Sensors for Machine Tool Monitoring

Balancing systems and vibration sensors allow machine tool monitoring and control.

A balancing system is composed of an electro-mechanical balancing head that is either mounted externally (or internally in the case of a grinding machine) to the machine spindle, a vibration sensor, and an electronic controller. In the case of grinding wheel balancing, acoustic sensors can be incorporated into the balance head and all functions can be controlled through a single multifunction device. Together, these combined components can provide a comprehensive solution for continuous automatic balancing, dress, gap and crash control.

Operating principle

Balancing - Vibration on a rotating spindle is generated by the sum of the variations in weight distribution on the various components assembled on the spindle. The resultant of the sum of these variations can be described as a force vector with a direction and an amplitude. The corrective action is to produce a force vector with an equal amplitude but opposite direction to cancel the imbalance condition. The first step to achieve this corrective action is to measure the vibration produced by the machine. This is measured using a vibration sensor that is composed of a seismic mass that is connected to a piezoelectric transducer (quartz type) which converts the vibration into an electric signal that can be processed by the balancer controller. When the machine control has determined that the machine is in a condition safe to execute the balance cycle, it gives the command to the balancer control to begin. The balancer controller then moves the masses inside the balance head following a certain algorithm to produce the corrective force required to maintain the machine vibration within the predetermined tolerance limits.

Monitoring - Tool monitoring systems based on acoustic technology monitor the noise emitted by the action between the tool and workpiece or wheel and dresser. This noise is created by the release of stored energy that occurs as the cutting action is carried out. This energy release is transmitted through the machine as ultrasonic waves. As these waves move through the various components of the machine, the frequency and amplitude are altered. By monitoring these waves, and comparing them to a baseline, changes can be detected and based on these changes decisions can be made for changes in machine actions. This could be used to determine contact between a tool and the workpiece, contact between a grinding wheel and a dresser tool. Changes in the acoustic emissions can indicate changes in forces due to tool cutting conditions and cycle adaptations can be made. In the case of grinding machines, the acoustic sensors can be located in the balancing head placing the sensor in the closest position to the machining interface.

Below is an illustration of a typical balancer and acoustic monitoring system application layout on a grinding machine: