# Photogrammetric Measurement and Analysis of the Shape Profile of Pneumatic Artificial Muscles
## 3 Procedure
### Image Acquisition  
For simplicity of implementation, image acquisition is performed with the single-image photogrammetry configuration. The test setup to perform the force response and shape profile characterization is shown in Figure 7. Photogrammetric data acquisition requires the addition of only a camera, a backdrop, lighting, and masking of components. A camera is placed in a fixed position, and photographic images are taken at a regular time interval while the PAM is quasi-statically stroked during force response characterization testing. The camera used is a Nikon D7500 DSLR Camera with a AF-P DX NIKKOR 18-55 mm f/3.5-5.6 G VR lens. This camera provides a high-resolution image (5568 \(\times\) 3712 pixels) along with manual control of settings including focus and image exposure. Figure 6: PAM with sensors, MTS fixtures, and masking applied prior to mounting in the MTS machine. Figure 7: (**a**) Front, and (**b**) Side views of the photogrammetric test setup (PAM, camera, lights, and backdrop). Careful placement of test setup components is required to obtain images that are sufficient for photogrammetric analysis. The scene of the image should be set to promote maximum contrast between the components of the image to be segmented. The foreground is separated from the background of each image using a threshold of relative brightness, and the end-fittings and braid of the PAM are separated in each image by using a hue threshold. Therefore, the goal for setting the image scene is to aid segmentation by maximizing the brightness differential between the foreground and background, and by maximizing the differential in hue between the end-fittings and the braid of the PAM. To determine the distance from the PAM of the camera placement, a balance must be realized between positioning the camera close to the PAM for maximum resolution, versus maintaining enough distance away from the PAM to minimize the angle-of-view and related perspective distortion. For the tested 236 mm long PAM, the camera was placed 1.52 m (5 ft) from the PAM. The height of the camera is set to a level that makes it level with the mid-height of the PAM in its resting length state. This results in a relatively small angle-of-view of the PAM of 8.84 degreesl when the PAM is at its resting length (Figure 7b). Contrast in brightness between the foreground and background is heightened by maximizing the distance between the foreground and background. A 1.5 \(\times\) 1.5 m (5 \(\times\) 5 ft) black cloth backdrop is placed 1.2 m (4 ft) behind the PAM. The PAM is front-lit by two fluorescent lamps (Smith-Victor 10 inch Adapta-Light) positioned to each side of the camera (Figure 7a). The lamps act to further increase brightness contrast between the foreground and background, and ensure complete, shadow-free lighting of the half of the PAM that is facing the camera. Black gaffers tape is applied to everything but the PAM in the foreground to enable these objects to be segmented out with the background by the brightness threshold (Figure 6). To maximize the contrast of hue between the yellow braid and the aluminum end-fittings of the PAM, the end-fittings are wrapped in blue tape (yellow's color complement). Non-reflective materials are selected for all applied surfaces to minimize specular reflections that can affect the image brightness and hinder hue identification of pixels (due to the aforementioned hue singularity conditions). Camera settings are adjusted to ensure accurate coloration and ideal exposure of images. The lens is set to its shortest focal length of 18 mm to maximize the size and resolution of the PAM's image. The camera's auto-focus functionality is used to obtain a sharp focus of the PAM in the image plane. However, a change in the focus would result in a change in the scaling of the image, so the focus must be set prior to scale calibration and testing. Additionally, the camera's auto-focus functionality is turned off prior to testing to ensure that the camera does not change focus by reacting to the changing shape of the PAM.