is actuated where the dispense valve is closed and the source valve opened.

8. A several dispenses are made before a weight verification is made assuring that the system is setup correctly.

Roller control can be used with various pumps and tubing sets as shown in the following examples where a MiniFlex pump, (left view) from Cole Parmer is used along with Watson-Marlow 313 and 114 pumps (right view). Other pumps and tubing can use the technology.

The system should be wired so that once the computer is removed, a trigger signal can used to the I/O to exorcise each dispense cycle (however, direct computer control of the motor position is within the scope of the present invention). The motor output needs to trigger a opto-amplifier which can handle the current necessary for each of the valve actuations. In the configurations shown in FIG. 5, the motor is coupled to the pump using a Helical coupler allowing a small amount of misalignment while not allowing rotational slip. FIG. 7 shows the model for the 100 system.

The Lexium motor was programmed through the RS485 port using custom Lexium code.

FIG. 8 is a wiring diagram for an embodiment of the present invention.

Roller control can be accomplished using external sensors such as a Banner reflector sensor as shown in FIG. 9. The sensor can count the number of rollers in a given system and then position rollers for dispensing.

Using roller control, a peristaltic pump can also be used as a “soft walled syringe pump” if the portion of the pump where the roller meets the pressure shoe has enough fluid volume. A Watson-Marlow 520 pump system can demonstrate the use of pulsation free dispensing while the roller is in contact with the pressure shoe. The 520 pumps use two rollers that come in contact with the pressure shoe. Preferred tube internal diameters showing volumes using 0.005 tube diameter for 0.0 to 0.02 ml, 0.008 diameter for 0.0 to 0.05 ml, 1.6 tube diameter for 0.0 to 0.2 ml, 4.8 tube diameter for 0.0 to 1.6 mL and 8.0 tube diameter for 0.0 to 4.5 mL. In FIG. 10, a 1.6 mm internal diameter silicon tube can be seen for one full revolution of the pump. The curve represents 5 dispensing cycles where points 1 through 7 can be used for a 0.02 mL pump. Linear pumps can be effectively used for this application.

Pulsation reduction can be achieved by adding more custom rollers to the pump rotor as the sixteen roller 313 Watson-Marlow pump, or the eight rollers to a 114 pump. A drawing of the pump is shown in FIG. 11. The rotors containing the rollers are made of aluminum and manufactured in one CNC Lathe setup. FIG. 11 shows a peristaltic pump 100 that contains a motor 103 and a shaft encoder 104. The motor 103 can turn a set of rollers 101 by turning its central rotor or shaft 102. FIG. 11 shows sets of four, six and eight rollers.

SUMMARY OF FEATURES

Some of the features of the present invention can be summarized as follows:

A precision fluid dispensing system that includes a peristaltic pump with two or more rollers mounted on a rotor configured to execute a sequence of single dispenses; a motor rotationally driving at a rotor containing the rollers; an encoder or external sensor cooperating with the rotor so that the encoder or external sensor determines an absolute circumferential position of the rollers with respect to the tubing. After each single dispense in the sequence of single dispenses, the motor positions the next roller to a position that has an identical angular position with respect to the tubing as the previous roller had before the dispense. The fluid dispensing system is set up so that each single dispense represents one movement of one of the rotor (pressing one of the rollers against the tubing). The precision fluid dispensing system is set up so the sequence of single dispenses along with an optional partial dispense results in a total dispense of a predetermined quantity of fluid. In this present invention, if a stepper motor is used, the stepper motor has a fixed number of micro-steps per revolution; each peristaltic pump has a fixed number of driven rollers, and the number of micro-steps in a single dispense equals the number of micro-steps per revolution divided by the number of driven rollers when this is an integer. In the present invention, if a stepper motor is used, the optional partial dispense has a number of micro-steps equal to the total number of steps required for the total dispense modulo the number of steps in a single dispense. Finally, when the number of micro-steps per motor revolution divided by the number of rollers is not an integer, the number of micro-steps in each single dispense is the number of micro-steps per revolution divided by the number of rollers truncated to the next lower integer for each single dispense except the last single dispense in a motor revolution, with the last single dispense in the motor revolution containing a number of micro-steps needed to bring the total number of micro-steps per motor revolution to the fixed number of micro-steps per revolution.

Parent U.S. Pat. No. 9,567,993 describes a way of holding the tubing during a dispense operation. The present invention adds a second motor and a second encoder in order to access the exact amount of stretch in the tubing. Optionally, in place of a motor, a negator spring may also be used with the encoder. This method is simpler and cheaper since the use of a motor requires a custom motor and encoder assembly; however, a motor with encoder is typically more accurate and controllable. A thin motor such as the Nanotec STF2818 requires extending the 3 mm shaft from the rear, attaching the second encoder thus making a custom motor assembly.

