Prime mover – motion control in medical applications10 April 2014
The advent of digital components has revolutionised systems, practices and techniques in many domains, and the field of motion control, particularly when it comes to medical applications, has embraced it as keenly as any. Roger Hollis of Sharp Concepts walks us through the digital evolution and explains how end users can ensure they best feel the benefits.
Remember the last time you went to the dentist? Hopefully it was nothing too serious and all you needed was a regular clean. What you probably didn't know was that experience was a great example of how technological advancements in the motion control industry have provided significant benefits to patients, doctors and hospital care over the years.
So, what do we mean by motion control? First, let me clarify that we are discussing electric motors and therefore rotary motion. There are linear electric devices, as well as pneumatic and hydraulic devices, but we will not be discussing those.
A motion control system is meant to perform any, or a combination of, the following parameters:
- Speed (velocity): in an electric motor, angular velocity, a measurement of the rate of change of angular position of an object over a period of time. Units in revolutions per minute (rpm) or radians per second (rad/sec).
- Torque: a measurement of how much a force acting on an object at a specific distance from the pivot point causes that object to rotate. Units in Newton metres (Nm), ounce-force-inches (ozf/in) or pound-force-feet (lbf/ft).
- Position: in an electric motor, measured in angular degrees.
- Acceleration: in an electric motor, the change in angular velocity as function of time. Acceleration refers to increasing velocity and deceleration as decreasing velocity.
In the past, motion control was achieved with a variety of analogue devices, cams, gears, cables and pulleys, rheostats, governors, clutches, brakes and the like.
For those of us old enough to remember, dentists' drills were powered by cables and pulleys, the drill ran slowly, took time to come up to speed, slowed down quickly under pressure and was quite uncomfortable.
Nowadays, dental drills are very fast, accelerate quickly and aren't bogged down under load, all because of advancements in the ability to control speed and torque - motion control.
Additionally, a wide variety of medical procedures in existence today either didn't exist or were laborious manual operations subject to human error. Vascular injection systems in existence today provide very precise rates and volumes of injector material at programmable pressure settings, a capability that didn't even exist many years ago in old analogue systems. Pill dispensing, once a manual process, can now be fully automated and programmable for high-volume users such as hospitals, freeing up nurses to do what nurses do best and eliminating the potential for human error.
What's in the box
The rapid development of the semiconductor has been the technology behind the advancements in motion control capability. While, in the past, an operator had to change some type of mechanical device such as a cam or gear, nowadays, it's a matter of changing a few lines of code or, even more likely, selecting from a list of drop-down options in a PC program. So now we have the world of programmable motion control.
The basic architecture of a programmable motion control system contains many elements, and each of those elements has evolved over the years from analogue to digital.
Each of these elements has a role in a motion system but not necessarily as a stand-alone piece of equipment. Early in the development of such systems, it was much more likely that each element was stand-alone primarily because the technology at the time required much larger components, and each element was connected via wiring. Over the last 20-plus years, components have become much smaller, and as a result many of the system requirements are now built into compact units.
Because of the rapid changes in technology and the ability to reduce sizing in all of the elements, a motion control system can be packaged in many different ways depending on each manufacturers' design intent and the ultimate application.
Some designs have all of the elements contained in one box, whereas others may have discrete boxes containing specific elements. The ultimate decision is based on application needs, cost constraints, space availability and project complexity.
It is not unusual - and this is certainly true in the medical industry - that some of the elements are incorporated into the electronic design of the overall package. If the design has electronics built into it for other features, such as diagnostics or doctor input for example, why not incorporate the motion control elements into the package? The amplifier (driver) and motion control board can be incorporated, thereby lowering overall cost, reducing space and providing greater user flexibility.
The servo motors themselves have now become integrated so that the amplifier (drive) and controller are a part of the motor with a built in communication port. The obvious advantages are reduced space, less wiring and greater reliability. They are also designed to be programmed much more easily.
God is in the details
While the rapid advancement in technology over the past 20-plus years has provided great benefits to end users in terms of lower costs, reduced footprints, more features, greater flexibility and opportunities for designs never before possible, it has also created more complex systems that require greater expertise.
The end user has to decide on how best to achieve the goals required for the specific design. Do they have the ability to design an entire system in house, requiring knowledge of software design, electronics design, controller and amplifier design, and expertise in electric motor selection?
Another option is to turn it over to a third-party integrator with the expectation that they have the requisite expertise. The third and often chosen option is a mixture of both, whereby they use their current in-house expertise and then work closely with a supplier or suppliers to integrate the total design.
While there is no 'right' option, it does become critical that the end user spend considerable time documenting the design criteria in detail and select the appropriate partners to work with. Because of the rapid advancements in technology (smaller, faster, feature-driven), the end user needs to work with suppliers that are up to date on the latest in technology in order to achieve the optimum design.
This means that the end user needs to specify the important performance criteria and describe them in terms appropriate to the system designer. They must first determine the classification of the system in terms of:
- controlled variable: position, velocity, torque
- operating mode: incremental or continuous
- load classification: kind of motion (rotational, linear) and coupling of load to motor (belt, gear, lead screw).
Once accomplished, it is necessary to specify the performance criteria for the operating mode that has been classified. For the incremental operating mode:
- define load requirements (friction, inertia)
- define the motion parameters (min and max speed, max acceleration and deceleration rates, duty cycle)
- define accuracy (short term and long term).
For the continuous mode:
- define load requirements exactly as above
- define disturbances such as impulse loading (maximum that system will see plus duration)
- error size and recovery time allowable
- drift - long-time error due to external variables.
For both systems, the following additional information needs to be included:
- life requirements
- environments (indoors, outdoors, clean room, industrial)
- ambient temperature (min and max)
- humidity (min and max)
- available power
- size and weight limitations
- any technical or regulatory standards that must be adhered to
- any other factors that would affect performance.
The greater the time spent analysing the requirements of the system up front, the less time will be spent on potential redesign, and wasted time and resources in the end. It cannot be emphasised strongly enough that the majority of the project time should be spent in the initial analysis of these systems.
As the technology continues to advance to provide greater flexibility and benefits to the end user, the more care and research in the beginning is required to take advantage of those benefits for the ultimate success of the project.