HMI Development Tips and Tricks:

Beckhoff TC HMI Image: https://www.beckhoff.com/en-us/products/automation/twincat-3-hmi/

HMI (Human Machine Interface) development is critical for any controls project allowing the operator to interface with the machine in the simplest manner possible.
Overall, there should be some framework in how the HMI interfaces with the PLC control variables via a control protocol interface (Modbus TCP, ADS, OPC-UA, Profinet etc.).
The selection of the control protocol interface between HMI & PLC is critical as it determines how easy an interface can be changed & expanded upon dependent on different equipment interfaces.
This will be a separate blog discussion later. 

HMI Variables used for both control and indication (bi-directional data) on the HMI should be separated from the variable data spaces used for only indication or only control.
Likewise, variables that are to be as used as retained configuration parameter variables (Config) for the machine should be in its own separate data space.
Polling of critical variables on the HMI should be dependent on the dynamics of the variable.
Typically process or motion control feedback variables such as displacement, speed, acceleration, rotation, flow, and pressure should be polled between 100 ms to 500 ms. State events or state transitions may be polled faster but one must consider that faster HMI polling will create general lag while running the HMI in debug or in run-time. Alarms and interlocks may be polled between 500 to 1000 ms. 

One should have a general screen structure or mockup of the HMI. 
An example Frac pump fleet control HMI mockup is shown below:

• An intuitive default boot-up or power-up screen showing the power-up states, overall machine state, and communication for mission critical equipment:​

• Operational screen with specific controls for equipment, in this case control of a pump:

• Configuration screen for operator instrumentation and sensor calibration as well as the ability to change set-point limits if necessary:

• Safety Interlocks screen showing specific interlocks preventing equipment from running or being enabled:

• Alarms screen with alarm event logger:

General HMI development tips:

• The operator should have access to the E-Kill (all power-off) or E-Stop buttons as well as some indication of the overall machine state at all times on the top banner of the HMI.
• The operator should always have access to redundant preferably SIL rated E-Stop/E-Kill push buttons in his control station/cab/area.
• Set-point limits on the HMI should be read by the HMI and should be dynamic if the limits configurations can be changed in the PLC code.
• Alarm naming should be explicit in terms of priority (critical, fault, alarm, error, informational, config error) and condition (Over Limit/Warning, Under Limit/Warning, Timed Out etc.) in an intuitive color scheme.
• Use ISA, IEC, and NFPA guidelines for equipment indication color scheming for alarms and equipment conditions (reference: Control-Systems-Engineering-Exam-Reference-Manual_A-Practical-Study-Guide_3rd Edition.pdf):

• Use standard ISA P&ID/PFD symbols with correct P&ID/PFD tags on the HMI where possible, for the operator to easily & correctly cross reference the HMI, P&ID, and electrical IO schematics during troubleshooting/commissioning: