Study of the chewing assistance mechanism in powered robotic dentures

2.1. Design of the chewing assistance mechanism

In general, chewing includes elements of cutting, crushing, and grinding, and the functions vary with the dentition. Among these, this study has focused on the two functions of the molar teeth i.e., crushing by biting into food materials and grinding via friction, and investigated the mechanisms to realize them.

Fig. 1 shows the structure of the assisting mechanism that consists of five parts, including a spring. When a force is applied perpendicular to the occlusal surface, the first part moves downward by rotating along the spiral shape. As it moves downward, the food is ground while it is kept pressed by the force in the spring.

A spring with spring constant of 6 N/mm was selected to ensure that the spring can be activated even by elderly people whose occlusal force has decreased. The occlusal surface was designed with parallel grooves to improve the grinding effect. A groove shape with a width of 1 mm and a depth of 1 mm was selected, for which favorable results were obtained in a previous study [2].

2.2. Prototype model of the chewing assistance mechanism

The modelling was done using 3D CAD software (SOLIDWORKS 2015, Dassault Systèmes) based on the specifications of a chewing assistance mechanism, and the prototype model was built using a Formlabs Form2 3D printer. The 3D printer utilized a stereolithography method to build the model in a spiral shape by orienting the model diagonally and using a stacking pitch of 0.1 mm. Since a 3D printer was used, acrylic material with a compressive strength of 120 MPa was used to build the model. The prototype spiral mechanism model consists of five parts, including the spring, as shown in Figs. 1 and 2. In Fig. 2 we can see, from the left, the upper part, the occlusion part, the spiral guide, and the base part. During assembly, the spring is inserted between the base part and the occlusion part. To verify the chewing assistance mechanism model using experiments, the size of the model is larger than that of an actual tooth. The model’s outer diameter is 45 mm, and the rotating occlusion part has an outer diameter of 25 mm to allow for testing by using peanuts as a chewing material. Fig. 3 shows the prototype model.

Fig. 1Structure of the assist mechanism

Fig. 2Component parts

Fig. 3Prototype model

3.1. Experimental method

To verify the chewing performance, four types of mechanisms were developed, as shown in Table 1.

The conditions for the experiment were established by referring to the Manly method [3] of testing the chewing function, in which the subject is made to chew 3 g of peanuts 20 times. The masticated food is then sifted through a 10 mesh sieve, its weight measured, and its ratio to the standard amount is determined as the chewing value.

Fig. 4 shows the experimental device. An air cylinder (PBDALS 10×30-M, KOGANEI Corporation, Japan) was used to apply pushing force as an occlusal force, and a load cell (LCN-A-2kN, Kyowa Electronics Company Ltd., Japan) was used for measuring the applied pushing force. The chewing assistance model was placed in a jig for measurements, and the air cylinder was adjusted such that vertical force could be applied to the model. Peanuts were used as a test material similar to the Manly method, and specimens of one half of a peanut kernel around 0.4 g in weight were selected for use in the experiments. The specimen was placed such that its cross-sectional surface faced the occlusal surface, and a maximum force of 50 N was applied successively thirty times. The normal occlusal force in healthy individuals with a normal chewing function is about 100 N. However, the value of force applied in the test was 50 N to account for elderly people. Next, the ground material was sifted through a 10 mesh sieve, and the chewing value was determined by calculating the ratio of the material passing through the sieve with respect to the standard material. The tests were repeated ten times for each condition, and the mean and standard deviation of the chewing values were calculated. These tests were also repeated with pushing forces of 30 N and 40 N to investigate the effect of differences in occlusal force on chewing value.

3.2. Discussion of experimental results

Table 2 shows the results of the chewing effect verification tests for mechanisms A through D for a pushing force of 30-50 N.

Fig. 5 shows the graph of chewing values for the mechanisms A through D. For a pushing force of 30 N, the mechanisms C and D with rotation exhibited values lower than those for mechanisms A and B without rotation. For a pushing force of 40 N, all the mechanisms exhibited similar values. For the pushing force of 50 N, mechanism C exhibited lower values when compared to the other mechanisms. For the 50 N tests, based on $t$-tests, a significant difference ($p$ < 0.05) was observed in the case of mechanism C when compared to the others.

