Honeycomb Structure

Design parameter

For the same material, different structures can make vast difference on mechanical properties.

Specifically, for a single honeycomb chamber, the figure above illustrates the difference of deformation about different wall thickness and with the addition of grooves under the same pressure and cross section area. The structure with a thicker wall appears are more stable however the addition of the inner grooves imparts additional flexibility to the structure.

Thus, we use flexibility and load bearing capacity as the evaluation metrics of mechanical property of the HPN manipulator. Flexibility is defined as the ratio of the reachable area of the manipulator's tip to the square of manipulator's original length. And load bearing capacity is defined as the maximum load moment under the condition that the manipulator's tip can hold stably at the same height with its fixed end. The definitions have relations with the original length of the manipulator, however, the connection can be approximately omitted.

Later, we will illustrate the model design procedure and use FEM simulations to estimate the relationship between flexibility, load bearing capacity and wall thickness, as well as groove depth. The result can be viewed on  the testing page.

Honeycomb structure 

The honeycomb structure we use to make manipulators consists of compressed hexagonal chambers. This structure has many advantages, such as a high elongation rate and crush resistance, When airbags inside pressurize, some of the chambers deform, so the structure elongates or bends.

 

Before making our model we must determine the design parameters. There are several dimensions that affect the actuator’s behavior: wall thickness, and groove depth.

 

structure_bend1

Honeycomb Structure

 

top_view

 

Model design CAD tutorial

This tutorial contains step-by-step instructions for making a solid model of a Honeycomb structure in SolidWorks 2015. The SolidWorks part files can be downloaded here. If you prefer to use a different software package, you should be able to apply the general tutorial steps to most solid modeling environments or you can refer to the dimensioned figures below.

In addition, there are other design parameters to be determined: chamber height, and overall number of chambers.

In this tutorial we will make an honeycomb structure with 2 columns, each column have 8 chambers. Each chamber is 3.5 mm long, 39 mm wide, 60 mm high, with 2 mm wall thickness. Of course, you can alter these parameters to change the morphology of your actuator yet general steps covered in the tutorial should still apply.

technical-drawing

Design Individual Chamber

There are many ways to make the honeycomb structure, in order to make it easy we will introduce it here step-by-step.

The general overview of steps entails first creating a fragment of one chamber, and complete it using symmetry operations. After that, the whole manipulator can be achieved by replication based on one chamber and patterning to achieve the network.

First step, launch Solidworks and create a new Part.

new_part

Next, select a datum plane and begin to sketch.

Next, we draw the sketch of a single honeycomb chamber. Because the chamber is a symmetrical hexagon, we only draw its quarter and then mirror it twice (using the Mirror Entities operator).

Now it's the sketch of one quarter of the hexagon shown below.

sketch_begin

Mirror it for the first time, and we get the upper half of the hexagon.

mirror1

Mirror again, and we get the whole hexagon.

mirror2

Then, we use the Offset Entities operator, with the distance of 2 mm, to get the chamber with desired thickness.

offset

Groove on the Corner

As discussed before, there are two parameters affecting the mechanical property of the manipulator: wall thickness and groove depth of the honeycomb structure. In this step, we will implement 2 mm depth grooves in sketch.

Below is the single chamber finished in last step.

single_chamber

We draw a rectangle with a depth of 2 mm and a width of 0.4 mm at the inner acute angle.

groove

Then we obtain grooves on two sides using the Mirror Entities operator.

groove_mirror

Pattern Sketch

The manipulator frame is composed of two lines, with 8 chambers on each line. In this page, we show how to sketch that using Linear Sketch Pattern and Move Entities operators.

Here is the sketch of a single chamber with inner grooves finished in last step.

ready_to_pattern

We use Linear Sketch Pattern operator and pattern it as below, with instances as 8 and spacing as 5.5 mm.

linear_pattern_p

Click Accept (green check mark on the left), and finish the pattern. The result is shown below.

linear pattern

Then, enable Move Entities operator, and select start point and end point at the red points shown below.

pattern_detail

pattern_p

Click Accept, and get the sketch below.

pattern

Trim

In this page, we trim the sketch using the Trim Entities operator based on last step. The result will be used for extrusion later.

As shown below, we have drawn the sketch of the whole honeycomb structure. 

ready to trim

First, select redundant lines and delete them by clicking delete button on the keyboard. We need mention that the two lines on the upper and lower boundaries cannot be directly handled in this way.

first step trim

So we use Trim Entities operator to cut the extended area on the two lines at boundary.

second step trim

Then, we also use Trim Entities operator in order to cut the redundant lines at the inner grooves, which can be observed in a magnified view.

before fourth trim

fourth step trim

finish third trim

At last, we use Fillet operator and smooth the structure's two edges as seen below.

fillet

Extrude

In this page, we illustrate the procedure of obtaining a 3D structure using the Extrude command.

The figure below is the trimmed sketch from last step.

fillet

We select Extruded Boss/Base command in 'Features' panel, and select the colored area in the figure below.

extrude

Set the height as 60 mm.

Click Accept (green check mark) and get the final 3D honeycomb structure.