This paper is the first in a two-part series on precision fluid handling and motion control. In the first installment we build up the fundamentals of liquid handling by looking at commonly used pumping technologies. In the second installment we focus on the higher level systems that utilize liquid pumping such as laboratory automation, patient treatment devices, and scientific equipment.
More and more motion control systems are dedicated to the precise movement of liquids. While pumps and compressors have been around for a long time, two applications have recently become powerful drivers of this technology. The first is laboratory automation - machines used in life science, chemical analysis, and pharmacology. The second is patient fluid processing including dialysis, drug infusion, plasma & platelet extraction, and more.
Pumping liquid in accurate measures is at the core of these systems. For dialysis machines and patient treatment devices such as drug infusers it's the beginning, and the end, of the problem to solve. As we will see in part II of this series however, for laboratory automation fluid pumping is just one element in a larger system that also incorporates the motion of test tubes, slides, cuvettes, and the various containers that provide and receive the liquid under study.
So, sit back, relax, and pour yourself a glass of your favorite liquid as we take a deep dive into precision fluid handling.
We want to… pump you up
Let's take a look at different pump types that are in common use in precision liquid handling systems. It's worth noting that all of these pump types can be used in non-precision applications as well. The difference between a precision application and a general pump application is often just the type of motion control and sensors that are used.
Figure 1 shows one of the most common types of pumps for liquid handling systems. Known as a piston or syringe pump, as the name suggests the liquid is dispensed or aspirated by moving a sealed plunger through a tube.
Figure 1: Syringe pump
Piston pumps are capable of very high precision. Most use a lead screw driven by a step motor or servo motor. The higher the motor's positioning resolution, the finer the dispensing resolution. Because of this piston pumps have wide application in liquid handling robotics, chromatography, drug infusion, chemical analysis, and more.
A single-piston pump has a fixed total displacement. While liquid can be ejected or drawn in, the total volume of liquid transfer is limited to the volume of the syringe. For continuous flow, piston pumps can still be used but two or more pistons must be arranged so that as one piston is dispensing, another is drawing in fluid. Devices known as check valves, properly arranged in line with the pistons, limit flow to one direction to provide continuous flow.
Key Motion Control Techniques
To deliver a programmable amount of fluid piston pumps require positioning control. So, the motor types we will use are either step motors or servo motors (Brushless DC or DC Brush) with position encoders. If a servo motor, we will control the motor's position with a PID (proportional, integral, derivative) loop. Whether a servo motor or step motor, we will need some sort of profile generator to command the syringe location from point A to point B.
In terms of mechanical dynamics this is a pretty simple system. So trapezoidal profiles (rather than more complex s-curve point-to-point profiles) should be fine. Exotic syringe applications requiring extremely high precision might consider a technique called lead screw mapping. In other applications, the compressibility of the fluid or of the containing vessels factor into the final destination position, in which case you may need to develop some fancy algorithms to compensate.
Can you draw me a diaphragm?
A variation of the syringe pump (perhaps more of a distant cousin) is the diaphragm pump, shown in Figure 2. The similarity to the syringe pump is that a seal moves in and out, thereby displacing the liquid. The difference is that the seal is driven in a reciprocating manner, similar to a gasoline piston engine, alternately drawing fluid in, and dispensing fluid out. If desired, check valves are used to maintain fluid flow in one direction.
Figure 2: Diaphragm pump
Because a simple spinning motor can generate the reciprocating motion needed for this type of device, diaphragm pumps are very popular and truly ubiquitous. They have even been used in artificial hearts, although in this case external air pressure rather than spinning motors cause the diaphragm to move in and out and thereby pump blood.
Coupled with positioning motion controls diaphragm pumps can deliver reasonable accuracies of measured liquid. However, a key strength of diaphragm pumps, particularly in patient treatment applications such as platelet extraction, is that the diaphragm cavity can be arranged to be separable from the drive motor linkage. This means that a brand new sealed, hygienic diaphragm/tube assembly can be swapped out for each patient.
Key Motion Control Techniques
In its basic operating mode of delivering a stream of liquid, the motion control task is a velocity control task. The motor's spin rate is proportional to the liquid flow rate. To deliver controlled pulses of liquid, which is a somewhat unusual application of a diaphragm pump, we go back to needing a full positioning system.
If we look a bit more closely at the operation of the diaphragm pump however, it becomes clear that a constant velocity of the rotary motion will not deliver a constant volume of liquid. And for that matter just the basic process of delivering a precise amount of liquid is not so simple. This is because of the linkage and its variable influence on flow rate. At the bottom and top of the motor arm stroke the rate of fluid delivery will be essentially zero, and at the mid-point the flow will be greatest.
One approach to managing this problem is to use a special profile known as cam profile. Cams consist of translation tables with their input typically being a master encoder datastream, and their output (the lookup table value) being a commanded motor position.
