Parametic equation for circular motion not giving desired effect

I have tried to use the parametric equations for circular motion to move an object around a circle. x(t) = A*cos(2pi * t), y(t) = B*sin(2pi *t)

The object moves in a circle when I run the code, but the orbit of the object moves downwards slightly with every rotation meaning it eventually goes off the screen. Obviously I do not want this to occur and I was it to revolve around the same point in space. I have worked out that this was due to gravity being set to 1 on the rigidbody. However when I set it to 0, it only completes a half circle and then stops.

Here is my code:

using UnityEngine; using System.Collections;

public class Parametric : MonoBehaviour {

 public float loadtime;
 float xpos;
 float ypos;
 Vector2 positionVector;
 
 // Use this for initialization
 void Start () {
     
 }
 
 // Update is called once per frame
 void FixedUpdate () {
     Debug.Log (Time.timeSinceLevelLoad);
     loadtime = Time.timeSinceLevelLoad;
     xpos = Mathf.Cos (2 * Mathf.PI * loadtime);
     ypos = Mathf.Sin (2 * Mathf.PI * loadtime);
     
     positionVector = this.transform.rigidbody2D.position;
     positionVector.x = xpos;
     positionVector.y = ypos;
     this.transform.rigidbody2D.position = positionVector;
     
 }

}