blending system

Hi all i've got a small blending system i wish to fit a less mechanical control system to! The existing system uses 5 interval timers (not sure if you call them that in the US), basically the operator sets each timer to the desired percentage and the feeder controlled by that timer runs for that time period followed by the next feeder. can i do this simply with a PLC? i dont want to use a HMI as the operators on this plant would damage it in a matter of minutes! they drive dump trucks etc so if it dont work they just push harder. Anyone with any idears?

Why do you want to change the system ?

Depends what you mean by simple.

Thumbwheel switches might be an answer if a HMI isn't an option.

How accurate does the percentage need to be? If it's to the nearest 1 percent you'll need 2 switches per timer, that's 10 switches, each using 4 inputs on the PLC (assuming BCD thumbwheels), that's 40 inputs. Or you can get clever and use multiplexing - e.g. use 10 outputs and 10 inputs and decade thumbwheels instead of BCD.

Another option might be to consider recipes. How many different blends do you have, i.e. how many different combinations of timer values do you need. One BCD thumbwheel switch could allow you 10 different timer combinations using only 4 PLC inputs. This option is more likely to cut down on operator error as well.

You did not offer much detail on the system but a PLC could offer some advantages...it depends.
1. You can offer a one button solution for each "blend". In other words you can have pushbuttons labeled for each blend you make..ie Blend 1 PB when pressed would move values (from a register) into the timer presets.

2. Instead of one feeder at a time it may be possible to run all the feeders, this depends on what they are feeding too and its capacity. Several options/posssiblities which could increase productivity because you now have the option to run the feeders when you want not in sequence.

Simple answer is YES you can do it and do not need an HMI but some HMI's are tougher than you think.