Move  
Iteration #1

Project partner- Salma Kashani

Introduction

Human locomotion is mainly characterized  by the sequence of lifting and releasing of the limbs in which  propel the center of gravity forward at each step. There are 6 possible configurations of limbs for a biped walker. (lift left leg, release left leg/ lift right leg, release right leg / lift both legs and release both legs together). Human can move naturally or under specialized training. These set of various moves are called human gaits. The natural gaits include 5 main categories of  walk, jog, run, skip and sprint are common between cultures. On the other hand the specialized gaits are being used for medical, training or entertainment purposes and require direct commands to start or modify the movement .

Displaying on a screen or using voice commands are among the most common ways, which are exploited to direct to specific gait position, although they may overuse the auditory and visual sensory input  and sometimes be misleading.

In this project we offer to use haptic feedback to convey the gait commands directly to the limbs that are involved in movement to have a fast and efficient communication. Physical perception and haptic feedback seems to work more transparent compared to auditory and visual feedback and need lower  level of cognitive processing. Unlike conventional haptic feedback which are placed on person's wrist or hand, we believe that the most effective communication will occur only when they target the designated limb.

Objectives

As mentioned earlier, in this project we will implement a local haptic feedback to provide highly transparent signal for gate modification. In our first iteration we will answer the following question:

1. What is the best way to measure the position of the leg accurately ?
2. Given various types of actuation, which one will be the most perceivable actuation by the user? 
3. Which method of actuation can better convey the message and lead to special movement without any training?
4. Where are the effective points of stimulation on the leg for certain movement and what  is the proper number of actuators?

First we will try to implement the position sensor  to measure the knee bending. Second, we explore different actuation both in case of feeling and the message that is conveyed. In the next step, based on the conclusion we got from the second step we will investigate the appropriate actuation to guide to the determined move.

STEP 1 

             Knee positioning:

Before starting any actuation and producing haptic feedback, we need to know the current position of the limb. It is necessary that this measurement doesn't interfere with the ongoing activity. We had two different types of sensors which can give us the amount of knee bending by changing their resistivity. Our first option was using a conductive stretchable fabric and sewing that to a knee pad. We observed that during the leg lift we have stretch of the knee pad in both horizontal and vertical direction. We observe 30% change of resistivity on fully lifted leg with full scale resistance of 3Ω. Our second option was using a flex sensor with 40 kΩ resistance. we also observed more than 40% change of resistivity for fully lifted and fully released leg. We opt out to use the flex sensor due to simpler readout circuit.

STEP 2

             Actuation principles:

We divided the actuation techniques based on their mechanism into 3 main groups:

2-1 Static pressure with compressibility.
2-2 Direct force and impact  by means of rigid actuator.
2-3 Vibration.  

To perform uniform experiments we defined three location on upper limb, lower limb and knee and exposed them to various stimulation. Our main focuses in this study were the feeling and the message which was conveyed. Since the magnitude of actuation is largely dependent on the leg situation and  the  amount of pre-stress during installation we will not discuss that in this project.

Fig1, Stimulation points on the left leg  (photo courtesy: Salma Kashani)

2-1 Compressible inflated balloon with static pressure behaviour.

Similar to the mechanism of compression socks, we made a static pressure by placing an inflated balloon between knee pad and the leg on 6 different points as depicted in Fig1. The reason that we haven't tried the dynamic pressure was the slow response of pneumatic pumps in small scale and other implementation difficulties. Another challenge that we faced during our experiment was the  high amount of prestress of knee pad which was dominant compared to amount of pressure caused by inflated balloon. Later we decided to replace that with a loose fabric.

 Fig2, Static pressure applied to the leg (photo courtesy: Salma Kashani)

Results:We tried these experiments on 3 different people (We ask one of our friends to join us) and observed that by placing the static pressure at any point except the knee it is hardly noticeable. The only feeling that one of us has experienced was in the situation where we put the balloon behind the knee and it prevents the full bend of knee during walking and guiding to stop.


2-2 Direct force and impact  by means of rigid actuator.

Being inspired by the gait modification methods in military and martial arts trainings, we decided to apply direct force to different parts of leg manually by means of wood sticks. At this point we could not only feel the impact and force, but also it directs us to different movements. By poking at different points of legs we observed that different messages such as lifting or releasing the leg or stepping back and forth can be conveyed, however these messages were not consistent from one person to another. We also asked another friend to join our experiment without telling her the meaning of each signal and wanted her to explain the task she wanted to do for each poke. 

