MOVE

Iteration #2

Project partner: Salma Kashani

Introduction:

Previously on iteration one, different actuation mechanisms were tested to design a haptic feedback in order to communicate  step commands.They include, Static pressure, direct force (impact) and vibration. We reached our paradigm, when we were applying poke stimuli using long sticks, which could also explain the reason that it was being used in martial art  and military training for gait modification. None of our experiments using pressure and vibration at constant frequency was as informative as poke experiment. We decided to make a similar vibration by exploiting the force impact feeling.

Motivation:

Our initial findings showed that having two body sites works more effectively compared to single body site. In addition one of our main objective is to make our haptic stimuli as close as we can to the poke feeling. These results have motivated us to expand our study on single vs two tractors and explore whether we can have more expressive force feedback using solenoids.

Objectives:

1) Tailoring the force impact by means of solenoid

Some of the main limitations of tractors such as small displacement and low produced force encourage us to see the effect of direct force on haptic perception. Although this force is much smaller than required force to move a leg but at least it is from the same family of poke.

2) One site vs two sites stimuli

 The effect of having redundancy in spatial stimuli mechanism is investigated for both vibration and force impact.

Step 1,

Solenoid vs Voice coil,

Although both of them are working based on electromagnetism forces there are differences in their mechanisms . A solenoid consists of a piece of ferromagnetic material which moves inside of a coil. When coil is energized using electrical current, the ferromagnetic part moves in order to increase the magnetic flux by filling the air gaps. A biased spring, will move back the metallic core when the coil is deenergized. They often are used  to create large displacement and forces very fast(1 time) but not at high frequencies.On the other hand voice coil is permanent magnet which is enclosed in a coil. Their main application is to create vibration so they work at high frequency and generate lower force and displacement compared to solenoids.

• Implementation of Solenoid:

To install our solenoid on site, We needed to make a holder that mechanically connect the solenoid to the velcro. After trying different flexible and rigid materials with different hole sizes we found out that mounting any flexible or rigid material at the head of solenoid can damp the mechanical impact and would not be as effective if we have other contacts to skin close the point of impact. TO have more free of contact space around our impact point we made the box as depicted in fig.1 a.

    

fig.1 a) Implementation of solenoid b)Electrical circuit for driving solenoid (http://electronics.stackexchange.com/)

First,we used a piece of balloon as a biased spring to bring the core of solenoid to initial position and later we removed that and benefit from the body stiffness to do the same job. We derived our solenoid by using a MOSFET and a push button circuit as depicted in fig.1.b.This system can be controlled both by arduino and push button. fig.2 shows the complete setup for one solenoid. In order to have more force,we slightly increased the maximum voltage since we were applying short pulses. (20 V and 500 mA).

Fig.2. Solenoid setup 

Step 2

At this part of our study we run some experiments on our friends to have more feedback on our system and have more unbiased results . Similar to iteration one we conduct our experiments on 6 differents points of the leg fig.3.

Fig.3. Stimulation points on the left leg  (photo courtesy: Salma Kashani)

for each point we tried different actuation and number of sites.the same signal of  vibration in iteration 1 was selected. Both vibration and force impact signals' length were less than a second. in each experiment we mount solenoid or tactor on velcro and make sure that there is enough initial stress that they can work perfectly.  Fig 4. shows the installation of tactor and solenoid on velcro.

fig.4.Mounting Solenoid and tactor on velcro 

In our first experiment we compared the behaviour of one site vibration to solenoid impact. We mounted both of actuators on velcro so the participant can switch between the force feedback immediately and compare them. we asked participant to step back and forth and bend and release her knee while we were  manually triggering the feedback system. A noise cancelling headphone was also used to eliminate the  effect of solenoid sound when being activated.

                                        

fig.5 Applying force feedback to participant's knee

We explained our experiment to the participants and asked them to tell us what was the message of the signal they had felt. if it took them so long to explain the message we provided them with some options and if they couldn't identify the message we considered that signal had no message. at each level we also checked whether they could feel the force signal or not. 

