A Laser-Based Direct Cable Length Measurement Sensor for CDPRs

Abstract

Accuracy improvement is an important research topic in the field of cable-driven parallel robots (*CDPRS). One reason for inaccuracies of *CDPRS are deviations in the cable lengths. Such deviations can be caused by the elongation of the cable due to its elasticity or creep behavior. For most common *CDPRS, the cable lengths are controlled using motor encoders of the winches, without feedback about the actual elongation of the cables. To address this problem, this paper proposes a direct cable length measurement sensor based on a laser distance sensor. We present the mechanical design, the first prototype and an experimental evaluation. As a result, the measurement principle works well and the accuracy of the measured cable lengths is within −2.32 mm to +1.86 mm compared to a range from −5.19 mm to +6.02 mm of the cable length set with the motor encoders. The standard deviation of the cable length error of the direct cable length measurement sensor is 58% lower compared to the one set with the motor encoders. Equipping all cables of the cable robot with direct cable length measurement sensors results in the possibility to correct cable length deviations and thus increase the accuracy of *CDPRS. Furthermore, it enables new possibilities like the automatic recalibration of the home pose.

Accuracy Improvement for CDPRs Based on Direct Cable Length Measurement Sensors

“…The proximal anchor points A 1 ,...,A 8 of the cable robot are displayed with Fig. 5: Measured poses to evaluate the DCLM correction controller [5] orange dots. The evaluated grid is quite small compared to the cable robot frame but due to the setup of the cable robot with the heavy end-effector the workspace is limited.…”

“…With this measurement method, a resolution of 0. The mechanical design of the DCLM-Sensors used in this paper is presented in [5]. Furthermore, an experimental evaluation of the DCLM-Sensors, comparing the cable lengths measured with the DCLM-Sensors and the cable lengths set with the motor encoders on the IPAnema 3, can be found in [5].…”

“…The mechanical design of the DCLM-Sensors used in this paper is presented in [5]. Furthermore, an experimental evaluation of the DCLM-Sensors, comparing the cable lengths measured with the DCLM-Sensors and the cable lengths set with the motor encoders on the IPAnema 3, can be found in [5]. The structure of this paper is as follows: In Section 2, the standard kinematic model of a CDPR and the kinematic of the DCLM-Sensor is defined.…”

Cable-Driven Parallel Robot Actuators: State of the Art and Novel Servo-Winch Concept

“…To succeed in such a task, one may rely on exteroceptive measurement devices directly providing EE pose information [39,40], state estimators [41,42], or forward kinematics based on cable length estimation. The latter approach is widely used thanks to well-established techniques in the solution to the forward kinematic problem and thanks to the fact that no sensors other than the ones embedded in the actuators for their low-level feedback control need to be added to the robot (additional sensors can be added to speed-up computation, improve accuracy [43,44], or if embedded sensors are not sufficient [45]). However, accurate pose information is achievable through forward kinematics if and only if (i) a cable model suitable to the application requirement is used [46][47][48], and (ii) there is a clear correlation between actuator displacement ∆θ and cable displacement ∆l, namely the actuation unit transmission ratio K = ∆l/∆θ.…”

An Approach for Predicting the Calibration Accuracy in Planar Cable-Driven Parallel Robots

“…Zhang et al [11] used only external measurement device to perform the iterative calibration method. Martin et al [13] used laser-based cable length measurement sensor to improve calibration quality. Little research has been done on the combination of different types of sensors in CDPRs.…”