When we have the freedom or the need to use both physical (hardware) and digital (software) controls for user interaction, it is a typical question how to make smart distribution of required features between them:
- Should change of the mode be done by software selector (like dropdown list) or by hardware button or both?
- Should we add joystick for this activity, or we can save money by making it from the touch screen?
- Is it needed to have a separate LED indicator for status, or better just to keep it in the display?
All these questions are important, even if we skip the primary concern — budget limitations, because hardware controls are in general more expensive than on-screen inputs. So, let’s concentrate on the usability/ergonomic reasons only.
There are several methods to distribute the functionality, that may be applicable to specific usage contexts and tasks.
Method A. Physical for physical things, digital for digital things
This approach was selected many years ago when I was designing the control panel for digital radiology machine. There was a need to remotely control the position of the patient and x-ray emitter/receiver, that was adjusted by using large servos before and during the medical research, and the settings of the x-ray imaging, like chosen preset, exposure time, voltage and so on. And it was quite organic, that the digital HMI of the touch screen was used for all the purely digital, “invisible” settings, like preset selection, and physical controls were used for hardware, mechanical and “visible” part of research.
Method B. Physical for critical, digital for secondary tasks
This approach is popular with transport machinery and after years of experiments and failures becomes more or less default with modern cars. The physical controls are used to control the systems, that are critical or important for the user during driving, while digital are used for all other secondary systems and features. So, if the user drives the car, it is much easier to operate physical control with tactile feedback, because it doesn’t require to look at them.
The problem here is to find a proper balance. For instance, many modern cars consider air conditioning and temperature control as something secondary, and make purely digital controls for them inside HMI display. But many users adjust these parameters quite often during driving, which increases distraction time and makes their usage dangerous, especially in dense dynamic traffic or while driving on winding mountain roads. So, recently some manufacturers announced that they are going to move back to physical controls for these types of tasks, like it was before any digital HMIs and embedded touch-screens.