The ultrasonic welding of carbon fiber-reinforced thermoplastic laminates received a wide interest from researchers in many fields, mainly in the aerospace and automotive industries. It allows the efficient joining of fiber-reinforced thermoplastic structural parts through the spot welding approach with a high level of automation. In this paper the differential ultrasonic spot welding method is briefly illustrated and a logical control system for this welding process is investigated. One of the main challenges of the differential ultrasonic welding is the overheating of the weld spot. To overcome this critical problem an active weld control method is suggested and investigated. The new method is based on the observation of the phenomena that the consumed power by the welder is in correlation with the temperature in the weld spot. It is assumed that if the power time derivative exceeds a certain value, then the ultrasonic displacement amplitude should be reduced by the controller. Consequently, the temperature in the weld spot can be indirectly regulated and a weld spot temperature in an acceptable range may be realized. The effectiveness of the control system is analyzed by means of weld temperature measurements and mechanical strength tests.