Classic Rocker

Design I

Team 7

NSF – Legless Rocker

Project Coordinator: Dr. Brooke Hallowell

Supervising Professor: Dr. John Enderle

10/21/05

Tom Dabrowski

Sarah Philo

Adam Rauwerdink

I. Discussion

Objective

The goal of this project is to design and implement a mechanism to automate a rocking motion of a legless rocking chair. This will be achieved through the use of a linear actuator to apply the necessary forces at the back of the chair. Initiation of the rocking motion will be through simple user-operated switch. The user will also have the ability to control a built-in sound system. For safety, caretakers will have master control over the rocking motion as well as the audio system.

Legless rockers are not a luxury furniture item-- they tend to be built fairly cheaply. For this reason many of the chairs that we have found lack any sort of solid wooden or metal frame, but rather have a flimsy plastic or Styrofoam frame. This makes the attachment of a mechanical rocking mechanism to the chair difficult because the forces that would result from supporting a rocking mechanism would destroy the chair. The few wooden framed chairs found were models only available online, and the manufacturers/retailers were not able to give much information on the frame design. For this reason the forces of rocking will be applied to an external frame.

The external frame will be built off of two standard rocking chair runners. A frame will be attached to these that will lock the legless rocker into the unit. The positioning and angle of the legless rocker will be a critical factor in creating a smooth and safe rocking motion. Because the use of rocking chair runners will create a larger radius of curvature than the legless rocker itself a higher natural rocking frequency will need to be controlled.

The rocking motion will be applied by a linear actuator attached between the two runners near their back end. This mechanism will need to be capable of handling the forces created when a 200lb. person, the heaviest user specified, uses the chair. Stroke length and speed of motion are the other two considerations that will be taken into account when deciding on an actuator. With proper positioning, an actuator with the correct specifications will create a smooth rocking motion.

The interchangeable controls for the user will be a touch pad and a squish switch. These controls will give the user the ability to start and stop the rocking motion and to turn on and off the sound in a fashion similar to a mute button. The caretaker will have master control over all aspects of the chair. The master controls will be on the back of the chair, out of reach of the suer. These controls wil include a switch to control the rocking motion, and access tho the CD player including controls for power, mute, volume, and track. For safety reasons there will be a master switch that cuts power to the entire chair possibly a keyed switch or remote control.

The audio component will be powered by a basic car CD player. This will provide amplification for the sound as well as anti-skip capabilities to limit CD skipping as the chair rocks. The CD player will be installed in the back of the chair on the frame panel. Speakers will be built into the headrest and. if budget allows. additional larger speakers can be installed into the sides of the chair. Though it is oddly shaped for the purpose, the addition of these extra speakers would allow the chair to be used in the place of a standard stereo, as it would provide decent sound quality.

The chair will use power from a standard 120 volt wall socket via an electric cord. A battery system was considered, but the high amperage of the actuator meant that the battery would either need to be charged frequently or be excessively large and therefore heavy. These factors would drastically lower ease of use of the chair. Because the CD player and actuator both run off of 12VDC, a power supply of proper wattage like those in a computer will need to be installed. An automatic cord reel, like in a vacuum cleaner, is a consideration. This would allow the external cord length to be kept to a minimum, also it would protect against damage to the internal connections of the chair, as someone yanking on the cord would pull out more cord rather than breaking the internal electrical connections of the chair.

The final unit will be a self-contained, easily moveable chair with all electrical and mechanical parts guarded as best as possible.

Figure 1: Finished Chair Side View

Figure 2: Chair 3D View

SUBUNITS

Frame

The lack of a hardwood frame in many of the commercially available legless rockers makes the attachment of a rocking mechanism to the chair very difficult. Even for the models which claim to have a hardwood frame, the frame design and its ability to handle attachment of a rocking mechanism is uncertain. For this reason, the mechanical forces necessary for rocking will be applied to a custom made frame which includes runners from a standard rocking chair. The legless rocker will be built into this frame, but will still maintain all of the comfort and ease of use found in the standard video rocker. Because the frame of the legless rocker does not need to be hardwood, a cheaper polyurethane filled vinyl rocker will suffice. This design will be soft and easy to clean and is also lighter than the solid, framed versions. This chair is also smaller, allowing it to fit easily onto the purchased rockers while still allowing room for the rocking mechanism and other accessories. The frame will be built on two standard rocking chair runners. These can be purchased from furniture restoration companies such as Van Dyke’s Restorers. Maple runners measuring 1 5/8” in height, 1 7/8” in width, 36 1/4” in length, and having an end rise of 4 ¼” are the largest runners available from Van Dyke’s and cost $26.99.

Figure 3: Maple Runners(Van Dyke’s)

Two large wooden dowels will be secured between the runners near their front end. The purchased legless rocker will rest on these dowels. The exact position and distance between these dowels will be dependent on the exact size and curvature of the bottom of the rocker we purchase. They must be strong enough to support a 25-45lb. chair plus an 80-190lb. person. Metal brackets and/or metal dowels could be used to supply additional support if deemed necessary.

