Synthetic aperture (SA) is a technique that enhances the image resolution by synthesizing information from multiple subapertures. The application of this technique for medical ultrasound imaging has been an active research area, but the resolution improvement is limited by the physical size of the ultrasound array transducer. With a large F number (depth to aperture-size ratio), it is hard to achieve high resolution at deep regions without extending the effective aperture size. In this paper, we investigate experimentally an approach to extend the available aperture size for SA by sweeping the ultrasound transducer using a precise robotic arm. Pose information from the robots kinematic for the calibrated probe is used to synthesize the signals received at different positions; therefore the available aperture is wider than the size of transducer. To experimentally validate this approach, a robot arm (UR5, Universal Robot) was used to hold a 64 elements phased array transducer (0.32 mm pitch, 2MHz central frequency), and in-plane lateral translational motion was applied. A line phantom as a point source and an ultrasound phantom with wire targets and anechoic region were used for evaluation. The full width at half maximum of a reconstructed point source improved a factor of 2.76 by moving five poses with 10.24 mm step size. For the ultrasound phantom, the contrast-to-noise ratio of anechoic region enhanced 12% by moving three poses with the same step. Results indicate that the technique to robotically extend aperture has potential to improve theimage quality for SA ultrasound imaging.