A negator spring can be used in conjunction with an encoder. With a simple negator spring as the top cover to the peristaltic pump, an encoder can read the negator spring advancement. The encoder can be added to the system using simple gearing directly on the outside of a holding device, or directly on the holding shaft, for example, a Nanotec CL3 encoder. In the case of a custom roller position motor/encoder, the encoder can be attached to the roller position controller either directly, or using a processor such as a Programmable Logic Controller (PLC).

Marprene tubing is in common use with peristaltic pumps throughout industry. However, the use of Marprene tubing requires stretching the tube a second time before commencing accurate dispensing. The encoder can indicate when the stretching of Marprene tubing for the second time is necessary (using a second period of dispenses). The encoder also indicates the start of a peristaltic run, and when tubing needs to be changed. The encoder in this case can determine the amount of stretch the tubing has undergone.

In any roller peristaltic system, there is a degrading of the fluid output slope in milliliters due to tubing stretch. The output of the encoder can offset the roller starting position to compensate for this stretch. In other words, the encoder can detect the amount of tubing stretch and compensate by adjusting the roller start positioning which becomes a new roller starting point. As previously stated, the system can be controlled by a PLC by connecting the hold device encoder to the roller starting position. The PLC both drives the motors in the system and reads the encoder.

To begin the process, the tubing is loaded into the peristaltic pump. Tubing can chosen from various tube types of tubing including silicon and marprene tubing. Marprene tubing takes much longer to stretch, but the present invention records the tube stretch and adjusts accordingly. The installed tubing typically has an unknown stretch. The cycle begins with the secondary motor used to provide enough torque to prevent the smaller tubing from going through a set of one way clutch rollers at the output of the pump.

The secondary motor outputs ample torque to create a force equal to the force of the negator springs. This one-way clutch motor has a stretch encoder (second encoder) attached to track the stretch. Once the encoder indicates that the tube stretch is showing a sign of limited to no change, the primary motor starts.

After one dispense sub-cycle, the volume is known from an independent weight scale and stored in memory. If an automated system is being used, the weight system is automatically seen at the PLC, and an adjustment as made to the roller position. The primary motor then makes a drop suction that is adjusted to account for the tubing size. Once the tubing suck-back takes place, the pump output is closed using a first solenoid. The tubing to the waste or supply tank is then opened by a second solenoid. The tube roller for the next roller position is adjusted to the correct position based on volume, and the process repeats.

This process is continued for each tubing type that is used. and a table is created and stored in memory. Marprene tubing undergoes a lengthy transformation, but if it is tied to a check weight system (a scale that records weight of the dispense), the system can undergo a repositioning to compensate. The cycle continues until the stretch encoder indicates a change that is indicative of a tubing change. If the tubing is shifted or replaced the calibration cycle repeats. If a weight check system is used, automatic weights can be loaded into the PLC or processor memory and corrected by entering, or reading on the encoder, the roller positioning. If not, then a user must manually enter the weights.

Embodiments of the present invention add a second (or first) encoder to the holding rollers to indicate when the initial tubing stretch is complete. If the tubing undergoes continuing stretching such as Marprene, the invention uses the encoder to reposition the next roller. In the case of silicon tubing, there is only an initial stretch which is seen by the holding clutches and encoder, so it is known when to start a run. If the tubing wears out after extensive run dispenses, the encoder indicates this, and notifies the user to change the tubing.

FIG. 12 shows a block diagram of a pre-stretch system. A primary motor 207 drives a peristaltic pump 201. A first encoder 200 is attached to the shaft of the primary motor 207. The primary motor 207 and first encoder 200 are both in communications with a processor such as a PLC 206. A secondary motor 202 is set up to stretch the tubing with a second encoder 203 that measures tubing stretch. A return solenoid 204 and output solenoid 205 control the pinch valves previously described. The secondary motor 202, second encoder 203 and the solenoids 204, 205 are also in communication with the processor or PLC 206. The PLC controls the pre-stretch process, controls the primary motor during dispense, records weights and makes roller position adjustments to compensate for weight errors and keep the dispense on target. The processor or PLC 206 has memory that can be disk, random access (RAM), read-only (ROM) and can optionally communicate on a network allowing remote control, remote updating and remote monitoring. The network can be wired or wireless. In particular it can be radio such as WiFi or any other network. The communications paths shown in FIG. 12 can be separate from the network (if one is used), or they can be part of the network.

