When a person touches this moving text display, the machine senses this contact and changes to interaction mode. It stills tries to write its messages by pushing the slider gently further left and right in a specific order. But the person can stop that motion, accelerate it forward, or push back. The human contact influences two main variables of the control code: nervousness and aggression. The machine’s nervousness increases steadily, the longer the hand remains on the display. This leads to more and more spelling errors by dropping characters or repeating them. Finally random characters are spread all over the display.

The other variable is aggression of the machine that grows any time the person blocks the movements of the display or even pushes it in the opposite direction. The further the person pushes the slider against the machine the harder it pushes back. But with each of these pushes the inner secret of the machine is gradually revealed: Its own program code is displayed line after line. The longer the person can withstand the force of the machine, the more source code the machine has to disclose. But the displayed lines step faster and faster after each other until the human eye can’t follow them any more and the characters smear to optical noise.

[…]
ld temp,Y
tst temp
breq TX20
dec temp
st Y+,temp
brne TX10
mov temp,COMstatus
cbr temp,0b00000010
mov COMstatus,temp
rjmp TX20
TX10: PUSHY
TX11: ldd tempB,Y+1
[…]

With higher and higher aggression values the machine starts to push in little hits back and forth. Also its motor makes louder and louder humming noises. The machine physically opposes the human being. In its highest state of wildness, the machine hits the slider left and right with all its power to repel the person.

The slider reminds of a typewriters carrier and also of a menetekel written in glowing red letters on the wall. The machine is old fashioned shaped. The used LED character display from the 80ies gives it a more ‘electrical’ look than a contemporary full color LCD displays, optimized for virtual worlds, could provide.

Go, touch, and fight with the display. This digital, analog, electromechanical hands-on object puts a point against a more and more styled and polished virtual reality that has arisen. It combines simple elements to a curious retro object. The movable text line is the focal point, or line of a man-machine interaction. This borderline of input-output streams is also the place where the physical fight between the machine and the person’s movements takes place. Haptic Opposition explores what happens when the apparatus claims physical space.

Technical Description

This project uses custom made hardware and software. The display is mounted on a linear bearing that is driven by a DC motor with a belt transmission. The power of this motor is controlled by a PID-loop. A motion model that is based on differential equations for a movable mass point with one degree of freedom, accelerated by attached springs, computes the gain position. It considers the machine’s gain position, applied forces from the person, as well as inertia and damping. The next higher software layer sets constantly varying force profiles along the motion axis of the linear slider. Depending on the shape of this profile, the person, who touches the display, either feels that the machine has a firm grip on the display, or just keeps it very weakly tied to a specific position. Asymmetrical force profiles provide a direction, where it is hard to push the slider towards the high force gradient, but it is easy to move it to the opposite direction. By changing the force profile, the machine acts on the display: By moving the position of the local minimum of the profile, the display gets moved as well; by rapidly offsetting the force profile, pushes against the human hand are generated; with small but fast oscillations of the profile humming sounds are applied to the motor.
The overall control of the machine takes the slider’s position, the applied force, and the passed time as input variables, and creates output in form of text display and force profiles that are applied to the slider via the PID-loop as described above. The two main variables that guide the system through different behavior states are aggression and nervousness. Both are integrals over time that increase and decrease depending how long and in which way the display is touched. The poles in this map of behaviors are ‘untouched state’, ‘nervous state’, and ‘aggressive state’. Some additional rules help to blur these modes; for example several random factors are added to make the machine more unpredictable.
All software runs on microcontrollers and is programmed in C and assembly.