Grip force sensors compatible with magnetic resonance imaging (MRI) are used in human motor control and decision-making research, providing objective and sensitive behavioral outcome measures. Commercial sensors are expensive, cover limited force ranges, rely on pneumatic force transmission that cannot detect fast force changes, or are electrically active, which increases the risk of electromagnetic interference. We present the design and evaluation of a low-cost, 3D-printed, inherently MRI-compatible grip force sensor based on a commercial intensity-based fiber-optic sensor. A compliant monobloc structure with flexible hinges transduces grip force to a linear displacement captured by the fiber-optic sensor. The structure can easily be adapted for different force ranges by changing the hinge thickness. A prototype designed for forces up to 800 N was manufactured and showed a highly linear behavior (nonlinearity of 2.37%) and an accuracy of 1.57% in a range between zero and 500 N. It can be printed and assembled within one day and for less than $300. Accurate performance was confirmed, both inside and outside a 3 T MRI scanner within a pilot study. Given its simple design allowing for customization of sensing properties and ergonomics for different applications and requirements, the proposed grip force handle offers researchers a valuable scientific tool.
Since its introduction to the research community in the early 1990s [
Human hand motor control and its central nervous regulation are explored in multiple research fields, such as the investigation of neural mechanisms underlying movement in the healthy brain [
The broad scope of application of force sensing devices in the scientific investigation of movement control underlines a need for MRI-compatible force transducers that can measure the full range of human voluntary grip force in a controlled and reproducible manner. Due to its unique and challenging environment involving strong (mostly 1.5 or 3 T) static and switching magnetic fields as well as radiofrequency pulses and a supine subject instructed to move as little as possible, there are specific constraints to be considered with regard to the design and fabrication of a force sensor for fMRI studies to ensure its safe and proper functioning (see also [
Force sensors taking into account the electromagnetic and spatial constraints of an MRI scanner have been developed (an overview on such systems is provided in [
The compliant structures used with fiber-optic based force sensing devices are mostly made of nonmagnetic metals (e.g., aluminum [
In the present paper, we describe and discuss the design and characterization of a low-cost, inherently MRI-compatible grip force sensor which is based on a commercially available intensity-based fiber-optic sensor and a custom compliant structure, printed on a low-cost 3D printer. The presented sensor is capable of measuring grip forces up to 800 N. We begin by describing its design and report results of a finite element method (FEM) analysis of the compliant structure of the sensor, including the design parameters that should be adjusted for different sensing ranges. Subsequently, we elaborate on a full characterization conducted on the manufactured device. Finally, we report a pilot study where the grip force sensor is used in a typical experimental condition of an fMRI setup, illustrating its inherent MRI compatibility.
The concept of the grip force sensor is based on a 3D-printable monobloc structure (handle) with integrated flexible hinges, which is ergonomic and can be held with a power grip. A 3D rendering of the design can be seen in Figure
Rendering of the developed grip force sensor with a partial cut in the monobloc structure to visualize the placement of the fiber-optic sensor and the mirror.
An affordable commercial fiber-optic sensor system (brass sensor head FUE 999C1004 and analog amplifier FWDK 10U84Y0, Baumer Electric, Frauenfeld, Switzerland, $260) was used for the grip force sensor. This system has already been successfully employed in earlier related work [
Concept of the reflected light intensity measurement. (a) The upper fiber-optic channel emits the light which is reflected on the mirror. The lower fiber-optic channel collects a fraction of the reflected light. (b) If the mirror is placed at a larger distance, the second fiber-optic channel collects more light.
The compliant structure was made of PLA (polylactic acid, an MRI-compatible, biodegradable thermoplastic, electric volume resistivity
The transversal compression of the grip force sensor due to the applied force
(a) Red: flexible structure before deformation due to the applied grip force
With this assumption a model can be derived for an individual hinge. According to Henein [
Using finite element method (FEM) in SolidWorks 2015 (Dassault Systèmes, Vélizy-Villacoublay, France), several grip force sensor designs were iteratively tested to match the order of magnitude of the desired force range. The ergonomic aspect limiting the outside dimensions was tested using styrofoam models and subjects with different hand sizes. The final design included ten hinges on each side and had the following dimensions: total length of 140 mm, height of 45.4 mm (with a curved surface of radius 22.7 mm), and width of 28 mm (which corresponds directly to the width of the hinges
As in the theoretical model described in Section
In order to explore the possible design parameter ranges to customize the grip force sensor, two constraints were defined:
Permissible dimension range (white) depending on grip force
Using a grip force
Furthermore, the FEM analysis with these parameters results in a total displacement
For the characterization of the manufactured grip force sensor (Figure
The developed grip force sensor. Total length 140 mm, height 45.4 mm (with a curved surface of radius 22.7 mm), and width 28 mm. The total weight of the sensor is 140 g.
