Medical Device Link.

DESIGN

Electro drives explained

Most medical devices are electrically powered. Consequently, all subsystems also rely on electricity as a power source. Electric drives of all kinds will, therefore, be the focus of this article. All electric drive systems are based on the well-known effect of electromagnetism. Electrical power is transformed into magnetism, which is then used to produce the rotating movement of a drive shaft (AC or DC motors) or the oscillating movement of a vibrating armature. Shaft and armature are connected to a diaphragm in the pump head where the desired flow and pressure is produced.

Table I. Different types of drive systems.

Table I shows the different types of drive systems that are typically employed. There follows an outline of the benefits of these drive systems with respect to performance in miniature pumps for specific medical applications. The systems are compared by their resulting pneumatic performance (in terms of maximum flow and maximum pressure or vacuum, size, weight, noise, lifetime and cost).

DC motors

DC motors are found in most of the miniature pumps used in medical applications. Their availability in a huge number of variants with different speed and torque characteristics, matched by a wide spread of qualities and prices, makes them the champions amongst the drive systems. All kinds of pneumatic performances can be realised with DC motors. They are ideal for low-pressure and vacuum applications that primarily demand flow, as well as applications where pressure and vacuum well above 300 mbar is required.

Figure 1. (click to enlarge) Forces in the front bearing of a DC motor shaft.

The size of the motors in relation to their electric performance can be described as small. A life-limiting element in all DC motors is the bearings of the motor shaft. The front bearing in particular will be subjected to dynamic loads, which may be high depending on the application. A mid-size eccenter-diaphragm pump with a 40-mm diameter diaphragm operated at a pressure of 700 mbar will create a dynamic force of almost 90 N. Figure 1 shows that depending on the relation between L1 (the distance between the point where the force (F) is introduced into the shaft and the front bearing) and the length of the motor (L2) this may result in much higher forces (F1) in the front bearing of the motor shaft.

Consequently, the selection of adequate bearings (sinter bearings or ball bearings, or a combination of both) in the DC motor will be the deciding factor in terms of the life of the pump. Lifetimes may reach 800–1000 h with sinter bearings in a free-flow, that is, no-load application, whereas ball-bearing equipped motors can reach more than 3000 h in the same application.

The electrical system of a DC motor consists of a static part (stator) and a rotating part (rotor). The stator holds a set of permanent magnets and the rotor holds a set of coil windings in which a magnetic field will be induced depending on the polarity of the current fed into the coils. The polarity of the current must be changed rapidly and continuously to make the rotor turn. Different systems to achieve this commutation (unidirectional current) are used and the choice is between DC motors in brushed and brushless designs.

Brushed DC motors

Figure 2. (click to enlarge) A partly disassembled brushed DC motor.

In brushed DC motors, the electrical current is passed from the static parts into the rotor with the help of brushes generally made from carbon or precious metal. Figure 2 shows a partly disassembled DC motor with the commutator partly pulled out of the brush assembly.

Figure 3. (click to enlarge) DC motor with rotor pulled out of the brush assembly.

Figure 3 shows the completely disassembled motor. The brushes are clearly visible in the left part. They are spring loaded so that they move forward when they become shorter as a result of wear. When the commutator is pulled out, the springs push them all the way forward.

The brushes in the motor are a source of noise and also wear because of the mechanical contact of the brushes with the moving rotor. The electric current is passed from the power source through the brushes into the rotor and the friction between the brushes and the rotating commutator will naturally cause wear and result in a limited life for the motor. This fact must be remembered when selecting a brushed DC drive for a pump. Depending on the design and the quality of the selected materials, brush life can vary significantly. As a guide, a lifetime of 3000 h can be considered as normal for a good-quality motor.

Brushless DC motors

Figure 4. (click to enlarge) Brushless DC motor.

Brushless DC motors overcome the limited life of brushed motors by eliminating the brushes. For example, the motor shown in Figure 4 has static windings and the permanent magnet is located on the rotating drive shaft. The commutation of the electric polarity is achieved electronically: the position of the rotor is detected by a sensor and the polarity of the electric current in the coil windings is controlled by the electronics on the integrated board.

Noise is lower than with brushed motors because there is no mechanical contact between static and rotating parts in the motor apart from the shaft bearings. The bearing design is the same as for brushed motors, therefore, brushless motors easily reach lifetimes far in excess of 10000 h. Diaphragm pumps will not live much longer than 10000 h even under favourable conditions. Thus, the purchase of an expensive motor that will outlive the pump and then be thrown away with 50% of its life remaining must be considered carefully!

Cost, in part because of comparatively small lot sizes, is the main reason why brushless DC-motors have not become standard in DC-driven pumps. Most of the applications do not demand a lifetime that makes a brushless drive compulsory. Only applications with continuous operation over a period of more than six months without the possibility of a scheduled maintenance removal of the pump will justify the cost of a brushless drive. Even then, the brushless motors face the competition of shaded-pole motors or vibrating drives if AC power is available and the performance requirements permit.

The built-in electronics of brushless motors allow a lot more control and regulation of the performance of the motor than simply controlling the commutation. However, the sophisticated electronic environment into which the pumps are usually placed make it an easy task to integrate these additional motor-control functions into the elctronics of the medical device itself instead of placing them in a small electronics board in the motor. This is one way to reduce the cost of brushless motors. It also guarantees that only the necessary control functions are built in, and avoid including, and paying for, all the standard functions that come with the motor, which may never be used.

