Introduction

Motorization of movements is quickly becoming a standard solution in more and more different types of medical applications, ranging from CPR to Dialyze machines. This motorization allow for extremely precise dosing and repetitive treatment to increase medical quality and comfort for the patients. For the personal in medical care an increased motorization means also increased efficiency, and the possibility to treat several patients in parallel.

A most interesting question is why we see this accelerated motorization since only the last 5-10 years. The DC motor were commercially available already in the1880’s, the hybrid stepper motor structure has been around for more than 60 years, BLDC motors are more recent but could be found on the market from the 1960’s. So if the motors themselves did not go through a recent revolution why this major increase in motorization of medical applications?

An important part of the explication can be found in the Si-revolution that has become so important for our daily life since the 1970’s. Integrated Circuit (IC) manufacturers still struggles to keep up with Moore’s law. The most common citation of Moore’s law dictates the doubling of transistors in integrated circuits every two years. With Moore’s law come also other positive side effects such as reduced size, reduced cost and increased speed. In motor drive applications it is in particular the reduced size that is of importance. Today we have reached a level of minimizations that allow for extremely compact design of modern motors drivers. Add to this the possibility to integrate, also in portable medical applications, more complex control units (so called ECU’s), and it becomes obvious that we see this increased motorization more due to the minimization of motor drivers than an evolution of the motors themselves.

Motor types: DC/BLDC/stepper

In medical applications there are three main motor types that are used. These are DC, BLDC and Stepper motors. The advantages and disadvantages of each type will be presented throughout this article, for a better understanding of the driver functionality it is important to know the basic functionality of each type.

The brushed DC electric motor generates torque directly from DC power supplied to the motor by using either stationary permanent magnets or rotating electrical magnets. The commutation between phases is guaranteed mechanically with brushes and commutators. Advantages of a brushed DC motor include low initial cost, and simple control of motor speed or torque that is directly dependent of the current. Disadvantages are low life-span for high intensity uses, and the inability to work in clean environments since the brushed commutators will leave a residue of graphite powder.

A BLDC motor has rotating permanent magnets and fixed coils supporting the winding, thus eliminating the problems of connecting current to the moving armature. An electronic controller that will be discussed later in this article replaces the brush/commutator assembly of the brushed DC motor, which continually switches the phase to the windings to keep the motor turning. The controller performs similar timed power distribution by using a solid-state circuit rather than the brush/commutator system.

BLDC motors offer several advantages over brushed DC motors, including more torque per weight, reliability, reduced noise, longer lifetime (no brush and commutator erosion), elimination of ionizing sparks and dust from the commutator, more power, and overall reduction of electromagnetic interference (EMI). With no windings on the rotor, they are not subjected to centrifugal forces, and because the windings are supported by the housing, they can be cooled by conduction, requiring no airflow inside the motor for cooling. This in turn means that the motor’s internal structure can be entirely enclosed and protected from dirt or other foreign matter.

The maximum power that can be applied to a BLDC motor is exceptionally high, limited almost exclusively by heat, which can weaken the magnets. (Magnets demagnetize at high temperatures. A BLDC motor’s main disadvantage is higher cost.

A hybrid stepper motor is a brushless, synchronous electric motor that can divide a full rotation into a large number of steps. The motor’s position can be controlled precisely without any feedback mechanism (see open loop control), as long as the motor is carefully sized to fit the application. Stepper motors, effectively have multiple “toothed” electromagnets arranged around a central gear-shaped piece of iron. The electromagnets are energized by a driver (see driver section). To make the motor shaft turn, first one electromagnet is given power, which makes the gear’s teeth magnetically attracted to the electromagnet’s teeth. When the gear’s teeth are thus aligned to the first electromagnet, they are slightly offset from the next electromagnet. So when the next electromagnet is turned on and the first is turned off, the gear rotates slightly to align with the next one, and from there the process is repeated. Each of those slight rotations is called a “step”, with an integer number of steps making a full rotation. In that way, the motor can be turned by a precise angle. For this reason Hybrid Stepper motor are often used in position control applications. Due to the high number of teeth (usually 200 steps per revolution), hybrid stepper present a high torque at low speed but show a limited potential of speed (typically under 1500 RPM). The main disadvantages of the hybrid stepper motor are their predisposition to produce noise and vibration as well as a poor efficiency when they are driven in open-loop mode. Combine with a LoadSense driver they can make a very good direct drive for application requiring low speed / high torque (as for example peristaltic pumps).

