Patent ID: 8687989

Claim:
A transfer unit, comprising: a) a rotatable static-dissipative member having a time-varying electrical property; b) a second member adapted to transfer toner to or from the static-dissipative member; c) a control system; d) a power source responsive to the control system for selectively producing an electrostatic transfer field between the static-dissipative member and the second member, so that toner is transferred between the static-dissipative member and the second member; and e) a charger spaced apart from the static-dissipative member and adapted to selectively deposit charge thereon in response to the control system; f) the control system being adapted to: i) successively drive a plurality of different selected voltages or currents through the charger and measure a plurality of respective resulting charger currents or voltages; ii) using the selected voltages or currents and the respective charger currents or voltages, automatically estimate a variation in the electrical property; and iii) cause the power source to produce an electric transfer field that transfers toner and compensates for the estimated variation.