Again, the stretch compensation process is as follows (a secondary encoder indicates stretch):

• • 1. First the initial tubing stretch occurs under control of a secondary motor. This requires a first, and possibly a second or additional cycles in order to settle the clutch output encoder signal so that the controller knows that the initial stretch is complete.
  • 2. Once the initial stretch has completed. a signal is provided to the primary motor, or to a PLC that controls the primary motor indicating that the system can proceed with a dispense. A full dispense usually requires several dispense sub-cycles and possibly a partial dispense sub-cycle.
  • 3. A dispense sub-cycle is made, and the amount of fluid dispensed of the particular type of fluid being used is determined by a weight scale, or within a special tube that has a check weigh system attached to the tube. The encoder reports primary motor shaft positions to the PLC system.
  • 4. The next step is a drip reversal. The amount of reversal of fluid is selected to compensate for the output pinch valve output closure.
  • 5. With the output pinch valve closed, another valve to the waste depository or the return to the supply container is opened. These two valves are typically controller by solenoids under control of the processor or PLC.
  • 6. The roller is then moved by the primary motor to the correct roller position for the next dispense sub-cycle. Subsequent repositioning of the roller can be made in either direction to minimize waste. The roller position can be determined by a separate encoder (first encoder) on the primary motor or on the roller shaft.
  • 7. The return to waste pinch valve is closed and the output pinch valve is opened.
  • 8. The dispense process continues with a dispense cycle or sub-cycle.
  • 9. Each new roller repositioning can have a small volume adjustment to keep the dispensing volume on its target by a continuous check weight system on the check weight scale. This information is reported to the primary motor control function of the PLC. This compensates for tubing stretch and other error factors.
  • 10. The dispense cycles or sub-cycles continue until a full dispense is complete.
  • 11. A series of full dispenses can be made until the holding tubing encoder begins to report an expansion in counts indicating tubing weakening, excessive stretch or complete tubing failure, each of which indicating that tubing needs to be replaced or at least shifted in position.

Summary of the Basic Dispense Cycle or Sub-Cycle

The system has a peristaltic pump with N rollers on a rotor, where N is a positive integer, and a primary motor driving the rotor. An encoder cooperates with the rotor determining the rotational position of the rotor. A deformable tube passes through the peristaltic pump connected at a first end to a fluid source and connected at a second end to a dispense point. This tube passes through a first pinch valve located between the fluid source and the peristaltic pump, and passes through a second pinch valve located between the peristaltic pump and the dispense point. The system makes a precision fluid dispense by:

a) opening the first pinch valve, closing the second pinch valve, and rotating the rotor to draw fluid into the deformable tube.

b) closing both the first and second pinch valves and rotating the rotor to a predetermined relative dispense starting angular position determined by the first encoder;

c) closing the first pinch valve, opening the second pinch valve, and rotating the rotor a predetermined number of degrees determined by the first encoder to dispense a predetermined amount of fluid;

d) moving the rotor a predetermined amount in an opposite direction to prevent a drip;

The system performs subsequent dispenses by executing steps a), b), c) and d) in order always bringing the rotor to the same predetermined relative angular dispense position determined by the first encoder in step b). The predetermined relative angular dispense position is reached by moving the rotor until one of the N rollers is at a predetermined angle with respect to a vertical direction. This predetermined angle allows each subsequent dispense to start with the rotors in the same geometric positions (the one roller at the predetermined angle will many times be a different roller, but the geometric pattern is the same. For example, if the first dispense starts with one roller at 10 degrees off vertical, every subsequent dispense will start with at least one of the rollers 10 degrees off vertical).

As discussed, new tubing needs to be pre-stretched. The present invention does this with a secondary motor and second encoder attached to it or to a negator spring that can be attached to the peristaltic pump. The pre-stretch can be repeated one or through a plurality of dispense cycles until the second encoder indicates that stretching has reached a minimum or a predetermined amount. In the case of silicon tubing, the stretching will reach a minimum; in the case of Marprene tubing, the pre-stretch will be up to a predetermined amount (Marprene tubing keeps stretching in use, where silicon and other tubing typically does not, or at least only a very small amount). In any case, the processor can keep track of the stretch during use, and as stated, notify the user when the tubing has stretched beyond its useful life.

As discussed, the predetermined angular start position can have a small angular adjustment to achieve a small volume adjustment to keep the dispensing volume to a predetermined amount or target value.