Modern Motor drivers

The story of modern motor drivers start’s in the 1970’s with a switching elements called MOSFET. This is an abbreviation for the more technical description Metal–Oxide–Semiconductor Field-Effect Transistor. MOSFET’s are today by far the most commonly used transistor type in both digital IC’s and in power applications. Unlike its predecessor the Bipolar Junction Transistor BJT, it is mainly used for ON/OFF switching. Using a switching element in power applications also fundamentally changed the way we generate and convert energy.

L/R Driver and PWM

One of the first, and also simplest, driver types used for stepper motors is what is commonly known as the L/R driver. The name comes from the phase resistance R and the phase inductance L of the motor. These two motor parameters depend on the motor winding i.e. number of turns in one coil and diameter of the copper winding used in each coil. The R and L can be chosen in such a way that the motor works well for a fixed voltage supply. The maximum phase current in one phase is simply limited by R and the motor dynamics (rise-time) can be controlled with the ratio of L/R.

The modern and more efficient technology that emerged from the introduction of MOSFET’s is what today is called PWM, or Pulse Width Modulation. The PWM technology is a simple and straight forward way to convert a direct/constant voltage (DC) into an alternating/sinusoidal voltage (AC).

To understand the working principles of a simple PWM driver, also called Chopper driver. Assume that we would like to transform a constant voltage A of 1 [V] to an alternating voltage B using switching elements. First step necessary is to define a switching frequency F [Hz] with a duty-cycle D [%], it is straightforward to see that a voltage B = 0.5 [V] can be generated by applying to A a duty-cycle of 50% with for example a switching frequency F = 20 [kHz]. This can be generalized for any duty-cycle D and from this an alternating, close to sinusoidal voltage B, can be generated.

Modern motor drivers all use the PWM technology to convert a DC voltage supply into sinusoidal motor currents. A modern driver for stepper motors have four MOSFET’s per phase. This is only possible with the reduced size and cost that has followed in the footsteps of Moore’s law. The advantage of PWM technique for converting DC voltage into AC motor voltages is higher motor efficiency and a maximum motor speed achievable for a given voltage supply. The PWM driver will also increase the torque output since a larger part of the winding armature can be energized simultaneously.

Open/Closed loop motor control

Any motor, stepper as well as BLDC motors, can be used in an open or closed loop configuration. While an open loop driver for BLDC motors is rare, the open loop has been, and still is, the standard driver type for stepper motors. The difference between an open loop and a closed driver is the pattern and more important when we apply phase current to the motor. The following two sections will explain the difference between these two driver types. Though stepper motors are very often used in open loop mode they are well adapted to run also in closed loop. For this reason we will use a stepper motor as example.

Open Loop Driver

An open loop driver imposes a specific current pattern into the motor phase to create a rotating magnetic field. The motor then follows this generated magnetic field more or less in the same way a paper clip lying free on a paper would follow a magnet that is drawn on the opposite side of the paper. It is easy to imagine that if something blocks the paper clip the magnet would quickly loose contact with the paper clip as the magnet continues its movement. This could also happen to a stepper motor and this condition is commonly known as stall or step loss. This is a severe fault condition since the expected movement no longer can be guaranteed. This is considered particularly serious in medical applications. Just consider the possibility that a dosing device or pump no longer works as expected.

Driving something in open loop and avoiding lost steps gets even more complicated if we consider applications that requires high accelerations and/or driving loads with high mass/inertia. Going back to our metaphor a paper clip is of course extremely lightweight, but what if it is attached to a load that we will be pulling using only the paper clip and the magnet. When the acceleration becomes much more important, we must take care not to move the magnet too quickly or we will lose the contact with the paper clip.

To avoid any possibility of a step loss, either due to torque fluctuations or high accelerations, stepper motors running in open loop is always over-dimensioned. A good engineering practice used in most applications is to use a stepper motor capable of pulling a load which is twice as large as the expected maximum load. This guarantees as safety factor two and also that full functionality is guaranteed for the life time of the application. An over-dimensioned system are of course not very cost-efficient and has also the disadvantages of increased heating and higher vibration / noise levels.

Closed Loop Driver

To understand the obvious advantages of a closed loop driver compared to an open loop driver, let’s once again go back to our example with a paper clip being pulled across a paper using a magnet on the opposite side. In the open loop example we concluded that it is necessary to use a very strong magnet (at least two times stronger than necessary) to overcome any obstacles that might be stopping the paper clip from moving freely.

