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Using control theory to model interaction with medical devices

Key points

  • We have developed a new approach for evaluating the user interfaces of medical devices based on techniques from aerospace engineering and robotics. It combines manual control theory with hybrid automata.
  • We have modeled both discrete and continuous human operator behaviour and, in particular, we have found a novel way to model a person using a medical device with up and down buttons. We can model both short button presses and holding down buttons.
  • We have integrated the model with the CHI+MED analysis toolset, PVSio-web, and have demonstrated the model’s utility by simulating a person entering values on an infusion pump, using up and down buttons.
  • A typical insight is the identification of user interface design flaws that would lead to a large overshoot when using up and down buttons to adjust numbers. It also helps an analyst to identify and reflect on different types of user behaviour.

Background
Interfaces are currently evaluated by getting some of the people who will use a device to try it out, noting problems they encounter. To uncover all the potential issues with the design of an interface requires running lots of user studies for a long time, however. This can be prohibitively time-consuming and expensive. A better, more automatic way is needed. Also to fully understand the use of machines that are controlled by people, we need to be able to model both human and machine.

The cruise control in a car or the thermostat control in a house are modelled and designed by engineers using control theory. In control theory, a system is modelled as a feedback loop that attempts to maintain it at a specific value (such as the temperature set) while the system is affected by a variety of disturbances (night falling, a window being opened, and so on). A variant of control theory, called manual control theory, also offers a powerful and flexible approach for describing human behaviour and analysing human-machine systems. It has been used to give models of pilots’ interactions with aircraft systems. Developed in the 1970s, these models are still used in aerospace engineering today.

Control theory and interaction design
We applied manual control theory to interactive medical device design, where both the human and the device are part of a feedback-loop control system. We needed a way of describing systems that involve both digital and analog processes and so uses an existing approach called hybrid automata that is designed specifically for this, combining it with manual control theory.

We first integrated our model with the CHI+MED analysis toolset, PVSio-web. It allows medical devices to be both simulated and analysed mathematically. This allowed us to link the control-theoretic model to existing models of medical device interfaces already developed as part of CHI+MED. The control-theoretic model then allows us to describe both discrete interactions, like pressing a button, as well as continuous interactions, like holding down a button to make a value change faster, within the toolset.

Identifying design flaws
A typical insight that can be found in this approach is to identify user interface design flaws, that would lead to a large overshoot when using up and down keys to adjust numbers. It also helps an analyst to identify and reflect on different types of user behaviour. Our approach can be used to evaluate new designs for medical device user interfaces when the design space is quite large and there are many design choices to be considered. Once any poor design choices have been eliminated, the best designs can then be evaluated using traditional user studies. We have made our model available as an open-source project.

We have compared the results of the model simulations with results from a lab study with human subjects, and showed that the model is a close approximation of human behavior. Because the model is computerised, it is very much faster, and cheaper, than using people. It can therefore be used for very fast and efficient evaluation of infusion pump designs.

Key people
Gerrit Niezen, Parisa Eslambolchilar

Publications
Niezen, G. (2014): Interaction loops. Figshare. http://dx.doi.org/10.6084/m9.figshare.1254311
Niezen, G. (2013). A continuous interaction approach to interactive medical device design. In MediCHI Workshop at CHI’13. Paris, France.

Submitted:
Niezen, G., Eslambolchilar, P. (2015). A human operator model for medical device interaction using hybrid automata. IEEE Transactions on Human-Machine Systems.

See also ...
Verification of User Interface Software of Infusion Pumps
The PVSio-web rapid prototyping tool