In order to determine the transfer function, sensitivity, nonlinearity, resolution, nonrepeatability, and hysteresis, a force sweep with six consecutive trapezoidal cycles, from zero to the designed full range 800 N, was performed. The whole force sweep (six cycles) lasted 10 min. Using the least-square best-fit straight line (LS BFSL) method, the mean and standard deviation (SD) of the absolute and relative nonlinearity in V and in percentage to the full scale output (FSO), respectively, were computed for the twelve cycle parts (up and down parts of one cycle are considered as two separate parts). Since the nonlinearity depends on the force range (on which the characterization is based), which in turn depends on the specific application the grip force sensor is used for, the mean and SD of the nonlinearity were calculated as a function of the force range maximum at which the data was truncated before fitting the LS BFSL. The sensitivity
Creep and recovery were examined by loading the grip force sensor with 800 N for 30 min and removing the complete load during the following 60 min. By taking the derivative of the estimated output force (i.e., of the output voltage
In order to test the developed grip force sensor, a pilot experiment with one healthy male subject (aged 27) performing a motor task inside, as well as outside, an MRI scanner was conducted.
In both conditions, the subject held the grip force sensor with the dominant hand using a power grip. During the motor task, the subject was instructed to generate a precise grip force level as displayed on a screen by a target area (±10% of the required force) on a vertical thermometer bar indicator. The level of the thermometer represented the subject’s applied grip force in real time. The subject was required to hold the grip force within the target area for 4 s. A set of different target forces (
In the first condition, the subject was lying in a running Philips Ingenia 3.0 T MRI scanner (Philips, Amsterdam, Netherlands) located at cereneo AG, Vitznau, Switzerland. A 32-channel Philips dStream head coil was used. The visual feedback was displayed on a screen placed at the end of the scanner bore, visible to the subject through a mirror mounted on the head coil. While performing the motor task, one run of functional data in a gradient echo
The experimental protocol and visual feedback were implemented in LabVIEW 2015 (National Instruments, Austin, TX, USA), as well as the acquisition of the grip force sensor output using a 12-bit-resolution data acquisition card (USB 6008, National Instruments, Austin, TX, USA). Prior to each condition, the sensitivity and offset of the grip force sensor were calibrated—simply by using no load and by placing a weight of 4.5 kg on the grip force sensor lying in the concave aluminum counterparts, in order to minimize effects due to viscoelasticity discussed in Section
The transfer function of the grip force sensor, as well as absolute nonlinearity, sensitivity, absolute resolution, and absolute nonrepeatability as a function of an application-specific force range maximum, is presented in Figure
Complete grip force sensor characteristics for the force ranges 800 N and 500 N.
Application | Sensitivity (mV/N) | Nonlinearity (LS BFSL) (%) | Nonrepeatability (%) | Hysteresis (%) | Resolution (N) | Accuracy (RMS) (%) |
---|---|---|---|---|---|---|
800 N | 0.843 ± 0.003 | 10.33 ± 0.10 | 0.76 ± 0.38 | 0.72 ± 0.38 | 2.219 ± 0.008 | 5.99 |
500 N | 0.978 ± 0.005 | 2.37 ± 0.22 | 1.02 ± 0.50 | 0.85 ± 0.09 | 1.911 ± 0.009 | 1.57 |
Characteristics of the grip force sensor determined in a force sweep from zero to 800 N for both, before (blue) and after (red) 800 N long-term load: transfer function, absolute nonlinearity, sensitivity, absolute resolution, and absolute nonrepeatability. Except for the transfer function, all characteristics are visualized as a function of a maximum force range, on which the characterization is based. Depending on the required force range for a specific application, the characteristics can be read off.
Creep and recovery response following a long-term load of 800 N. The dashed line represents the applied force by the material testing machine, and the solid line represents the estimated force, as well as its derivative, based on the sensor reading and sensitivity. The decay rate
Grip force sensor response to a force step from 0 to 500 N within 0.2 s and from 500 to 0 N. The dashed line represents the applied force by the material testing machine, and the solid line represents the estimated force based on the sensor reading and sensitivity.
The comparison of the force means and standard deviations for the two measurement conditions inside and outside the MRI scanner is illustrated in Figure
The outcome measures force mean (a) and standard deviation (SD) (b) for the comparison of the two conditions inside and outside of the MRI scanner are presented for each target value separately. Note that, according to the statistical tests, both conditions can be considered as being equivalent regardless of the outcome measure.
This paper proposes an easily customizable, inherently MRI-compatible grip force sensor composed of a commercial intensity-based fiber-optic distance sensor and a compliant structure printed with a low-cost 3D printer. The comprehensive characterization of the grip force sensor showed that it can be safely used for measurements up to 800 N, which is comparable to the force range of commercially available grip force sensors (e.g., Biopac Systems TSD121C). The range 0–500 N shows the best properties with a very high linearity (nonlinearity of 2.37%) and accuracy of 1.57%, comparable to commercially available products (e.g., 1% for the Biopac Systems, ADInstruments MLT004/ST).