Upon comparing the results of the tests for 30 N and 40 N pushing forces, it was observed that the mechanisms of A and B without rotation showed a tendency where the chewing value increased with an increase pushing force. Although the values under 40 N pushing force for mechanisms C and D with rotation were both quite high, no significant difference was observed in comparison to the other mechanisms. One reason for this is because the ratio of the material with sizes smaller than 1.7 mm passing through the 10 mesh sieve was used to calculate the chewing value, the amount of materials with sizes in the below 1.7 mm category could not be determined, resulting in an inability to determine the effect of frictional grinding on the chewing value. Hence, further tests using sieves lower than 10 mesh are necessary in order to determine the effect of grinding due to friction.

Fig. 4Experimental device

Fig. 5Chewing value by Mechanism

Fig. 6Measured waveform with 50 N pushing force

To identify the factors associated with the lower chewing values in the mechanisms C and D with rotation, the measurement waves formed at less than 50 N pushing force were captured and analyzed. Fig. 6 shows the rise in the waveform for the mechanisms A through D. The rise in the waveform was more gradual for mechanisms C and D with springs, when compared to those for mechanisms A and B without springs. The results suggested that the rate of transmission of the pushing force decreased due to the springs. When the area under the curve (integral) of pushing force over time was calculated, the value was observed to be 9-12 % lower for the C-D group when compared to the A-B group (the time taken to reach the set value was taken as 50 ms). These observations suggested that it was likely that the impact was lower in C and D mechanisms when compared to the A and B mechanisms, thereby decreasing the effect of grinding due to impact and subsequently leading to the lower chewing values that were observed.

4.1. Motor-driven prototype model

The results of various experiments with the chewing assistance mechanism indicated that the improvement in chewing ability was affected by the compressive force along with the grinding effect due to rotation. Also, the possible applications of ultrasonic motors as actuators for the powered robotic denture was investigated.

A small ultrasonic motor was used to realize the vertical and rotational movements around the lower and upper axes (Fig. 7. small TR motor) [4]. Miniaturization is more effective this way because of the simple mechanism where the shaft (rotor) is inserted into a hole bored through a dice-shaped metal (stator). A piezoelectric element is attached to the surface of the stator. Upon applying a voltage in the ultrasonic range to this area, an ultrasonic progression wave is generated on the inner surface of the stator hole, thereby enabling axial rotation and translation of the shaft.

As shown in Fig. 8, incorporating the mechanism of vertical movement in the inner part of a molar tooth will enable chewing in an occlusal condition.

Fig. 7TR motor (Translation-Rotation motor)

Fig. 8Molar tooth model occlusal condition

Fig. 9Driving experiment equipment

a)

b)

Fig. 10Preload-output Characteristics

Fig. 11Frequency characteristics (Preload 9.8 N)

4.2. TR motor driving experiments and discussions

An experimental setup (shown in Fig. 9) was created to measure the output and frequency characteristics of the motor. The torque and thrust force were measured by using a thread and weight hung from a shaft shown in the figure. In the TR motor used in the experiments, phosphor bronze (C5191) was used as stator material and stainless steel (SUS303) was used as rotor material. The stator was shaped in the form of a 14 mm cube with a 10 mm hole bored through it. The shaft (rotor) was 150 mm long and weighed 92.3 g. The inner and outer diameters of the stator were 10.009 mm and 9.998 mm, respectively. The texture of the contact surface had an average roughness 3.2 μm. The impedance characteristics in a range of 60-90 kHz were measured using an impedance analyzer．

Fig. 10 shows the preload-output characteristics of rotation and thrust force in the direction of translation. The maximum torque was 16.7 mN with a preload of 14.5 N in the direction of rotation, and the maximum thrust was 1.96 N with a preload of 9.8 N in the direction of translation. Based on these results, the optimum preload was set at 9.8 N. Fig. 11 shows the frequency characteristics at the optimum preload. For both rotation and translation, the results showed a slight change in speed at a frequency band higher than the frequency at which maximum speed is obtained. This indicated that a frequency band higher than the resonance point is suitable for speed control.