We  tested our two sites  stimuli on a new participant, similar question were asked. In addition we asked our participant to tell us her preference between vibration and force impact and which one she found more effective in conveying the gate modification message. fig.6 shows the two sites stimuli in action. 

fig6. a) Two sites force impact stimuli b)Two sites vibration stimuli 

we ran all of the experiments on our third participant, however as we got closer to the end of experiment our participant seems to be bored and gave the same answers. Due to our lengthy experiment, we decided to just study one subject on each participant in future.

Discussion:

Although our initial experiments on us were suggesting that force impact is more informative than vibration , running the same  experiments on the participant gave unpredicted results. It is still too soon to generalize our results due to limited number of participants and subjectivity of our study, but we reach to following findings based on common answers of our participant.

•  Force impact vs vibration

Two of our participants were mentioning that for single point of contact they could barely feel the force impact and found vibration to be more informative. We expected that our force impact to be at least more sensible because it has higher force and displacement amplitude, but in practice, since vibration was carefully selected from a library of vibration. it could evoke the feeling by working in certain range of frequency, on the contrary,  force impact is working similar to impulse signal and resonant all of the frequencies; however, this full spectrum of resonance can  cause ambiguity in receptors and may have smaller amplitude at certain range of frequency.

• One vs two sites of stimuli

Two of our participants in addition to us found two sites of stimuli to be more transparent in conveying a message signal compared to one stimuli,regardless of the nature of it.  This means that, they got stronger messages of the same type when using two stimuli.  Sometimes  when they were using only one force feedback the message was vague and they could only feel that there was a vibration or impact in comparison with two stimuli that they could realize the message. The conveyed messages were not consistent from one participant to the other but for most of them the back of the leg stimulations had more propelling meaning while in the front leg was more dragging and stopping.  

fig.7. Conveyed messages at different points and different states (standing/walking)

As shown in fig.7 both position and timing of stimuli are important in cognition part. These massages can vary based on position of stimuli and state of the leg/activity. This is one example of different messages during Standing/Walking state.The question mark means no message. 

Why conveying a message using two stimulus is more direct and effective?

Based on our experience and asking our participants we believe that having just one stimuli can be precepted more accidental compared to two stimulus. Having two sites of vibration or impact make a redundancy to our signal and in most of the cases make the user think "for sure there is a message and what should I do? What is that message? " 
The further question that we should respond is the intensive or extensive nature of each of these stimulus. As we know the  intensive property does not depend on the stimuli amplitude and amount, nor the size and extent of actuator , while we have additive relationship in an extensive property.

Source of errors

• Actuators placement
• number of participants 
• Solid design of  experiment
• physical limitation

Conclusion 

In the iteration two, we tried to replicate the similar poking feeling by means of solenoid and expand our study on direct haptic messaging using local stimuli/stimulus. We designed studies to find the differences between 1) Vibration and force impact and 2) Single and two sites of stimuli.
Although we couldn't realize the communication of exact messages to each individual, but our findings show that having vibration or impact at the back part of the leg is more propelling while on the opposite site is more dragging and prohibiting. Perception of vibration is more stronger than force impact when we are dealing with only one site of stimuli, while two sites of force impact is overtaking two vibrators. We could also discover that having having redundancy in our stimuli can help better precepting of the signal and enhance expressiveness of our commands. 

In the next step, instead of exploring different actuation and sequence of activation that have great transparency and conveying a solid message to every user, we decided to study more the intensive and extensive properties of  each stimulus to find the answer that can explain why having two sites of stimuli works better than single site. Both temporal and spatial properties should be tested by means of  designing studies in which placement, amplitude and timing varies. The next question that we are curious about is to know if this is just because we are doubling the amplitude by having two stimulus or we have sensory integration similar to multimodal system.

My contribution

We were developing each part of the project together after having conversation and sharing ideas with each other, so it is hard for me to draw line between my work and Salma's.
I think because of my background, I was mainly involved in hardware developing and experimenting the best design of the solenoids, includes different mounting setups, different solenoid head sizes and mechanisms to bring the solenoid back to its initial position.
I was also involved in designing our study to meet our objectives, plus, synthesizing the results of our study.In addition, I was helping participants to mount the setup on their legs during study.