It is easy to realize that it would be much easier to make the paper clip follow the magnet if we were using a transparent paper instead of a standard opaque paper sheet. If we at all times could see both the magnet and the paper clip at the same time then we could for example slow down when we come to an obstacle. We could also make the paper clip easily creep around obstacles since we have the information of where the magnet is situated with respect to the paper clip. So what has transparent papers and paper clips to do with closed loop motor control? Well, a great deal, the key in closed loop motor control is to know at all times the position of the rotor with respect to the magnetic field we are generating. This can be achieved using a position sensor. The position sensor can be compared to the transparent paper. The transparent paper gives us the information of the relative position magnet / paper clip. In a similar way a position sensor return the relative position between phase current / rotor. For a BLDC motor a low resolution position sensor is sufficient. A position encoder for a closed loop stepper motor driver must on the other hand have a very high precision due to the high (200) number of steps.

Having the position information of the motor allow us to slow down when the load increases, it further allow us to have complete control of the acceleration. If we ‘see’ that motor do not follow the acceleration we try to apply. Then we simply slow down and wait for the rotor to catch up, the same way we would do if we saw through the transparent paper the paper clip were unable to follow the movement of our magnet.

The use of position sensor and a closed loop motor driver means that is not necessary to have the same factor two security margins necessary in an open loop driver. With a closed loop control the current level can be modulated and adapted automatically to fit the load. In the end the result is a much smaller stepper motor for pulling the same load or being able to reach significantly higher accelerations. This is an undeniable advantage in medical applications where size of the motor unit usually takes up a large part of the apparatus.

Position Encoders

A position sensor or position encoder can be realized in many different ways. The most commonly used types in motor control are optical or magnetic Hall effect sensors.

The simplest type of optical sensor is the incremental disc encoder. Light from a photodiode shines through slits in a rotating disc. A light sensitive element on the other side of the disc captures any rotation of the disc. This type of encoder has very high precision and can be found in resolutions up to 20’000 increments / revolution. It is however sensitive to condensation and dust and can only be used in well controlled environments.

A more robust and cheaper alternative to the optical encoder is a magnetic Hall sensor. It is based on Hall effect elements normally stacked in number of 2-4 in an integrated circuit. A simple magnet is all that is necessary on the motor shaft. This encoder design is much more robust to dust, condensation and other impurities. The resolutions is usually less impressive than that of an optical encoder but today the precision of a magnetic LoadSense encoder is sufficiently precise to be able to run a stepper motor with 200 steps / revolutions in fully closed loop. The encoder can be combined with integrated driver electronics or it can be used separately to replace e.g. expensive optical encoders. As compared to an optical encoder the advantage of the LoadSense encoder is also the small size and a complete integration into the motor. This makes the final solution extremely compact and robust.

Vector Control

For many years the DC motor has been, and still is for some applications, the workhorse for position and speed control applications. This is mainly due to its simplicity of use, the habitude of design engineers and also because there have been few competitive alternatives. Medical industry need however to find alternatives that are maintenance free for the entire >20’000h of life cycle, and this cannot easily be achieved with a DC motor due to the re-changeable commutators and brushes.

Once again motor drivers changed in an irreversible way with Moore’s law. The introduction of fast microprocessors such as DSP’s and cheap high-performance power semiconductors over the last 20 years made a new trend of motor drivers possible. Speed and position control applications that previously was only possible with DC-motors are now realized with BLDC or Stepper motors. This is mainly thanks to the introduction of the vector control theory in the beginning of the 1980’s.

In a DC-motor the speed/torque can easily be varied by controlling the stator current and if applicable also the armature current. This is not as simple in the case of AC-drives such as the BLDC or Stepper motors. In these types of motors it is also necessary to control the phase angle between the phase current and the induced voltage. In a DC-motor this angle is fixed mechanically by the commutator and the brushes. For AC-drives this angle has to be controlled by the electronic hardware and the control loop. This means that the current vector, amplitude and angle, is commanded by the control algorithm. The name “Vector Con- trol” originates directly from this strategy. If the current vector is not properly controlled in an AC-drive the result will be a non-optimal operation and intolerance to disturbances in the load torque. By using vector control, BLDC and Stepper motors can replace DC-motors in demanding drives, thus avoiding all the disadvantages of the DC-motor such as periodic maintenance and inability to operate in corrosive environments. Furthermore the DC-motor cannot be used in clean environments such as medical applications or clean rooms. There are many ways to control the current vector of an AC-drive.