When comparing more closely the characteristics of the proposed grip force sensor for a force range of 500 N and 800 N, the only significant difference is the nonlinearity, since the slope, that is, sensitivity, of the transfer function decreases with increasing forces. This decrease in sensitivity (as well as the increase in sensitivity in the range of 100–300 N) is most probably attributable to the increasing nonlinearity of the fiber-optic sensor itself. In order to obtain a complete characterization including hysteresis not only for the maximal range (0–800 N) but also for the more linear range (0–500 N), the characterization was repeated for a force cycle profile going up to 500 N, only.
The characterization showed minimal hysteresis and nonrepeatability during nominal use. The sensor characteristics, such as linearity and sensitivity, are only marginally affected by the long-term load and, therefore, the sensor will deliver a repeatable output. Yet, during long-term load, the sensor presented some creep and a relatively slow recovery, as previously shown in similar research [
The results from the force step applied to the grip force sensor showed a good dynamic response of the sensor. Thus, it could be used for motor control tasks with rapidly changing forces and to assess reaction and force rise/decrease times, which is an advantage over most commercially available MRI-compatible force sensors relying on air pressure measurement.
As the proposed sensor consists entirely of nonmagnetic and, except for the optical head, nonconducting materials and the sensing is performed by reflected light intensity measurement, it is inherently MRI-compatible. The optical head is made from brass and might lead to deformations of the magnetic field. However, this effect is small and highly localized due to the low magnetic susceptibility of brass [
Moreover, based on theoretical model and FEM analysis, we proposed equations and a recommended dimension range in order to customize the grip force sensor for applications requiring different sensing ranges. The theoretical model and the FEM analysis correspond well (approximately 20%) and therefore provide a cross-validation. The existing small difference (between the two analysis methods) in total displacement of the sensor head with respect to the mirror may be attributed to various assumptions made to simplify the model (perpendicular direction of force application, rigid outer bars, linear behavior of the hinges, and uniform distribution of the force).
For force ranges of 0–800 N the sensor characteristics can be obtained directly from the presented graphs. Yet, for applications requiring forces smaller than 100 N, we recommend using the sensor characteristics of a force range of at least 100 N. This is due to the fact that for low force ranges the relative nonlinearity of the sensor output is high (as the absolute nonlinearity is divided by a smaller number, i.e., force range) and the number of data points available for the LS BFSL fit is limited. As a consequence, there are large variations in the slope of the LS BFSL fit (i.e., sensitivity) and the resolution for low force ranges. In order to obtain more precise measurements with the presented grip force sensor for forces beyond 500 N, the transfer function can be described with a higher-order parametric function, or a lookup table of the nonapproximated transfer function can be used, in order to take the increasing nonlinearity into account. For other force ranges or higher SNR requirements in specific applications, the dimensions of the compliant structure can be adjusted to optimally match the fiber-optic sensor operation range. With the theoretical model and the parametric model derived from the FEM analyses, adapting the grip force sensor to a specific force range becomes very simple and can be done by primarily changing the hinge thickness. However, change of the hinge thickness may require a full characterization with the modified compliant structure and should be done within the permissible dimension range. Note that the lower end of the permissible dimension range is limited not only by internal stress but also by 3D-printer resolution. Nevertheless, for the presented designs the printer resolution of state-of-the-art low-cost 3D printers (in the range of 0.4 mm) is largely sufficient and the structure can be printed accurately. Furthermore, today’s 3D-printing technology allows simple manufacturing of custom ergonomic shapes. Thus, grooves on the handle could be added, in order to make the subjects’ grip type and location more consistent or to adapt it to different hand sizes.
The grip force sensor presented in this paper can be fabricated within less than 24 hours requiring only access to a FDM 3D printer. Furthermore, the total material cost (including fiber-optic sensor, amplifier, and 3D-printed structure) is less than $300.
This paper presented the design and characterization of an inherently MRI-compatible, 3D-printed grip force sensor, based on a commercially available intensity-based fiber-optic sensor and a compliant structure used to transduce force into displacement. The sensor can be fabricated with any low-cost FDM 3D printer within one day and provides accurate MRI-compatible grip force measurement covering a high range of forces at very affordable cost. Researchers can reproduce the presented design and use the sensor according to reported characteristics or adapt the sensing range to custom applications by changing a single parameter of the 3D-printed structure. The characteristics, affordability, and customizability make this sensor a valuable tool for the scientific study of motor control, decision-making, and related topics within and outside the MRI scanner.
The authors declare that there are no competing interests regarding the publication of this paper.
Tobias L. Bützer and Mike D. Rinderknecht contributed equally to this work.
The authors would like to thank D. Woolley and U. Baumgartner for their valuable input regarding the first designs, A. Luft and K. Lutz from cereneo AG for providing access to the MRI scanner, J. Snedeker for allowing the use of his measurement infrastructure, and T. Vögeli for his help with the measurements. This research was supported by the ETH Zurich Foundation in collaboration with Hocoma AG, by the ETH Research Grant ET-17 13-2, as well as by the Swiss National Science Foundation through the National Center of Competence in Research (NCCR) on Robotics.