Sensorless Force Estimation for a Three Degrees-of-Freedom Motorized Surgical Grasper1

Abstract

Robotic minimally invasive surgery (R-MIS) has gained in popularity due to its advantages of improving the accuracy and dexterity of surgical interventions while minimizing trauma to the patient. However, because of the loss of direct contact with the surgical site, the surgeon cannot perceive tactile information, which may adversely affect surgical efficiency and/or efficacy. The lack of haptic feedback is seen as a limiting factor in existing R-MIS technology [1].To solve this problem, researchers have incorporated force sensors on the surgical tools to measure the tool–tissue interaction forces [2,3] (Figs. 1 and 2) and reproduce these at the surgeon console. However, the employment of force sensors leads to other problems limiting their practical application. For example, they can make the tools bulky, and the harsh conditions required to sterilize surgical tools may damage the sensors [4].The objective of this research is to sensorlessly estimate the tool–tissue interaction forces based on motor current. Previous work has been done to decouple motions/forces for the three degrees-of-freedom (3DOF) grasper [5,6] and prove the feasibility of the force estimation method [7]; this paper demonstrates the continuation of this work by presenting the force estimation results for grasp, pitch, and yaw DOFs.A 3DOF surgical grasper prototype has been fabricated using 3D printing at approximately 3:1 scale, and each DOF is driven by a DC motor (Faulhaber 2224U012S in combination with 66:1 planetary gearhead) through braided polyethylene cable (Fig. 3). All the joints in the prototype are equipped with ball bearings to reduce friction. A force-sensitive resistor (FlexiForce A201, 4.4 N force range) was used for force measurement, to make sure the force is uniformly distributed on the sensor, a spherical-jointed intermediate pad was placed between the jaw and the sensor. A 3DOF master robot equipped with position sensors on each joint was also fabricated to control the grasper prototype.Experiments have been done on the prototype to test the force estimation for grasp, pitch, and yaw DOFs separately. The experiment setup for the three experiments is shown in Fig. 4. The grasp force is estimated by averaging the force estimations from the driving motors of the two independent jaws based on motor current, the pitch force is estimated from the driving motor of the jaw that is in contact with the force sensor, and the yaw force is estimated from the motor that drives the yaw DOF. The motor current signal is obtained from the motor driving unit (NI 9505) at 2 kHz and is then filtered by a low-pass filter with a cut-off frequency of 3 Hz.To check the reliability of this force estimation method, steady inputs lasting more than 10 s were manually applied to the grasper input cables for producing force estimations for the grasp, pitch, and yaw DOFs. Figures 5–7 show the results by comparing the force estimations with respective force measurements; the force shape comparisons are shown versus time in (a), and the repeated testing results are shown in (b).It is noticed that the performance is similar on all three DOFs; the force estimation has an initial peak at the beginning, due to dynamic effects; then, the amplitude decreases slowly and finally settles to a steady state, which is slightly larger than the force measurement, due primarily to the friction in the mechanism. The repeated testing results demonstrate that the performance of this force estimation method is relatively robust.To test the time response of this force estimation method, periodic inputs were manually applied at about 2 Hz on the grasp, pitch, and yaw DOFs separately, since voluntary surgical motions lie in the 0–2 Hz range [8]. Again similar results have been obtained on all three DOFs. Figure 8 shows the test result on the grasp DOF; the force shape comparisons between force estimation and force measurement are shown in (a), and two input cycles are shown in detail in (b)