Automata with sensing heads

Automata that can detect coincidence of heads (sensing heads) are investigated. It is shown that several types of storage can be used for simulating sensing heads with ordinary heads. Then a general technique for separating automata with an increasing number of sensing heads on unary input alphabets is presented. Finite automata with sensing heads, pebble automata, and bounded counter automata are compared. On unary input pebble automata and finite sensing head automata are equivalent which implies a hierarchy of languages accepted by automata with an increasing number of pebbles. Finally decision problems for one-way finite sensing head automata on unary input alphabets are classified