Video 1, Applying single direct force

Results:This method seems to give us more reliable outputs with enough transparency to follow the instructions without thinking about them. However we didn't have a general perception for each impact.After a discussion we agreed on the following instruction:

Fig3, Different impact points and their suggested actions (photo courtesy: Salma Kashani)

• Front of the leg
• Upper: State:lifted ,Action: release
• Knee:  bent the knee. 
• Lower: stepping back. 
• Back of the leg
• Upper: State:released   Action:lift 
• Knee: Sit. 
• Lower: stepping forward.

In our next experiment we increased the number of impact points to two. We noticed that at some level, The user needs to think about the action and the message is ambiguous. Our intuition was that just by using a single impact, we are just sending alert signals and it can be interpreted that it happened accidentally.  By having two points of stress, the message was more clear and consistent between different users and they don't need to think before taking actions. Just by poking at the lower limbs the user starts to walk by evoking the push feeling and poking at the sides we could stop the motion by creating gripping feeling.

Video 2, Applying direct forces at two points

2-3 Vibration.  

By adding an unbalanced weight to a DC motor we made a very powerful vibration system. At this point, the frequency and magnitude of vibration were kept constant.We mounted the vibration system on different points of the leg and vibration continued until the full action is fulfilled.

                                         Video 3, Mounting vibration system on upper limb.

Results:

Vibration is shown to be a powerful techniques to alert the user, but at this level was unable to convey a motion message unless the user is being told about the desired action. It appeared that for some cases, the feeling was unpleasant and causes pain or stress so user was just obeying to relieve from that.


STEP 3

In step 2, we studied different mechanisms of actuation and whether they can convey a certain message. Among the plausible actuation (inflation of balloon, poking and vibration) we selected vibration and poking based on their perception by the user. In step 3 we will study more deeply on different variables of vibration and we will try to deliver a message by applying a designed class of vibration.
Our first experience of applying direct force by means of wooden sticks was successful in conveying the desired motion messages. We consider this effect in our selection of vibration signals.First we started by using a vibration generator app called HIVExport designed by Oliver Schneider from SPIN lab to generate vibration with different frequencies and time lengths. We connect the output signals to an amplifier which drives a tractor and tried different vibration. Although we could tell the difference between them but none of them could really convey a motion message. So we decided to use more complex vibration signals by using a predesigned vibration library (VibViz) by Hasti Seifi from SPIN lab. We selected 8 different vibrations based on the feeling and closeness to poking experiments.

                            Fig 4, Selection of 8 vibration signals.(photo courtesy: Salma Kashani)

Then we mounted our tactor/tactors on a piece of Velcro fabric strip and rounded it around different parts of the leg so the tactor be placed on predefined stimulation points with moderate tightness. Similar to our direct force experiment we tried effect of one and two points of actuation.

Fig 5, Mounting tactor/tactors on a peach of Velcro fabric

Fig 6, In situ testing of vibration tactors

By in situ testing of vibration signals on our legs, we narrowed down the list of vibration to 3 and make them just a single beat.

Fig 7,  Final selection of vibration signals

Results:

1. One Tactor :    While we expected to see similar effect of one impact in direct force, we could only feel the vibration which didn't lead to a specific motion. At this level vibration was not unpleasant or causing stress but it was hard to tell the message by its own.                                            
2. Two Tactors at 10 cm distance  : At this level we could feel the vibration not only in two parts but also in the area between two tactors. These vibrations seems to be able to resonant the skin and create the haptic illusion. Even the larger area of vibration couldn't lead us to certain movement.
3. Two Tactors at 15 cm distance : By increasing the distance between two tactors we could better distinguish two separate vibration which sounds to be more expressive such that by feeling them, user will try to change the state of leg to the other.For example if the leg is in lifted position and vibrations are being applied it will direct to release and vice versa.

Conclusion

In our first iteration we studied different actuation mechanisms to grant natural feeling for gait modification. Among different methods of actuation(pressure, impact and vibration), applying direct force and vibration are considered as good candidates to communicate our gait commands.While we could only recognize the alert signal from vibration, applying direct force and impact could lead us to specific moves more voluntarily and seems to be processed at lower cognitive level. later, a class of vibration signals is introduced to mimic the same effect of impact. Further we investigate the effect of having two vibrations at different distances. We observed that by having 2 vibrations at far distances relative to each other, our vibrations are more expressive for the purpose of motion so they can lead to change the state of the gait;however there is still large gap between the feelings which are evoked by vibration compared to impact signal.