The most commonly used transformation for 2- or 3-phase system is the Clarke/Park-transformation. This transformation is really only a change of coordinate system, a change between a stator fixed coordinate (ab) system onto a rotating coordinate system (dq). The key to simplification is to fix the dq-coordinate system to the rotor. This operation will transform all sinusoidal phase currents into a representation where the currents are constant. Since everything in the new dq-representation is DC we are back into the simple control structure used so successfully for DC motors.

Load-Sense Technology

Going away from DC motors in medical applications has undeniable advantages when it comes to durability and life time. With the recent advances in closed loop control the choice is now more between a BLDC or a Stepper motor. BLDC motors have a high efficiency but rather low torque / size ratio. It is therefore increasingly interesting to consider Stepper motors for all application that requires high precision of speed or position. Stepper motors are furthermore capable of driving everything from small to high loads. While BLDC motors needs in most applications a gear, a Stepper motor solutions can in many cases be designed as a direct drive, totaly without a gearbox. This is of course extremely beneficial for life time and durability.

Today’s state of the art in stepper motor drive is a combination of well establiched techniques such as PWM driver and new innovation technologies such as closed loop Vector Control with high precision Hall encoders. Going away from the traditional open loop control for stepper motors is a large break-through in terms of reduced heating, reduced motor size, noise, vibration and increased motor efficiency. A closed loop Vector Control also eliminates all risk of stall or lost steps. The motor can simply not loose any steps since the rotor position is at all times known.

An innovative example of this new closed loop driver is the fully integrated LoadSense driver from Sonceboz SA as shown in Figure 6. This intelligent driver permanently adapts the phase current to meet any changes in the load torque. This fully automatic adaptation can be achived with the elegant integration of a precise Hall sensor encoder directly on the electronic PCB board. A simple magnet on the motor shaft garantees the continuety of the position information. The LoadSense driver comes furthermore with a wide range of customisable communications interfaces such as CAN, PWM, Clock & Direction, RS485/RS232, I²C. The LoadSense driver is well adapted for both position- as well as speed control applications.

Future challenges in motor control

Recent advance in LoadSense technologies open up a lot of new possibilities. Until now, only a few of them were implemented and available on the market. The first application that comes to mind is for sure the position control. Based on the information computed and available in the microcontroller we will be able to keep an exact position with the minimum amount of current, noise and heat dissipation. We will also see a clear trend in order to extend the speed above the 2000 rpm range. The load sense technology as of today is already capable of virtually 0 to 1500 RPM.

There will most certainly be a multiplication of this technology applied to both small (<0.2 Nm) and large (>6Nm) motors in direct drive. This will for example open a new possibility in higher torque applications. It is already clear that high torque direct drive units are very attractive for long life time applications. In the past the heat dissipation was always a clear disadvantage of big stepper motors. Using the new LoadSense technology will virtually solve this old known problem

Using a stepper motor as well as combining it with an accurate sensor give the possibility to build fault robust system. In very high demanding application it is also thinkable to use in the future one of these units as an independent control unit for its neighbor.

Using the calculation power of modern microcontrollers could also give the possibility to use the motor not only as driver but also in the same time as sensor. In fact it should be possible to compute in real-time the torque seen by the motor. This torque information can then be used to program a preventive maintenance or for security reasons.

Conclusion

In the past decade we have seen the number of electrical actuators per machine being multiplied by factors. We have also seen more and more DC motors being replace by BLDC or Hybrid stepper in order to extend the lifetime of high-end machine using the new possibility offered by modern electronic components

As far as we see today complexity of treatment, machine and regulation (security EMC ect…) will continue to drastically increase over the next years. One way to face this complexity is to use the old Roman strategy: Divide et Impera or Divide and Conquer. Probes and intelligent actuators arranged in a network and exchanging data over a BUS system via sequenced command are becoming the basis for the efficiency, safety and comfort of modern technical systems. Using independent systems not only improved flexibility, reliability and help to solve EMC trouble but also give the OEM the ability to concentrate on what really matter for him, the machine as a whole as well as the elaboration of new treatment.

If this politic was already known in the Roman times the evolution of electronic hardware will also give an economic sense of this solution in more and more application.

Sonceboz SA is convinced that the evolution seen in the past decades in electronics driver solution is only the start of a fascinating travel.