A coiled tubing injector comprises a drive system for independently driving a plurality of chains independently but otherwise retarding relative motion between the driven chains when a chain begins to slip uncontrollably. A write current is initialized to an initial write current INIT_Iw, and an overshoot is initialized to a maximum MAX_OS. Each electronic shopping cart has an indication of items currently within the electronic shopping cart and billing and shipment information. As a result, carriers can climb over the barrier and move from the source region to a drain region, and thus, a drain current flows. Each of the side frames includes a guide groove for guiding a shaft of at least one of the plural rolls through an opening which is open in a direction orthogonal to an axial direction of the plural rolls.