AC motors

Of all the different types of AC motors, only shaded-pole motors play a significant role as pump drives in medical applications. Usually they are used for devices with an AC power supply of 230 V/50 Hz or 120 V/60 Hz such as stationary compression devices, suction devices, nebulisers and autoclaves. The difference in performance between a 50-Hz or 60-Hz power supply and the variety of voltages used worldwide will make special motors for different countries a necessity.

Figure 5. (click to enlarge) Shaded-pole motor with rotor partly pulled out.

The AC in the coil winding is inducing a changing magnetic field in the “poles” of the motor, which causes the rotor to turn (Figure 5). The commutation of the polarity does not have to be created artificially, but it is already there because of the nature of AC.

Therefore, brushes and electronics are not necessary in an AC-driven pump, which gives the shaded-pole motor a clear advantage over DC motors as far as noise and lifetime are concerned. Prices are also extremely competitive, because the motor consists of only a few simple parts and it is used in huge quantities in other industrial applications such as fans.

Unfortunately, shaded-pole motors are comparatively large, heavy and inefficient. They produce a lot of heat and usually have to be equipped with a cooling fan and a thermo switch to prevent overheating. But apart from the lack of efficiency, all desired pneumatic performances can be reached. High flow with low pressure as well as high pressure and vacuum performance is possible.

Vibrating drives

Vibrating drives need AC to work; this current induces a magnetic field in a coil. In front of the poles of the coil a vibrating armature is placed with a permanent magnet facing the coil poles. The changing magnetic field causes the armature to vibrate in the same frequency (50 or 60 Hz) as the AC. The vibration of the armature pulls and squeezes the diaphragm, which causes the air to flow through the valves of the pump head.

Figure 6. Rotating armature drive and linear drive.

Depending on the basic design principle of the pump, two types of vibrating armature pumps are employed: pumps with a rotating armature and so-called linear pumps in which the armature makes a linear movement, as shown in Figure 6.

The characteristics of the oscillating movement limit the pneumatic performance of these types of pump to pressures of up to approximately 300 mbar. Therefore, prime candidates for the pumps are applications needing flow, not pressure.

Figure 7. (click to enlarge) Vibrating armature pump with rotating drive.

Vibrating drive systems (Figure 7) are contact-free and, therefore, extremely quiet with long lives. They are used in applications such as airbeds or compression therapy. The lifetime of a pump equipped with a vibrating drive will be limited by wear of its parts (mostly the rubber parts such as the diaphragm and valves), not by the drive system; however, 10000 h or even more can be expected. If the design of the pump allows easy access to the worn parts, then pump life can be doubled or even tripled after exchanging the worn-out diaphragm and valves.

If vibrating drives are to be used with a DC power supply, a DC–AC converter has to be used. The price of a converter will lead to a significant increase in the cost of a pump. Therefore, the use of a pump with DC current has to be considered carefully.

Attention must be paid to the fact that a vibrating drive works because of the vibration. Vibration is an extremely unwanted effect in any machine. The fixation of pumps with this type of drive in a casing must be designed with special attention to avoid the transfer of the vibration into the casing, which creates unwanted noise. Most pumps are supplied with rubber feet for fixation.

Another factor to take into consideration is that a vibratory drive system works best and most efficiently within its range of resonance. This is the point where the amplitude of the armature will be at its maximum, and this will be dependent on the exiter frequency and design parameters such as the stiffness of the diaphragm and the mass of the armature. Most pumps will be optimised for operation at 50 or 60 Hz because these are the two main AC frequencies used worldwide. Of course, a pump optimised for 50 Hz will not work properly if it is plugged into a 60-Hz power supply. The complete vibrating system consisting of diaphragm, armature and coil must be modified to optimise performance at another frequency.

Figure 8. (click to enlarge) Maximum amplitude Xp at 50 Hz (top) and at 60 Hz (bottom).

This type of vibrating system can be described using differential equations and the maximum amplitude of the armature can be calculated. Figure 8 shows the maximum amplitude (Xp) of the armature of a linear pump as a function of diaphragm-stiffness (CM) and armature mass (MA) for 50 and 60 Hz. It is clearly visible that the performance of this pump at 50 Hz will be much different from its performance at 60 Hz: the amplitude peaks, which are the points of resonance, are much higher and they occur for different combinations of armature mass and diaphragm stiffness. For operation at 60 Hz, for example, an armature with a different mass can be used to shift the operating point of the pump into the area of an amplitude peak.

Table II. (click to enlarge) Vices and virtues of drive systems ranging from ”11“ (good) to ”22“ (poor).

Vices and virtues

The selection of a miniature pump for a medical application focusses on issues such as pneumatic performance, size, noise and price. The drive system of a pump will significantly contribute to the overall performance of the pump with regard to these requirements. This discussion has highlighted the characteristics of different drive systems in miniature pumps and Table II summarises the vices and the virtues of the discussed systems ranging from “++” signifying good to “ - -” signifying poor.

Dr Detlev Borstell is Technical Director at Schwarzer Präzision GmbH & Co. KG, Steeler Strasse 477, D-45276 Essen, Germany, tel. +49 201 316 9720, e-mail: d.borstell@schwarzer.com , www.schwarzer.com.