Technical note Exploiting robotic link deflection for low-cost force measurement in manufacturing B.T. Gibson ⇑ , C.D. Cox, W.R. Longhurst, A.M. Strauss, G.E. Cook Vanderbilt University, 2301 Vanderbilt Place, PMB 351592, Nashville, TN 37235-1592, USA article info Article history: Received 4 April 2011 Received in revised form 31 August 2011 Accepted 19 September 2011 Available online xxxx Keywords: Force Measurement Manufacturing Friction Stir Welding Robot abstract A custom, low-cost force measurement system for manufacturing is presented that exploits robotic link deflection for measurement purposes instead of utilizing a commercially pur- chased load cell. Force measurement can be an important aspect to many manufacturing processes, as it can allow for force feedback control or other process-planning related oper- ations, such as tool-workpiece autozero or through-the-tool joint tracking. This system is demonstrated on a machine that is used for Friction Stir Welding research, and a Finite Ele- ment Analysis is also conducted on the robotic link that is used for axial force measurement purposes. This system may be particularly attractive to small businesses or low-volume manufacturers whose smaller operating budgets may normally prohibit them from imple- menting force measurement systems. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Force measurement and feedback control can be impor- tant aspects in a variety of manufacturing processes [1]. In machining, force signals have been used to detect the occurrence and the severity of tool breakages [2]. In Friction Stir Welding (FSW), force signals can lend insight into weld quality, and force feedback control can be used to expand the applicability of FSW and even manipulate weld charac- teristics [3,4]. A low-cost force measurement method is presented here in a FSW application that exploits the deflection of the manufacturing robot itself as a means of measuring force, instead of utilizing a commercial load cell. FSW is relatively new joining technique that is rapidly expanding to a wide range of industries. It was developed in 1991 at The Welding Institute (TWI) of Cambridge, England [5]. The process involves a rotating non-consum- able tool, comprised of a shoulder and probe, which plunges into and traverses the joint line. Material is heated, plasticized, and transferred before being forged together at the rear of the tool probe. The operation is either per- formed using standard industrial robots or custom ma- chines that are designed specifically for FSW. Forces can be measured using commercially available load cells, but researchers have also developed custom alternatives [6,7]. An application of a custom, low-cost force measure- ment system is presented here that involved instrumen- ting the FSW robot itself with strain gages in order to measure axial force during the FSW process. 2. Materials and methods A custom force measurement system was developed for use at the Vanderbilt University Welding Automation Laboratory (VUWAL), specifically to be implemented on a Milwaukee #2K Universal horizontal milling machine that has been converted to a vertical orientation with a Kearney and Trecker head attachment for Friction Stir Welding pur- poses. This machine is pictured in Fig. 1. The motion drives on the machine have been retrofitted with motors and position sensors to allow for automatic welding programs and custom control schemes to be utilized. The machine is controlled by a Dell Vostro 230 PC with a custom user-interface. 0263-2241/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.09.012 ⇑ Corresponding author. Tel.: +1 615 322 3322. E-mail address: brian.t.gibson@vanderbilt.edu (B.T. Gibson). Measurement xxx (2011) xxx–xxx Contents lists available at SciVerse ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement Please cite this article in press as: B.T. Gibson et al., Exploiting robotic link deflection for low-cost force measurement in manufacturing, Measurement (2011), doi:10.1016/j.measurement.2011.09.012