Several descriptions and illustrations have been presented to aid in understanding the present invention. One with skill in the art will realize that numerous changes and variations may be made without departing from the spirit of the invention. Each of these changes and variations is within the scope of the present invention.

Claims

1. A precision fluid dispensing system comprising: a peristaltic pump with N rollers on a rotor, where N is a positive integer;a primary motor driving the rotor;a first encoder cooperating with the rotor, the first encoder adapted to determine a rotational position of the rotor;a deformable tube passing through the peristaltic pump connected at a first end to a fluid source and connected at a second end to a dispense point;the deformable tube passing through a first pinch valve located between the fluid source and the peristaltic pump;the deformable tube also passing through a second pinch valve located between the peristaltic pump and the dispense point;the system constructed to make a precision fluid dispense by a) opening the first pinch valve, closing the second pinch valve, and rotating the rotor to draw fluid into the deformable tube; b) closing both the first and second pinch valves and rotating the rotor to a predetermined relative dispense starting angular position determined by the first encoder; c) closing the first pinch valve, opening the second pinch valve, and rotating the rotor a predetermined number of degrees determined by the first encoder to dispense a predetermined amount of fluid; d) moving the rotor a predetermined amount in an opposite direction to prevent a drip;wherein, the system is constructed to perform subsequent dispenses by executing steps a), b), c) and d) in order always bringing the rotor to the same predetermined relative angular dispense position determined by the first encoder in step b), andwherein the predetermined relative angular dispense position is reached by moving the rotor until one of the N rollers is at a predetermined angle with respect to a vertical direction.

2. The precision fluid dispensing system of claim 1 wherein the sequence of single dispenses, along with an optional partial dispense, results in a total dispense of a predetermined quantity of fluid.

3. The precision fluid dispensing system of claim 1 further including a scale that determines weight of a dispensing volume of fluid in a single dispense.

4. The precision fluid dispensing system of claim 3 wherein the scale determines the weight of each dispense.

5. The precision fluid dispensing system of claim 1 wherein the primary motor is a stepper motor that has a fixed number of micro-steps per revolution; the peristaltic pump has a fixed number of rollers on the driven rotor, and the number of micro-steps in a single dispense equals the number of micro-steps per revolution divided by the number of rollers.

6. The precision fluid dispensing system of claim 1 further comprising a secondary motor with a second encoder attached to the deformable tube constructed to pre-stretch deformable tube before dispensing begins.

7. The precision fluid dispensing system of claim 6 wherein said pre-stretch is repeated through a plurality of dispense cycles until the second encoder indicates that stretching has reached a minimum or a predetermined amount.

8. The precision fluid dispensing system of claim 4 wherein the predetermined angular start position has a small angular adjustment to achieve a small volume adjustment to keep the dispensing volume to a predetermined amount.

9. The precision fluid dispensing system of claim wherein a total dispense includes a plurality of single dispenses; the primary motor being a stepper motor having a fixed number of micro-steps per revolution, and the peristaltic pump having a fixed number of rollers; and wherein, the number of micro-steps per revolution divided by the number of rollers is not an integer, the number of micro-steps in each single dispense is the number of micro-steps per revolution divided by the number of rollers truncated to the next lower integer for each single dispense, except for a last single dispense, with the last single dispense containing a number of micro-steps needed to bring a total number of micro-steps in the total dispense to the fixed number of micro-steps per revolution.

10. A method of making a precision fluid dispense with a peristaltic pump system, the peristaltic pump system having peristaltic pump with a primary motor driving a rotor with N rollers, where N is a positive integer, a first encoder on the rotor; a deformable tube running from a fluid source, passing through the peristaltic pump, and running to a dispense point, the method comprising: a) drawing fluid into the deformable tube by rotating the rotor;b) rotating the rotor so that the nth roller is at a predetermined dispense starting angle determined by the encoder, where n is a positive integer greater than or equal to 1 and less than or equal to Nc) making a partial dispense by rotating the rotor through a dispense angle determined by the encoder causing only the nth roller to pinch the deformable tube;d) closing a pinch valve on the deformable tube located between the peristaltic pump and the dispense point;e) rotating the rotor so that the n+1 roller is at the predetermined dispense starting angle determined by the encoder, where n+1 is a positive integer greater than or equal to 1 and less than or equal to N, and is n+1 is greater than n by one unless n=N, wherein if n=N, n+1=1;f) opening the pinch valve;g) letting n become n+1;h) repeating steps b) through g) until a predetermined total dispense of fluid has taken place.

11. The method of claim 10 wherein a second motor with a second encoder pre-stretches the deformable tube before dispensing starts.