The legless rocker will be prevented from side to side and fore/aft motion by encasement of the chair by a plywood frame. The frame will be made from ½” plywood because this thickness is fairly light but will still be capable of providing needed support. This frame will also serve as a panel for the caretaker controls. Plywood frames will be attached to the side of each runner and will follow the curve of the rocker. The plywood will only progress part way up the sides of the chair, so as to not give the seat hard edges. This part of the frame will prevent any side to side motion of the legless rocker. The fore/aft motion of the frame will be controlled through the front and back pieces of the frame.

Before any of these frame parts can be attached, the ideal legless rocker angle and position in relation to the runners will need to be determined. A safe and comfortable motion will be dependent on the user not falling or feeling as if they are going to fall, forward out of the chair. Therefore, the legless rocker will need to be placed in a backwards leaning position. This position will need to be chosen so that at the most forward leaning angle of the mechanized rocking motion the user will still be resting on the back of the seat and not falling forward out of the seat. To account for this, the position of the rocking mechanism will have to be considered along with position of the legless rocker. Analysis in Working Model 2D has shown that incorrect placement of the legless rocker can result in the rocking mechanism lifting off of the ground as the chair rocks forward. One of the major reasons that this occurs is that the low center of gravity of the user in contrast to the high radius of curvature of the rocking chair runners leads to a very high frequency. Careful calculation will be needed to correctly match the position and angle of the legless rocker to the size and location of the rocking system. Initial calculations of this sort are included later in this report in the rocking mechanism section.

With the exact chair position determined, a front part of the frame can be attached that will abut the front of the legless rocker. This part of the frame will be padded and covered in fabric in order to act as a continuation of the rocker seat. This part of the frame will secure the side pieces of the frame and prevent forward movement of the now enclosed legless rocker.

The back of the frame will be attached to the runners and abut the top, back section of the legless rocker. The top section of the frame will tie together the back and sides atop of the legless rocker to further prevent movement of the legless rocker within the frame. The caretaker’s controls for the chair and the CD player, if not wireless, will be attached to the back frame piece. As this part of the frame covers many of the mechanical and electrical components of the rocking mechanism, it will be an important safety feature.

Figure 4: Frame Diagram

Linear actuator rocking mechanism

The primary rocking mechanism will be a linear actuator attached to the rear of the runners. The actuator piston will push directly on the ground and supply the necessary displacement for rocking. Analysis of a standard rocking chair demonstrates that the typical traveling distance for the back of a rocking runner should be between 2 and 4 inches, depending on the user’s position in the chair and the amount of force applied to initiate rocking. The actuator will need to be attached in the middle of the two runners in order to prevent side to side tipping of the chair. The use of two actuators, one for each runner, would result in less force on each actuator, but the need to ensure that the actuators move exactly in unison would create a more complicated and therefore less reliable rocking mechanism. Also, the cost of two small actuators would not be much less than one large actuator, so the effect on our budget would be minimal. Analysis of a standard rocking chair also demonstrates that the average comfortable rocking frequency is about 30 rocks per minute. An old NSF motorized rocker project used a rocking frequency of 6 rocks per minute. Most linear actuators capable of supporting a high load as needed for our project work at speeds of less than 0.5”/sec. This would give us a frequency of only 7.5 rocks per minute with the standard 2” displacement. To achieve a frequency of roughly 20 cycles per minute we need an actuator speed of about 1 ¼” per second. For this reason we will need a high speed actuator. The actuator could be attached to a mechanical system which increases the speed at which a second piston, which provides the force on the ground, moves; but this would amplify the load on the actuator which increases amperage and decreases life.

Figure 5: Life vs. Load of linear actuator (Power Drives Inc.)

Figure 6: Working Model 2D model

The force on the linear actuator will determine which model will be purchased. To determine the forces, a simple free body diagram was drawn for the chair at its most forward and backward positions. To accomplish this, a model providing the greatest accuracy possible of the completed chair was created with the program Working Model 2D. Data from this the program was combined with Anthropomorphic Data from Dr. Enderle’s Introduction to Biomedical Engineering textbook. The segment weight and center of mass were determined for the upper body, thigh, and lower leg. Dr. Hallowell informed us that patient weights would vary from 80-190 lbs. To be safe 200 lbs. was used as the maximum weight in the calculations.

Trunk, arm, head / Thigh / Foot and Leg
Subject Weight (lbs) / Center of mass / Weight (lbs) / Center of mass / Weight / Center of mass / Weight
80 / 0.626 / 54 / 0.433 / 16 / 0.606 / 10
200 / 0.626 / 136 / 0.433 / 40 / 0.606 / 24

Table 1: Anthropometric data

With the anthropometric and Working Model 2D data, a free body of the forces on the runners was constructed and moment analysis was performed. This was completed for two positions of the rocking cycle: the highest and lowest positions of the device. The highest user weight , 200 lbs, was used in the analysis as this will produce the most force on the actuator.

HIGH/FORWARD ANALYSIS