The anatomy of a Human Interface Device

by Praveen Kumar , TechOnline India - July 14, 2011

This article describes the basics of a Human/Machine interface and the ingredients of a Human Interface Device (HID). Specifically, a user interface is the medium of communication or the ‘language’ between a human and a machine and the language popular right now is ‘Touch'.

With the advent of such innovative and user-friendly products as smart phones and tablets, consumer expectations regarding the user experience have significantly changed. This article describes the basics of a Human/Machine interface and the ingredients of a Human Interface Device (HID). 

Specifically, a user interface is the medium of communication or the ‘language’ between a human and a machine and the language popular right now is ‘Touch.’ As in any communication system, both the human and the machine need to speak the same language, which is equivalent to encoding and decoding in the machine world. The effectiveness of a user interface depends on how well the HID gathers input from the user and the system responds with feedback. Note that HID is used as a generic term in this article and does not mean the HID class as defined in the USB protocol.

Any signal in the real world is analog in nature. Even though the world is becoming digital rapidly, inputs to a system are still and will continue to be analog. Hence, we have to make the system adapt by converting the nature of the input from analog to digital with Human Interface Devices (HID). HIDs act as a bridge between humans and machines to decode human actions (touch, gesture, etc.) into machine understandable instructions.
 
Any Human Machine Interface will have the following sequence of operational steps:

* User action
* Identifying/decoding the user action
* Converting the user action into a machine control parameter

* Machine Acknowledgement / Feedback

 

                            

Figure 1 – Human Machine Interface –The Ecosystem

                             

Figure 2 – HID – Generic Architecture

 

Sensors are a primary part of any HID, translating any form of user signal into a machine understandable electrical signal. The output of the sensor is predominately analog and in most cases requires conditioning such as filtering or amplification before converting the signal into a digital representation. The Analog to Digital converter must differentiate noise from a signal, and is a key component of the HID. The simplest analog-to-digital converter in the world is a mechanical switch that simply converts the analog finger action (push, toggle, etc) to a digital ON or OFF.

The digital back end is responsible for receiving the digital data and sending it to the CPU in a format the system expects and understands. The CPU then responds back to the user with some form of feedback.

The physical location of the HID between the human and the machine is of high significance in the architecture of the HID. For example, in a tablet, the HID is a touch screen, which is a part of the system itself. In a game console like Nintendo Wii, the HID (Wii Remote) is in the hands of the user and is far removed from the centre console. There are some key parameters that will be considered during the design of a User Interface/HID. Some of them are ease of use, ease of design, ease of manufacturing, power, form factor, accuracy, size, cost, speed, resolution, scalability, and precision.

 

                              

 Figure 3: Capacitive touch-based HID

  
Human touch is the user action and the physical connection for the HID. The HID senses the capacitance change caused by the user action. The change in capacitance is then converted into digital counts which are then further processed to determine if there was a finger touch or not. The processed output is then sent to the host controller for further actions through a serial protocol such as SPI, I2C, or USB, etc.

 

                              

 

 Figure 4. Finger navigation-based HID

 

Human movement over a sensor is the user action and the physical connection for the HID. The HID senses the light due to the reflection of the human movement and converts that into (X, Y) coordinates. These coordinates are then communicated to the central console through wireless protocols such as iR, a proprietary RF standard, or Bluetooth, based on the system design.

 

                             

 Figure 5. Speech-based HID

 

Voice is processed and sampled to generate secured/unsecured bit streams
and sent to the central processor for further processing.

 

                             

Figure 6. Movement-based HID 

 

 
An accelerometer senses 3-axis movement made by the user and gives an analog output for each of the axes, which is then conditioned and converted using an appropriate ADC. The raw digital (X, Y, Z) coordinates or the processed action/controls are then transmitted through iR/RF to the central console for further processing. 

Different user interfaces have become popular at different period of times for different reasons and the current trend is capacitive touch sensing. What is evident in any of the scenarios discussed above is that a mixed-signal platform is a requirement for any kind of HIDs. 

Programmability is also a key requirement for such a platform in order to quickly adapt the platform for different types of user interfaces.  A programmable mixed signal system-on-chip platform such as PSoC provides a rich array of analog and digital building blocks, industry-standard processors, and wired/wireless interfaces that give the ability to create precisely the chip that is required for a specific HID design.

Mixed-signal SoCs also remove the barriers faced by fixed-function MCUs and discrete analog and digital components by providing flexibility, integration, and analog functionality while meeting the need of providing the functionality required for a HID all in a single device.

 
About the author:

Praveen Kumar holds a B.E in Electronics & Communications Engineering from College of Engineering, Guindy, Anna University, India. He is currently working as a senior applications engineers at Cypress.
 

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