But cams are a flexible generous purpose approach to arbitrarily translating reference frames, so for a diaphragm control application the input stream can just be time (a steady system clock tick generated internal to the motion controller) rather than a constantly changing encoder position. When used in this way, to increase velocity we increase the clock increment and to decrease velocity we lower it.
Note also that we only need to encode the output command position for one motor rotation cycle. Cam profiles are frequently programmed to automatically wrap from the end of the translation table to the beginning at a specified value of the input datastream. Figure 3 shows an example cam translation table.
Figure 3: Cam profile
Depending on the application, in addition to linearizing the output liquid volume the cam may also be used to execute a rapid 'intake' stroke, thereby significantly smoothing out the flow rate. If a double diaphragm pump is implemented, either via mechanical linkage of two chambers from a single motor or via synchronized cam action of two separate motors, the liquid flow from this pump type can be continuous or nearly continuous.
You had me at peristaltic
Figure 4 shows a completely different kind of pump, which, like the syringe pump and the diaphragm pump, has a wide range of applicability in medical, chemical, and general scientific liquid handling applications. Known as a peristaltic pump, this type of pump uses a roller to squeeze a liquid-containing flexible tube, thereby displacing the liquid in the direction of the roller movement.
Figure 4: Peristaltic pump
The big advantage of this type of pump is the separation of the pumping mechanism (the roller) from the medium (the tubing) that holds the liquid. This is ideal for applications where liquid contained in sterile tubing, catheters, or other packaging must be pumped without contacting the contents of the packaging. Dialysis machines, blood transfusion machines, and many similar applications use this pump for that reason.
For precision liquid handling the main disadvantage of peristaltic pumps is a lack of accuracy. Flexible tubes are elastic, which means the volume of delivered liquid for a programmed amount of motor rotation may vary. In addition, the liquid is delivered in 'packets' consisting of the space between two roller engagement points. So, the fluid flow tends to be somewhat pulsed.
Key Motion Control Techniques
Providing control of the rotating peristaltic pump motor is, in the face of it, a simple matter. Typically, a direct drive or geared servomotor is used, while a simple profile generator and PID controller advance the rotor angle.
But looks can be deceiving. The first challenge is that the reflected torque on the drive motor varies significantly depending on where in the tube engagement cycle each pinch roller is. This can lead to variations in velocity, which can accentuate the pulsed flow output. So, one essential need here is a high-performance low velocity control loop.
The larger challenge however is managing the variability of fluid flow rate, particularly for hematological applications where patient-specific variables such as blood pressure exist. There is no magic bullet for these challenges, but a good place to start is using a motion control system with an excellent motion control trace facility along with good motion analysis tools.
Continuously recording motion control parameters such as servo lag and command output torque allows observers to be constructed which can assist with maintaining accurate blood flow rates and pressures for a broad range of operational conditions.
You make an impelling argument
Next on our tour of pumps commonly used in laboratory automation and patient treatment applications is the centrifugal pump, specifically the sealless centrifugal pump. The major elements of this pump type, shown in Figure 5, are the inlet, the magnetic coupling, the impeller, and the outlet.
Figure 5: Centrifugal pump
Fluid flow occurs due to the action of centrifugal force as liquid enters, is 'spun out' by the rapidly rotating impeller, and then exits via the outlet. The magic of sealless operation is achieved by magnets located inside the impeller which couple to rotating magnetic fields driven by a motor shaft external to the impeller assembly. In some designs the impeller floats freely once rotation starts, held in place by the hydrodynamic forces created during rotation.
Like peristaltic pumps, sealless centrifugal pumps have the advantage of complete hermetic isolation. In addition, they are exceptionally durable because unlike peristaltic pumps there are no tubes being contacted and wearing down. This is one reason why centrifugal pumps have been developed as candidates for artificial hearts.
On the negative side these pumps are not designed to deliver a precise quantity of fluid or even a precise flow rate, so their application is limited to patient treatment devices where those variables, if important, can be measured or inferred separately.
Key Motion Control Techniques
The primary motion task here is to spin a Brushless DC motor fast and with the highest possible efficiency. Heat and excess energy consumption are the enemy of patient treatment devices, and so the key motion technology that we use here is FOC (Field Oriented Control).
FOC is a technique, that while algorithmically complex, operates BLDC motors more efficiently then 6-step trapezoidal commutation and forms the basis of nearly all modern motion controllers when driving high speed spindles and turbines such as pumps of this type.
We're operating in a vacuum
Finally, for precision fluid handling, it is possible to use general purpose air pumps coupled with sensors to deliver surprisingly precise measures of fluid. This type of arrangement is shown in Figure 6.
Figure 6: Air pump
Here's how it works. A general purpose air pump that can create positive or negative pressure is connected to the working fluid tube. Sensors measure the actual air pressure imparted by the pump. To determine the amount of liquid transferred, the air pressure is monitored at regular intervals and a previously created lookup table that converts pressure differential to flow rate is used to sum the total amount of liquid transferred.
The advantage of this arrangement is compactness and low cost. Air pumps can be very cost effective and come in a wide range of shapes and sizes. At the same time, electronic pressure sensors are continuously becoming smaller and less expensive. So, for many applications, this basic pumping system is perfectly adequate.
Key Motion Control Techniques
Air pumps can be viewed as 'cousins' of liquid pumps. For example there are piston air pumps, diaphragm air pumps, and gas versions of most of the pump types discussed above. One important difference however is that liquids are generally not compressible while air and other gasses most certainly are.
This often means that pressure sensors play a central role in precision air pumping applications. While there is typically still a motor (often one that drives a turbine to pump the air) in the system, an outer control loop is often used to precisely maintain a desired pressure profile.
Figure 7: Outer loop control flow
Figure 7 shows a typical outer loop control flow for systems such as this. The outer loop receives a commanded pressure and combines this with the actual measured pressure and after passing through a servo filter outputs a commanded velocity (motor spin rate) to the downstream velocity loop, which in turn sends a desired torque to the motor amplifier controller.
As it turns out outer loop controllers are quite common, not only in the medical/laboratory world but in a broad range of industrial processes as well. In addition to controlling pressure, outer loops are used to control temperature, liquid level, reaction rates, and more. Outer loop controllers vary in their complexity, but happily there are off-the-shelf IC and module products which can integrate a complete outer loop control function.
Pumps that move liquid precisely are at the heart of a broad array of patient treatment devices and laboratory and scientific automation. The keys to machine design success here are understanding what type of fluid pumps will work best in your motion control application and what type of motion control techniques will give you the fluid pumping accuracy that you need.
In part II of this series we will show how we can put the pumps detailed in this article together to create a complete laboratory automation application. This broad and growing class of automation include devices such as blood analyzers, DNA sequencers, chemical assayers, and a range of related machines used in life sciences, pharmacology, chemicals, and agricultural industries.
PMD Products Used To Control Pumps
Performance Motion Devices has been producing motion control ICs that provide advanced position and torque control of step motor, DC Brush, and Brushless DC motors for more than twenty-five years. Since that time, we have also embedded these ICs into plug and play modules and boards. While different in packaging, all of these products are controlled by C-Motion, PMD's easy to use motion control language and are ideal for use in pick & place machines, laboratory equipment, liquid handling, and a wide variety of other high performance motion control applications.
The Juno family of ICs are perfect for building your own low cost, high performance pump controller. Juno's excel at velocity and torque control, with features such as FOC (Field Oriented Control), profile generation, high/low switching amplifier control signal generation, leg current sensing, and more. Available in packages as small as 7mm x 7mm and costing $12 in quantity, these ICs are an ideal solution for your next pump controller design.
The MC58113 series of ICs are part of PMD's popular Magellan Motion Control IC Family and provide advanced position control for step, BLDC, and DC Brush motors alike. Standard features include support for cam profiles, trapezoidal & s-curve profiling, direct encoder & pulse & direction input, and much more. MC58113 ICs have an advanced trace capability that lets you collect critical pump performance data as fast as twenty times per mSec, or as slow as once a day. Whether used for pump control, laboratory automation, microscope controllers, or general-purpose automation, the MC58113 family of ICs are the ideal solution for your next machine design project.
ION® Digital Drives combine a single axis Magellan IC and an ultra-efficient digital amplifier in a compact rugged package for control of step motors, DC Brush, and Brushless DC motors. In addition to advanced S-curve & trapezoidal profiles, cam profile support, and intelligent motor performance analysis software, IONs are loaded with safety functions such as over current, over voltage, and over temperature detect. IONs are easy to use plug and play devices that will get your next pump controller, laboratory equipment, patient treatment, or automation design project up and running in a snap.
Pro-Motion is PMD's easy-to-use Windows-based exerciser and motion analysis program. It offers ready-to-go capabilities your entire development team will be able to share. A step-by-step axis wizard allows designers to quickly and easily tune position loop, current loop, and field-oriented control motor parameters. Advanced users can access a complete motion analysis package with Bode plot generation and auto-tuning.
Related papers and resources:
- Feedforward in Motion Control - Vital for Improving Positioning Accuracy
- Precision Fluid Handling, Part II: Optimizing Lab Automation Mechanics
- Motion Control Techniques for Improved Liquid Handling
- Motion Control Technology Trends for Medical and Laboratory Applications
- Improve Liquid Handling Robot Throughout with Direct Path Planning
- Optimize A Control Architecture for High Accuracy Syringe Dispensing