An HMI is a large purchase, so it's important to understand exactly what is required of it.HMIs typically perform one of three primary roles: button replacer, data processor, or supervisor. Push-button replacement HMIs replace LEDs, on/off buttons, switches, or any mechanical device that performs a control function. Data processors are used in applications that require constant feedback and monitoring. Typically, these data processors are equipped with mass storage. Supervisors work with SCADA and MES, which are centralized systems that monitor and control entire sites or large complexes of systems distributed over a large area. The supervisor HMI is usually linked to the database and software program of the SCADA system.
Consider first the other components necessary to make a manufacturing control system work. A production line consists of all the machinery that performs the work necessary to produce a product. Next, consider the various input/output sensors that monitor temperature, speed, pressure, weight, and feed rate. Third, decide on a programmable logic controller (PLC) that will receive data from the input/output sensors and convert the data into logical combinations.
HMIs are initially separated by their display size; typically from 4.3" to 15" diagonal. Within these sizes, HMIs are separated by their features (i.e., communication capabilities). HMIs with multiple COM and/or Ethernet ports typically cost more, but allow supervisory types of applications to connect well to multiple PLCs. make sure that your PLC is using a communication protocol that can connect to the HMI by first verifying the PLC's protocol and ensuring that it is present in the HMI Communications and Connectivity Guide.
If your system has many very simple and repeatable processes, you may not need an HMI with a large memory capacity. in this case, you can select an HMI based on size and choose an HMI with the right price point.