Moving liquid, and moving containers that hold liquid, represent two critically important tasks in the fast-paced field of laboratory automation. Particularly machines used in healthcare, life sciences, pharmacology, chemicals, scientific instrumentation, and also agriculture and packaging are important users of motion control technology.
The keys to machine design success are understanding what type of fluid pump will work best in your motion control application and selecting motion controls that give you the fluid pumping accuracy you need.
There are two overall motion control tasks related to fluid handling. First is the actual movement/control of the fluid itself. This means precisely delivering a particular volume of fluid, a particular flow rate, or a particular pressure. Second is the movement of the containers which hold these fluids, such as test tubes, microscope slides, and a variety of other carriers.
As it turns out liquid handling automation has been around for years, and at the core, these machines share some common characteristics. In particular, they require the ability to move to various points on a work surface, draw in (aspirate) fluid from a container, and deliver (dispense) this liquid to some other container.
Figure 1 shows the liquid handling portion of such a system schematically. The major elements are the pump, the connective tubing, the working fluid, and the aspirate/dispense head (often called the tip).
Along the way we need to control the amount of liquid being drawn in or dispensed very accurately, and isolate this liquid sample from previously handled samples to avoid contamination.
The amount of fluid drawn is always less than the volume of the dispensing tip, ensuring that no sample fluid enters the pump connective tubing. When the pump expels air, the positive pressure pushes the sample fluid out of the tip.
Many liquid handling systems support multiple tips so that liquid can be drawn in and dispensed to more than one port at a time. Some models provide more than 1,000 such ports!
Pumping liquid in accurate measures is at the core of fluid handling system, so let’s take a look at a few different pumps that might be used. It’s worth noting that all of these mechanical pump types can be used in non-precision applications as well. The difference between a precision application and general pump applications is often just the type of motion controls that are used.
Figure 2 shows one of the most common types of pumps for high precision liquid handling. Known as a piston or syringe pump, the liquid is dispensed or aspirated by moving a sealed plunger through a tube.
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, not only for liquid handling robotics such as described above, but for a host of other applications such as liquid chromatography, drug infusion, chemical mixing, and much more.
A variation of the syringe pump is the diaphragm pump, shown in Figure 3. 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 engine, alternately drawing fluid in, and dispensing fluid out.
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, for our general purpose liquid handling system described above, a major disadvantage is that liquid flows in only one direction. So for bi-directional pumping two diagram pumps, one to draw liquid in, and one to dispense liquid, are used.
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 the 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.
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 motion control 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 these pumps for this reason.
For precision liquid handling the main disadvantage of peristaltic pumps is a lack of accuracy. Flexible tubes are elastic, which means their volume over a particular distance 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 pulsed. There are arrangements of rollers that minimize this effect, and control loops that can handle such variable loading dynamics are a must, but compared to syringe pumps fluid output is not as constant.
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 5.
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 are inexpensive 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.
Pumps that move liquid precisely are at the heart of a broad array of patient treatment devices and laboratory and scientific automation. The key to machine design success is understanding what type of fluid pumps will work best in your motion control application. There are different types of pumps, each characterized by different properties and advantages of use.
What does Garnet offer? Garnet has a series of products used to control pumps, ideal for laboratory equipment, pick & place machines and for a wide variety of motion control applications for precise liquid handling.
Juno PMD Integrated Circuits: The Juno family of integrated circuits is perfect for building low-cost, high-performance pump controllers. Juno stands out for its speed and precise torque control.
PMD Magellan Family ICs: Magellan is a high performance motion and positioning control IC. It is the ideal solution both for pump control and laboratory automation, and for general purpose automation.
ION PMD digital drives: ION® digital converters combine a single-axis Magellan IC and an ultra efficient digital amplifier in a robust and compact package for the control of stepper motors, DC Brush and DC Brushless motors. Easy to use, they are ideal for pump control, in laboratory equipment, and other applications in the motion field.
Content inspired from “Precision Fluid Handling: It’s All In The Pump” article, by Chuck Lewin’s , on pmdcorp.comFor more information contact us at firstname.lastname@example.org